Oil and Gas Terms Beginning with “D”

An individual trained in the science and art of intentionally drilling a well along a predetermined path in three-dimensional space, usually involving deviating the well from vertical and directing it in a specific compass direction or heading. The directional driller considers such parameters as rotary speed, weight on bit, control drilling and when to stop drilling and take surveys of the wellpath, and works closely with the toolpusher.

A treating system used to remove hydrogen sulfide [H2S], carbon dioxide [CO2] and carbonyl sulfide from a gas stream. The acid gases are absorbed by the diethanolamine (DEA), and sweet gas leaves at the top of the absorber.

The difference in the arrival times or traveltimes of a reflected wave, measured by receivers at two different offset locations, that is produced when reflectors dip. Seismicprocessing compensates for DMO.

A seismicprofile recorded specifically to study the lower crust, the Mohorovicic discontinuity and the mantle of the Earth, typically using refraction methods. Most standard seismic reflection profiles record only a small fraction (typically, on the order of 10 km [6 miles]) of the Earth's crust, which is 5 to 75 km [3 to 45 miles] thick.

Well tests conducted with the drillstring still in the hole. Often referred to as DST, these tests are usually conducted with a downhole shut-in tool that allows the well to be opened and closed at the bottom of the hole with a surface-actuated valve. One or more pressure gauges are customarily mounted into the DST tool and are read and interpreted after the test is completed. The tool includes a surface-actuated packer that can isolate the formation from the annulus between the drillstring and the casing, thereby forcing any produced fluids to enter only the drillstring. By closing in the well at the bottom, afterflow is minimized and analysis is simplified, especially for formations with low flow rates. The drillstring is sometimes filled with an inert gas, usually nitrogen, for these tests. With low-permeability formations, or where the production is mostly water and the formation pressure is too low to lift water to the surface, surface production may never be observed. In these cases, the volume of fluids produced into the drillstring is calculated and an analysis can be made without obtaining surface production. Occasionally, operators may wish to avoid surface production entirely for safety or environmental reasons, and produce only that amount that can be contained in the drillstring. This is accomplished by closing the surface valve when the bottomhole valve is opened. These tests are called closed-chamber tests.Drillstem tests are typically performed on exploration wells, and are often the key to determining whether a well has found a commercial hydrocarbonreservoir. The formation often is not cased prior to these tests, and the contents of the reservoir are frequently unknown at this point, so obtaining fluid samples is usually a major consideration. Also, pressure is at its highest point, and the reservoir fluids may contain hydrogen sulfide, so these tests can carry considerable risk for rig personnel.The most common test sequence consists of a short flow period, perhaps five or ten minutes, followed by a buildup period of about an hour that is used to determine initial reservoir pressure. This is followed by a flow period of 4 to 24 hours to establish stable flow to the surface, if possible, and followed by the final shut-in or buildup test that is used to determine permeability thickness and flow potential

The characteristic plot of pressure versus time obtained from the mechanical recording of pressure gauges in a DST tool. Pressure rises as the tool is lowered into the hole and the hydrostatic head above the tool increases. The pressure stabilizes when the tool reaches bottom and then moves when the packer is set. Pressure drops immediately upon opening of the downhole valve to match the pressure in the drillstring, and then rises as fluid flows into the string. When the downhole valve is closed, the pressure buildup period begins immediately and continues until the valve is closed again.

Units of atm, cm3/s, cp and D, as originally used by Darcy in flow experiments.

The drilling line from the crown block sheave to the anchor, so called because it does not move.

A method for the measurement of fluid saturations in a core sample by distillation extraction. The water in the sample is vaporized by boiling solvent, then condensed and collected in a calibrated trap. This gives the volume of water in the sample. The solvent is also condensed, then flows back over the sample and extracts the oil. Extraction continues for a minimum of two days until the extracted solvent is clean or the sample shows no more fluorescence. The weight of the sample is measured before and after extraction. Then the volume of oil is calculated from the loss in weight of the sample minus the weight of the water removed from it. Saturations are calculated from the volumes.

Removable, hard-steel, serrated pieces that fit into the jaws of the tongs and firmly grip the body of the drill pipe, drill collars, or casing.

The power supplied to a drilling rig by diesel engines driving electric generators.

Tools utilized in achieving directional drills, including whip stocks, BHA configurations, three-dimensional measuring devices, mud motors, and specialized drill bits.

An equation used to calculate the interval velocity within a series of flat, parallel layers, named for American geophysicist C. Hewitt Dix (1905 to 1984). Sheriff (1991) cautions that the equation is misused in situations that do not match Dix's assumptions. The equation is as follows:

A type of gas detector tube that quantitatively measures a gas that is passed through the tube by the length of the stain it generates chemically in the tube. Drdger tubes are used in Garrett Gas Train tests for sulfides and carbonates.

Heavy, thick-walled tubes used between the drill pipe and the bit in the drill string, used to stiffen the drilling assembly and put weight on the bit.

Heavy seamless tubing used to rotate the bit and circulate the drilling fluid. Joints are generally approximately 30 feet (9.5 metres) long.

A heavy duty wire rope hoisting line, reeved back and forth through the sheaves of the crown block and travelling block.

The unit of measurement to compare the relative intensity of acoustic or electrical signal, equal to one-tenth of a bel, named for American inventor Alexander Graham Bell (1847 to 1922). The logarithm of the ratio of the sound or signal to a standard provides the decibel measurement. The symbol for the unit is dB. Humans typically hear sounds in the range of 20 to 50 dB in conversation, and upwards of 90 dB when exposed to heavy machinery or aircraft.

Natural or induced production impairments that can develop in the reservoir, the near-wellbore area, the perforations, the gravel-pack completion or the production pipelines, such as the tubing. Natural damage occurs as produced reservoir fluids move through the reservoir, while induced damage is the result of external operations and fluids in the well, such as drilling, well completion, workover operations or stimulation treatments. Some induced damage triggers natural damage mechanisms.Natural damage includes phenomena such as finesmigration, clay swelling, scale formation, organic deposition, including paraffins or asphaltenes, and mixed organic and inorganic deposition. Induced damage includes plugging caused by foreign particles in the injected fluid, wettability changes, emulsions, precipitates or sludges caused by acid reactions, bacterial activity and water blocks.Wellbore cleanup or matrix stimulation treatments are two different operations that can remove natural or induced damage. Selecting the proper operation depends on the location and nature of the damage.

The area surrounding the wellbore that has been harmed by the drilling process, generally as a result of mud or cement-filtrateinvasion. Near-wellbore damage can significantly affect productivity and is typically easier to prevent than it is to cure. Although almost always present, a lightly damaged zone around the wellbore can be bypassed by perforation tunnels to create connecting conduits from the wellbore to the undamaged reservoirformation. More severe cases of damage may require a matrix-acidizing treatment to restore the natural permeability, or a hydraulic fracturing treatment to create a new high-conductivity flow path to the reservoir.

The opposition, slowing or prevention of oscillation, or decreasing vibration amplitude, as kinetic energy dissipates. Frictional damping can be important in the use of geophones for seismic surveys, since a vibrating instrument is difficult to read. Eddy currents can produce electromagnetic damping. The classic example of damping from physics is the slowing of a swinging pendulum unless it has a steady supply of energy.

A standard unit of measure of permeability. One darcy describes the permeability of a porous medium through which the passage of one cubic centimeter of fluid having one centipoise of viscosity flowing in one second under a pressure differential of one atmosphere where the porous medium has a cross-sectional area of one square centimeter and a length of one centimeter. A millidarcy (mD) is one thousandth of a darcy and is a commonly used unit for reservoir rocks.

A device dropped or pumped through a tubing or coiled tubing string to activate downhole equipment and tools.

The rate at which measurements are transmitted between logging tool and surface. In measurement-while-drilling (MWD), if the data rate is low in comparison with the drilling or tripping speed, the sampling interval or the amount of data transmitted must be reduced or else information will be lost. In wireline logging, the data rate can limit the logging speed or the number of tools in the tool string.

A depth reference point, typically established at the time the well is completed, against which subsequent depth measurements should be corrected or correlated.

A value added to reflection times of seismic data to compensate for the location of the geophone and source relative to the seismic datum.

The depth to which pressures are corrected to adjust for differences in elevations at which pressure measurements are made in different wells or at different times.

The daily cost to the operator of renting the drilling rig and the associated costs of personnel and routine supplies. This cost may or may not include fuel, and usually does not include capital goods, such as casing and wellheads, or special services, such as logging or cementing. In most of the world, the day rate represents roughly half of the cost of the well. Similarly, the total daily cost to drill a well (spread rate) is roughly double what the rig day-rate amount is.

Oil at sufficiently low pressure that it contains no dissolved gas or a relatively thick oil or residue that has lost its volatile components.

The unit of measurement to compare the relative intensity of acoustic or electrical signal, equal to one-tenth of a bel, named for American inventor Alexander Graham Bell (1847 to 1922). The logarithm of the ratio of the sound or signal to a standard provides the decibel measurement. The symbol for the unit is dB. Humans typically hear sounds in the range of 20 to 50 dB in conversation, and upwards of 90 dB when exposed to heavy machinery or aircraft.

A fault surface parallel to a mechanically weak horizon or layer, or parallel to bedding, that detaches or separates deformed rocks above from undeformed or differently deformed rocks below. Decollements, or decollement surfaces, are typical of regions of thrust faulting such as the Alps.

A mathematical operation that uses downhole flow-rate measurements to transform bottomhole pressure measurements distorted by variable rates to an interpretable transient. Deconvolution also can use surface rates to transform wellhead pressures to an interpretable form. Deconvolution has the advantage over convolution that it does not assume a particular model for the pressure-transient response. However, the simplest form of deconvolution often gives a noisy result, and more complex approaches may be computing intensive.

A particular type of induction log that was designed to read deep into the formation while maintaining reasonable vertical resolution. The deep induction log (ID) is based on the measurement of a 6FF40array and was combined with a medium induction array to form the dual induction tool. Versions built after 1968 had a small extra transmitter coil to reduce the borehole effect on the medium induction while changing the deep response very little. The midpoint of the ID integrated radial geometrical factor is at 62 in. [157 cm] radius for high resistivities, reducing to 45 in. [114 cm] at 1 ohm-m. ID receives very little signal from within 20 in. [50 cm] of the tool. The vertical resolution is about 8 ft [2.4 m] but varies with local conditions.

A seismic profile recorded specifically to study the lower crust, the Mohorovicic discontinuity and the mantle of the Earth, typically using refraction methods. Most standard seismic reflection profiles record only a small fraction (typically, on the order of 10 km [6 miles]) of the Earth's crust, which is 5 to 75 km [3 to 45 miles] thick.

A method of marineseismic acquisition in which a boat tows a receiver well below the surface of the water to get closer to features of interest or to reduce noise due to conditions of the sea. Deep tow devices are used for some side-scan sonar, gravity and magnetic surveys.

A perforating charge designed to provide a long perforation tunnel, such as may be required to bypass any near-wellbore damage. Specially designed deep-penetrating charges achieve this additional length while retaining a medium-sized entrance hole, an important consideration in high-shot density applications.

Exploration activity located in offshore areas where water depths exceed approximately 600 feet [200 m], the approximate water depth at the edge of the continental shelf. While deep-water reservoir targets are geologically similar to reservoirs drilled both in shallower present-day water depths as well as onshore, the logistics of producing hydrocarbons from reservoirs located below such water depths presents a considerable technical challenge.

(noun) A production logging tool that uses a deflector plate or vane positioned in the flow stream within a wellbore to measure the velocity and direction of fluid flow. The force exerted on the deflector by the flowing fluid is proportional to the flow rate, enabling downhole flow profiling across producing intervals.

A thinning agent used to reduce viscosity or prevent flocculation; incorrectly called a "dispersant." Most deflocculants are low-molecular weight anionic polymers that neutralize positive charges on clay edges. Examples include polyphosphates, lignosulfonates, quebracho and various water-soluble synthetic polymers.

A clay-based, water mud that has had its viscosity reduced with a chemical treatment; incorrectly, called a "dispersed" mud. The chemical used is a deflocculant, not a dispersant. A well-known and effective clay deflocculant is lignosulfonate. The mud, after being deflocculated, usually shows much improved filter-cake qualities with lower yield point and gel strengths. Filter-cake quality is improved because when clays are deflocculated, the platelets become detached from each other and can lie flat to form a thin, low-permeability filter cake. Lowering yield point and gel strength may not always be desired and can be adjusted by the amount of deflocculant added in each treatment. If yield point and gels are lowered too far, suspension and cutting capacity of the mud are impaired.

The act of reducing the viscosity of a suspension by adding a thinning agent, also known as a deflocculant.

A mud additive used to lower interfacial tension so that trapped gas will readily escape from mud. Mechanical degassing equipment is commonly used along with defoamer. Octyl alcohol, aluminum stearate, various glycols, silicones and sulfonated hydrocarbons are used as defoamers.

In a separator, a series of inclined parallel plates or tubes to promote coalescence, or merging, of the foam bubbles liberated from the liquid.

A device that removes air or gases (methane, H2S, CO2 and others) from drilling liquids. There are two generic types that work by both expanding the size of the gas bubbles entrained in the mud (by pulling a vacuum on the mud) and by increasing the surface area available to the mud so that bubbles escape (through the use of various cascading baffle plates). If the gas content in the mud is high, a mud gas separator or "poor boy degasser" is used, because it has a higher capacity than standard degassers and routes the evolved gases away from the rig to a flaring area complete with an ignition source.

A unit of measurement established by the American Petroleum Institute (API) that indicates the density of a liquid. Fresh water has an API density of 10.

To remove water from a substance. The substance may be crude oil, natural gas or natural gas liquids (NGL).Fluid dehydration is needed to prevent corrosion and free-water accumulation in the low points of a pipeline. In the case of gas, it is especially important to avoid hydrate formation and also to meet pipeline requirements. Typical maximum allowable water vapor content is 7 pounds of water per million standard cubic feet. In colder climates, this threshold value could be 3 to 5 pounds per million standard cubic feet. Water vapor can also affect the sweetening and refining processes of a natural gas.Dehydration of crude oil is normally achieved using emulsion breakers, while gas dehydration is accomplished using various liquid desiccants such as glycols (ethylene, diethylene, triethylene and tetraethylene) or solid desiccants such as silicagel or calcium chloride [CaCl2].

The loss of water from cementslurry or drilling fluid by the process of filtration. Dehydration results in the deposition of a filter cake and loss of the slurrys internal fluid into a porousmatrix. The cement is not completely dehydrated because sufficient water remains to allow setting of the cement.

A device used to remove water and water vapors from gas. Gas dehydration can be accomplished through a glycol dehydrator or a dry-bed dehydrator, which use a liquid desiccant and a solid desiccant, respectively.Gas dehydrators are designed to handle only water and gas vapors. If liquid water or oil enters the dehydrator, the device cannot work properly.

Consideration paid to the lessor by a lessee to extend the terms of an oil and gas lease in the absence of operations and/or production that is contractually required to hold the lease. This consideration is usually required to be paid on or before the anniversary date of the oil and gas lease during its primary term, and typically extends the lease for an additional year. Nonpayment of the delay rental in the absence of production or commencement of operations will result in abandonment of the lease after its primary term has expired.

Tests in an oil or gas well to determine its flow capacity at specific conditions of reservoir and flowing pressures. The absolute open flow potential (AOFP) can be obtained from these tests, and then the inflow performance relationship (IPR) can be generated. A deliverability test also is called a productivity test.

An area of deposition or the deposit formed by a flowing sediment-laden current as it enters an open or standing body of water, such as a river spilling into a gulf. As a river enters a body of water, its velocity drops and its ability to carry sediment diminishes, leading to deposition. The term has origins in Greek because the shape of deltas in map view can be similar to the Greek letter delta. The shapes of deltas are subsequently modified by rivers, tides and waves. There is a characteristic coarsening upward of sediments in a delta. The three main classes of deltas are river-dominated (Mississippi River), wave-dominated (Nile River), and tide-dominated (Ganges River). Ancient deltas contain some of the largest and most productive petroleum systems.

A log that shows the magnitude of the correction applied to the long-spacingdetector of a density measurement. When delta rho is above a certain value, typically +/- 0.15 g/cm3, the correction may no longer be accurate, and needs to be examined in more detail. Delta rho is also used as a qualitative indicator of boreholerugosity.

A feature on a sonic log caused by low signalamplitude that results in erroneously long traveltimes. Sonic logs that do not record waveforms measure the acoustic traveltime between transmitter and receiver by detecting the first signal at the receiver above a certain threshold (first motion detection). The threshold is small so that the signal is detected just after it crosses the zero signal baseline. However, if the threshold is set too high, or the signal is too small, the system will not trigger at the sharp zero crossing but at some later point on the waveform. This increases the apparent transmitter-receiver time. Delta-t stretch is more likely at the far receiver, where signals are weaker, so that the apparent traveltime calculated between receivers is too large. In the extreme case, the system triggers on the next cycle of the waveform, known as cycle skipping.

A chemical used to break emulsions (that is, to separate the two phases). The type of demulsifier selected depends on the type of emulsion, either oil-in-water or water-in-oil. Demulsifiers are used in the chemical analysis of oil and synthetic muds and to treat produced hydrocarbons.

An instrument that measures the specific gravity of a mixture of gas, liquid and solids. This device is also known as a densitometer.

A device installed on a mixing or pumping system manifold to measure the density of fluids. The density of fluids pumped into a well is frequently a key operating parameter, requiring constant monitoring and control. This is especially true when mixing slurries and transport fluids for solids, such as fracturing or gravel-pack fluids.

Mass per unit of volume. Density is typically reported in g/cm3 (for example, rocks) or pounds per barrel (drilling mud) in the oil field.

The variation in the mass per unit volume of rocks, which affects the local gravitational field of the Earth. A density contrast also contributes to an acousticimpedance contrast, which affects the reflection coefficient.

A measurement of the bulk density of the formation based on borehole-gravity measurements. As the gravitational attraction between two bodies is dependent upon their masses and their separation, it follows that its measurement also can be used to make a direct determination of density. The density thus measured is highly accurate but averaged over a large volume.

A series of gravity measurements made along a line or over an area of a locally high topographic feature to remove or compensate for the effect of topography on deeper density readings.

A graph that shows the effects of environmental factors on the ideal response of a measurement. The name comes from the departure of the actual response from the ideal. The term is used most commonly in relation to the effect of hole size, mudresistivity, bed thickness, invasion and other factors on electrical logs.

In a nuclear magnetic resonance (NMR) measurement, the loss of synchronization of hydrogen atoms precessing at different speeds about the static magnetic field. When the signals from individual atoms are not synchronized, they are out of phase and the total signal is reduced. The dephasing occurs either because of inhomogeneities in the static magnetic field or through molecular processes. Dephasing due to inhomogeneities is known as the free-induction decay and is corrected by the CPMG sequence. Molecular dephasing is known as transverse relaxation.

An isolated section of reservoir in which the pressure has dropped below that of adjacent zones or the main body of the reservoir formation.

The drop in reservoirpressure or hydrocarbon reserves resulting from production of reservoir fluids. At times, a strong waterdrive will maintain reservoir pressure to a substantial degree so that reserves diminish without a corresponding pressure decline.

An assembly of pressure-control equipment that enables the running and retrieval of long tool strings on a coiled tubing string in a live wellbore. The deployment system is configured to provide two barriers against well pressure as the tool string is assembled and run into the wellbore. Once fully assembled, the coiled tubing equipment is connected and the tool string is run into the wellbore. The process is reversed for tool retrieval.

The area of thickest deposition in a basin.

The action of moving sediments and laying them down.

The relative kinetic energy of the environment. A high-energy environment might consist of a rapidly flowing stream that is capable of carrying coarse-grained sediments, such as gravel and sand. Sedimentation in a low-energy environment, such as an abyssal plain, usually involves very fine-grained clay or mud. Depositional energy is not simply velocity. For example, although glaciers do not move quickly, they are capable of carrying large boulders.

The area in which and physical conditions under which sediments are deposited, including sediment source; depositional processes such as deposition by wind, water or ice; and location and climate, such as desert, swamp or river.

The three-dimensional array of sediments or lithofacies that fills a basin. Depositional systems vary according to the types of sediments available for deposition as well as the depositional processes and environments in which they are deposited. The dominant depositional systems are alluvial, fluvial, deltaic, marine, lacustrine and eolian systems.

The practice of ensuring that all measurements taken in a borehole are matched to the "base depth," normally the depth determined with the resistivity log.

A device used in acquisition of marineseismic data that keeps streamers at a certain depth in the water.

The process of transforming seismic data from a scale of time (the domain in which they are acquired) to a scale of depth to provide a picture of the structure of the subsurface independent of velocity. Depth conversion, ideally, is an iterative process that begins with proper seismic processing, seismic velocity analysis and study of well data to refine the conversion. Acoustic logs, check-shot surveys and vertical seismic profiles can aid depth conversion efforts and improve correlation of well logs and drilling data with surface seismic data.

The process of comparing and fixing measured depths with known features on baseline logs of the wellbore tubulars and the surrounding formation.

(noun) A fixed reference elevation from which all depth measurements in a well are calculated, typically the kelly bushing, rotary table, drill floor, or mean sea level. Establishing a consistent depth datum is essential for correlating geological and engineering data between wells in a field.

A two-dimensional representation of subsurface structure with contours in depth that have been converted from seismic traveltimes.

A magnetic mark placed on a loggingcable as a reference for depth measurements. The marks are placed on the cable at regular intervals, usually 100 ft [30 m] or 50 m [164 ft], under a certain tension in a workshop. The intervals may change slightly as a function of tension downhole, but this change can be corrected for. During logging operations, the marks are detected by a magnetic mark detector, and then used to check and correct the depth read by the depth wheel.

Pertaining to two or more logging curves that have been aligned in depth. Logs recorded on different runs will not be exactly aligned at all depths because of the difficulty of perfect depth control. If the two logs are offset by the same amount throughout the log, then only a simple depth shift is required. If the offset varies, then the logs need to be depth matched.Depth matching is simplest if both runs contain the same type of log, such as a gamma ray. The two gamma rays can then be aligned, either manually or with software, and the other logs shifted by the same amount. Otherwise the alignment is based on two logs that respond in a similar fashion, such as a neutron porosity and a shallow laterolog.Depth matching also may be needed for logs recorded on the same run. Although there is a fixed distance between the measure points and the depth reference, the apparent distance will vary if the tool moves unevenly up the hole, due to stick and slip or yo-yo effects. Depth matching is then necessary.

The practice of shifting depths of various data sets to a measurement that is known to be on depth. The general standard that is usually used is the first resistivity logs run, because those logs usually underwent the most rigorous depth control.Depth matching is usually applied to all wireline data, cores, boreholeseismic data, and any other data taken in a well. Depth matching is a vital process in any well evaluation or any reservoir characterization exercise, so much so that, in its absence, accuracy and validity of the exercise must be questioned.

A step in seismic processing in which reflections in seismic data are moved to their correct locations in space, including position relative to shotpoints, in areas where there are significant and rapid lateral or vertical changes in velocity that distort the time image. This requires an accurate knowledge of vertical and horizontal seismic velocity variations.

The distance from the borehole wall that the mudfiltrate has penetrated into the formation. The depth of invasion affects whether a log measures the invaded zone, the undisturbed zone or part of each zone. The term is closely related to the diameter of invasion, the latter being twice the depth of invasion plus the borehole diameter. Depth of invasion is a more appropriate parameter for describing the response of pad and azimuthally focused measurements such as density and microresistivity logs.The term is well-defined in the case of a step profile of invasion. In the case of an annulus or a transition zone, two depths must be defined, corresponding to the inner and outer limits of the annulus or transition zone. When the invasion model is not specified, the term usually refers to the outer limit of invasion.

A distance that characterizes how far a logging tool measures into the formation from the face of the tool or the borehole wall. The depth of investigation summarizes the radial response of the measurement in one or more directions. For nuclear and resistivity measurements, the depth of investigation should be associated with the percentage of signal received from within that depth, typically either 50% or 90%. Most quoted depths of investigation assume a homogeneous formation with certain properties, such as a given resistivity or fluid content. The depths of investigation can vary considerably in inhomogeneous conditions, and at different values of the properties concerned. They should be considered only a qualitative guide to tool response.For other measurements, the depth of investigation is either well-defined by the tool physics (in the case of nuclear magnetic resonance), or else can be given only approximately, an accurate value being too dependent on formation properties (in the case of acoustic and electromagnetic propagation).The term is used for all measurements but is most appropriate for azimuthally focused devices such as nuclear logs. For azimuthally symmetric devices such as resistivity logs, the term radius of investigation is more appropriate.

A point on the surface for which the depth to a horizon has been calculated in a refractionseismic survey. The term is commonly misused as a synonym for common depth point.

The point in a well from which depth is measured. Alternatively, the depth reference is the point at which the depth is defined as being zero. It is typically the top of the kelly bushing or the level of the rig floor on the rig used to drill the well. The depth measured from that point is the measured depth (MD) for the well. Even when the drilling rig has been removed, all subsequent measurements and operations in the well are still tied in to the same depth reference. However, for multiwell studies, the depths are normally shifted to the permanent datum. The depth reference and its elevation above the permanent datum are recorded on the log heading. In some contexts, the term may refer to any point from which depth is measured.

A point within the wellbore from which accurate depth measurements can be made, such as the end of the tubing string, or a nipple or similar completion component.

A display of seismic data with a scale of units of depth rather than time along the vertical axis. Careful migration and depth conversion are essential for creating depth sections.

A calibrated wheel used to drive the depth recording system in wirelinelogging. The wheel is pressed against the logging cable as the cable is spooled onto the drum and therefore turns as the cable is run in and out of the borehole. After zeroing the depth on surface, the depth wheel provides the depth input to the recording system. Small errors in calibration and slippage can cause the wheel to introduce systematic errors in the depth recorded. For this reason, the depth is checked and corrected using depth marks. The depth wheel is also referred to as a depth encoder. Modern encoders have two wheels so that slippage can be detected by differences between the two measurements.

Referring to a borehole-compensation scheme for sonic logs that combines measurements taken when the logging tool is at two different depths in the borehole. In normal borehole-compensation schemes, the effects of caves and sonde tilt are minimized by combining measurements from a second transmitter (T2) above a pair of receivers with those from the first transmitter (T1) below the receivers. This arrangement makes the logging tool unacceptably long for the long-spacing sonic log. In the depth-derived system, T2 is located below T1, at a distance equal to the receiver spacing. T1 is fired and the transit time between the receivers at depth z (TT1z) is recorded as usual. Then when T1 and T2 are at depth z, both are fired sequentially and the difference in time for their signals to reach one of the receivers is recorded (TT2z). The average of TT1z and TT2z is borehole-compensated since the acoustic signals traveled in opposite directions for the two measurements.

Logs that have been calculated from other logs to find the rate at which a log is changing with depth. For example, the derivative caliper (rugosity) calculates the rate at which the caliper is changing from one depth to another. Bad hole conditions that cause the density log to produce incorrect measurements are usually more closely related to the rugosity of the hole than the hole size, so the rugosity curve is the more useful in this regard.

The structure used to support the crown blocks and the drillstring of a drilling rig. Derricks are usually pyramidal in shape, and offer a good strength-to-weight ratio. If the derrick design does not allow it to be moved easily in one piece, special ironworkers must assemble them piece by piece, and in some cases disassemble them if they are to be moved.

The member of the drilling crew in charge of the mud-processing area during periods of circulation. The derrickman also measures mud density and conducts the Marsh funnel viscosity test on a regular basis when the mud is circulating in the hole. The derrickman reports to the toolpusher, but is instructed in detail by the mud engineer on what to add to the mud, how fast and how much. His other job is to handle pipe in the derrick while pulling out or running into the hole.

A hydrocyclone device that removes large drill solids from the whole mud system. The desander should be located downstream of the shale shakers and degassers, but before the desilters or mud cleaners. A volume of mud is pumped into the wide upper section of the hydrocylone at an angle roughly tangent to its circumference. As the mud flows around and gradually down the inside of the cone shape, solids are separated from the liquid by centrifugal forces. The solids continue around and down until they exit the bottom of the hydrocyclone (along with small amounts of liquid) and are discarded. The cleaner and lighter density liquid mud travels up through a vortex in the center of the hydrocyclone, exits through piping at the top of the hydrocyclone and is then routed to the mud tanks and the next mud-cleaning device, usually a desilter. Various size desander and desilter cones are functionally identical, with the size of the cone determining the size of particles the device removes from the mud system.

A substance used in a gas-dehydration unit to remove water and moisture. The desiccant can be liquid, such as methanol, glycol (ethylene, diethylene, triethylene, and tetraethylene). Dessicants also can be solid, such as silica gel or calcium chloride [CaCl2].The most common gas-dehydration system (glycol dehydrator) uses liquid desiccants such as diethylene, triethylene and tetraethylene, which are substances that can be regenerated. Regeneration means that the water absorbed by these substances can be separated from them. Some liquid desiccants such as methanol or ethylene cannot be regenerated.Solid desiccants are also used for gas dehydration. They are placed as beds through which wet gas is passed. The main limitation of the use of solid dessicants is that they absorb only limited quantities of water. When the desiccant saturation point is reached, the solid dessicant must be replaced. Another limitation is that sometimes water cannot be removed from it.

A hydrocyclone much like a desander except that its design incorporates a greater number of smaller cones. As with the desander, its purpose is to remove unwanted solids from the mud system. The smaller cones allow the desilter to efficiently remove smaller diameter drill solids than a desander does. For that reason, the desilter is located downstream from the desander in the surface mud system.

To remove sulfur or sulfur compounds from an oil or gas stream.

A log with a depth scale chosen to show sufficient detail of the formation. The most common scales are 1/200 or 5 in./100 ft.

The minimum thickness necessary for a layer of rock to be visible or distinct in reflectionseismic data. Generally, the detectable limit is at least 1/30 of the wavelength. Acquisition of higher frequency seismic data generally results in better detection or vertical resolution of thinner layers.

A sensor or receiver, such as a geophone or hydrophone, gravimeter or magnetometer.

The ability of a chemical agent to remove a contaminant from a solid surface. For example, in enhanced oil recovery, a surfactant can be used to remove an oil phase from a mineral surface. At least two mechanisms can occur: a) The surfactant adsorbs on the contaminated surface and presents its hydrophilic group to the contacting liquid. Thus, the surface behaves hydrophilically and repels macroscopic oil drops. b) The surfactant adsorbs to the contaminant. It is energetically more favorable for the combination of surfactant and contaminant to be in solution than to be attached to the surface so the contaminant is solubilized, exposing the mineral surface.

The use of deterministic methods to solve problems or find solutions to data sets.

A type of inverse filtering, or deconvolution, in which the effects of the filter are known by observation or assumed, as opposed to statistical deconvolution.

Techniques that use equations or algorithms that have been previously developed for similar situations. These methods do not involve stochastic or statistical approaches. Deterministic methods are generally easier and faster to apply and readily lend themselves to computer applications. However, they may not provide the most detailed or the most accurate reservoir models.

A cord containing high-explosive material sheathed in a flexible outer case, which is used to connect the detonator to the main high explosive. This provides an extremely rapid initiation sequence that can be used to fire several charges simultaneously.

A device containing primary high-explosive material that is used to initiate an explosive sequence. The two common types of detonators are electrical detonators (also known as blasting caps) and percussion detonators. Electrical detonators have a fuse material that burns when high voltage is applied to initiate the primary high explosive. Percussion detonators contain abrasive grit and primary high explosive in a sealed container that is activated by a firing pin. The impact force of the firing pin is sufficient to initiate the ballistic sequence that is then transmitted to the detonating cord. Several safety systems are used in conjunction with detonators to avoid accidental firing during rig-up or rig-down. Safety systems also are used to disarm the gun or ballistic assembly if downhole conditions are unsafe for firing.

Pertaining to particles of rock derived from the mechanical breakdown of preexisting rocks by weathering and erosion. Detrital fragments can be transported to recombine and, through the process of lithification, become sedimentary rocks. Detrital is usually used synonymously with clastic, although a few authors differentiate between weathering of particles, which forms detrital sediments, and mechanical breakage, which produces clastic sediments.

The phase of petroleum operations that occurs after exploration has proven successful, and before full-scale production. The newly discovered oil or gas field is assessed during an appraisal phase, a plan to fully and efficiently exploit it is created, and additional wells are usually drilled.

A wellbore that is not vertical. The term usually indicates a wellbore intentionally drilled away from vertical.

The angle at which a wellbore diverges from vertical. Wells can deviate from vertical because of the dips in the beds being drilled through. Wells can also be deliberately deviated by the use of a whipstock or other steering mechanism. Wells are often deviated or turned to a horizontal direction to increase exposure to producing zones, intersect a larger number of fractures, or to follow a complex structure.

The process of removing water from water-base drilling mud. Dewatering can involve chemical treatment for the flocculation and aggregation of solids followed by mechanical separation, such as centrifugation, or mechanical treatments alone.

The pressure at which the first condensate liquid comes out of solution in a gas condensate. Many gas condensate reservoirs are saturated at initial conditions, meaning that the dewpoint is equal to the initial reservoir pressure. Condensate dissolution is called retrograde condensation because this is counter to the behavior of pure substances, which vaporize when the pressure drops below the saturation pressure under isothermal (constant temperature) conditions.

Pertaining to a strike-slip fault or right-lateral fault in which the block across the fault moves to the right. If it moves left, the relative motion is described as sinistral. Clockwise rotation or spiraling is also described as dextral.

The initial stage of alteration of sediments and maturation of kerogen that occurs at temperatures less than 50°C [122°F]. The type of hydrocarbon generated depends on the type of organic matter in the kerogen, the amount of time that passes, and the ambient temperature and pressure. During early diagenesis, microbial activity is a key contributor to the breakdown of organic matter and generally results in production of biogenic gas. Longer exposure to higher temperatures during diagenesis, catagenesis, and metagenesis generally results in transformation of the kerogen into liquid hydrocarbons and hydrocarbon gases.

A type of secondary porosity created during diagenesis, commonly through dissolution or dolomitization or both. Diagenesis usually destroys porosity, so diagenetic porosity is rare.

The distance from the borehole wall into the formation that the mudfiltrate has penetrated. The term assumes equal invasion on all sides of the borehole. It is the diameter of the circle thus formed, with the center being the center of the borehole. The diameter of invasion affects whether a log measures the invaded zone, the undisturbed zone, or part of each zone. The term is closely related to the depth of invasion, being twice the depth of invasion plus the borehole diameter. Diameter of invasion is a more appropriate parameter for describing the response of azimuthally symmetric measurements such as induction, laterolog and propagation resistivity.The term is well-defined in the case of a step profile of invasion. In the case of an annulus or a transition zone, two diameters must be defined, corresponding to the inner and outer limits of the annulus or transition zone. When the invasion model is not specified, the term usually refers to the outer limit of invasion.

A distance that characterizes how far a logging tool measures into the formation from the axis of the tool or borehole. The term is similar to depth of investigation but is appropriate only for azimuthally symmetric measurements such as resistivity.

A tool for drilling rock that works by scraping industrial grade diamonds against the bottom of the hole. The diamonds are embedded into the metal structure (usually a sintered or powdered carbide base matrix) during the manufacture of the bit. The bit designer has virtually unlimited combinations of bit shape, the placement of hydraulic jetting ports, the amount of diamonds and the size of the diamonds used (usually expressed as diamonds per carat). In general, a diamond bit that drills faster has a shorter lifetime. Similarly, a bit designed for extremely long life will typically drill at a slower rate. If a bit has a relatively high number of diamonds compared with other bits, it is said to be "heavy-set" and has higher durability. A "light-set" bit, on the other hand, drills more aggressively, but wears out faster because fewer diamonds do the work.

A relatively mobile mass that intrudes into preexisting rocks. Diapirs commonly intrude vertically through more dense rocks because of buoyancy forces associated with relatively low-density rock types, such as salt, shale and hot magma, which form diapirs. The process is known as diapirism. By pushing upward and piercing overlying rock layers, diapirs can form anticlines, salt domes and other structures capable of trapping hydrocarbons. Igneous intrusions are typically too hot to allow the preservation of preexisting hydrocarbons.

A microscopic, single-celled, freshwater or saltwater algae that has a silica-rich cell wall called a frustule. Diatoms are so abundant that they can form thick layers of sediment composed of the frustules of the organisms that died and sank to the bottom. Frustules have been an important component of deep-sea deposits since Cretaceous time. Diatomite is the sedimentary rock that forms from diatom frustules.

A soft, silica-rich sedimentaryrock comprising diatom remains that forms most commonly in lakes and deep marine areas. Diatomite can form an excellent reservoir rock. The Belridge diatomite in the San Joaquin basin, California, USA, is a prolific oil-producing formation.

A type of salt in which chromium atoms are in the plus-7 valence state, such as potassium dichromate, K2Cr2O7.

A material used in a capacitor to store a charge from an applied electrical field. A pure dielectric does not conduct electricity.

The degree to which a medium resists the flow of electric charge, defined as the ratio of the electric displacement to the electric field strength. It is more common to use the relative dielectric permittivity.

A log of the high-frequency (on the order of 25 MHz) dielectric properties of the formation. The log usually includes two curves the relative dielectric permittivity, symbolized by epsilon which is unitless, and the resistivity in ohm-m. At the frequency used, water molecules have a strong effect on the dielectric properties, so that both relative dielectric permittivity and conductivity increase with the volume of water present. Relative dielectric permittivity can be used to distinguish hydrocarbons from water of any salinity. However, the effect of salinity is more important than the salinity effect with the high-frequency electromagnetic propagation log, and the interpretation is more complex. The advantage of the dielectric propagation log is that the lower frequency permits a larger depth of investigation and therefore an analysis of the undisturbed zone.

The resistivity of the formation derived by combining the attenuation and phase shift of a propagation resistivity measurement. Common practice is to transform attenuation and phase shift independently to resistivity, assuming a certain transform between permittivity and resistivity. These relations lose accuracy at high resistivity. However, by combining the two measurements, both the dielectric permittivity and resistivity can be determined without need for a transform. The dielectric resistivity extends the range of measurement, typically up to 3000 ohm-m.

An oil-base mud with diesel oil as its external phase. Diesel-oil mud is the traditional oil mud and has a history of excellent performance for drilling difficult wells. It has been used because the base oil is low-cost and widely available motor fuel. In-gauge holes can be drilled through all types of shales, salt, gypsum and other difficult strata using diesel-oil mud systems. It is often the mud of choice for drilling high-pressure, high-temperature zones. Diesel-oil muds usually contain from 5 to 40 vol.% emulsified brine water (except those that are specially designed to have none). The water phase usually contains 20 to 40 wt.% dissolved calcium chloride for shale control. Diesel-oil muds have been replaced in land drilling by mineral-oil muds and offshore by synthetic-fluid muds. These newer muds have fewer health, safety and environmental concerns compared to diesel oil.

An oil-base mud with diesel oil as its external phase. Diesel-oil mud is the traditional oil mud and has a history of excellent performance for drilling difficult wells. It has been used because the base oil is low-cost and widely available motor fuel. In-gauge holes can be drilled through all types of shales, salt, gypsum and other difficult strata using diesel-oil mud systems. It is often the mud of choice for drilling high-pressure, high-temperature zones. Diesel-oil muds usually contain from 5 to 40 vol.% emulsified brine water (except those that are specially designed to have none). The water phase usually contains 20 to 40 wt.% dissolved calcium chloride for shale control. Diesel-oil muds have been replaced in land drilling by mineral-oil muds and offshore by synthetic-fluid muds. These newer muds have fewer health, safety and environmental concerns compared to diesel oil.

A map that represents the change from one map to another, such as a reservoir map of an area made from two different seismic surveys separated in production history (one possible product of 4D seismic data), or an isochron map that displays the variation in time between two seismic events or reflections.

The spontaneous potential (SP) measured between two electrodes placed close together in the borehole, as opposed to the normal SP, which is measured with one electrode in the borehole and one at surface.

A phenomenon that occurs after the deposition of some sediments such that different parts of the sedimentary accumulation develop different degrees of porosity or settle unevenly during burial beneath successive layers of sediment. This can result from location on an uneven surface, such as near and over a reefstructure, or near a growth fault, or from different susceptibility to compaction. The porosity in a formation that has experienced differential compaction can vary considerably from one area to another.

The difference between two pressure measurements. For production wells, the differential pressure is the difference between average reservoir pressure and bottomhole pressure, and for injection wells, it is the difference between injection pressure and average reservoir pressure.

A technique in nuclear magnetic resonance (NMR) logging that is based on the difference between the T2 distributions, or spectra, acquired at different polarization times. The technique often is used to detect gas or light oil. These fluids have long T1 that exceed 1 s. A measurement made with a long polarization time will polarize much of these fluids and give significant signal at the appropriate T2. A measurement made with a short polarization time will polarize little of these fluids and will give a much smaller signal. Other fluids, with shorter T1, will be polarized in both cases, so that a difference in signal at the appropriate T2 identifies gas or light oil.

A situation in which the drilling assembly (pipe, drill collars and bottomhole assembly) is stuck in filter cake that was previously deposited on a permeable zone. The pipe is held in the cake by a difference in pressures between the hydrostatic pressure of the mud and the pore pressure in the permeable zone. The force required to pull the pipe free can exceed the strength of the pipe. Methods used to get the pipe free, in addition to pulling and torquing the pipe, include: (1) lowering hydrostatic pressure in the wellbore, (2) placing a spotting fluid next to the stuck zone and (3) applying shock force just above the stuck point by mechanical jarring, or (4) all the above. The most common approach, however, to getting free is to place a spot of oil, oil-base mud, or special spotting fluid.

A type of static correction that compensates for delays in seismicreflection or refraction times from one point to another, such as among geophone groups in a survey. These delays can be induced by low-velocity layers such as the weathered layer near the Earth's surface.

A record of the difference in temperature between two vertical points in a well. Most differential-temperature logs are obtained by differentiating a normal temperature log with respect to depth. Some are obtained by recording the difference in temperature between two vertically displaced sensors. Note that the differential-temperature log and the radial differential-temperature log are not the same.

Pertaining to a cement that is not easily dispersed by a material known as a dispersant. This term is commonly abbreviated as DTD.

Pertaining to cement that is not easily dispersed by a material known as a dispersant when the slurry is mixed with water containing a high concentration of salt. The term is commonly abbreviated DTDS.

A type of event produced by the radial scattering of a wave into new wavefronts after the wave meets a discontinuity such as a fault surface, an unconformity or an abrupt change in rock type. Diffractions appear as hyperbolic or umbrella-shaped events on a seismicprofile. Proper migration of seismic data makes use of diffracted energy to properly position reflections.

The process by which particles move over time within a material due to their kinetic motion. The term is most commonly used in pulsed neutron capturelogging and in nuclear magnetic resonance (NMR) logging. In a pulsed neutron capture log, the term refers to the spread of neutrons away from the neutron generator. In NMR logging, diffusion refers to the movement of gas, oil or water molecules within the pore space.

A fundamental differential equation obtained by combining the continuity equation, flow law and equation of state. Most of the mathematics of well testing were derived from solutions of this equation, which was originally developed for the study of heat transfer. Fluid flow through porous media is directly analogous to flow of heat through solids. Solutions used in well testing usually assume radial flow and homogenous, isotropic formations.

In a nuclear magnetic resonance (NMR) measurement, the loss of coherent energy by hydrogen atoms as they move within the pore space. Hydrogen atoms that move significantly within the pores during a NMR measurement will encounter different magnetic fields and hence will precess at different rates, or dephase. Dephasing contributes only to T2 and is most significant in gas or light oils. The magnitude depends on the field gradient, the echo spacing and the diffusion coefficient of the fluid. Diffusion relaxation can be induced in water by using long echo spacings. This is the basis of the enhanced diffusion technique.

An intrusive rock that invades preexisting rocks, commonly in a tabular shape that cuts vertically or nearly vertically across preexisting layers. Dikes form from igneous and sedimentary rocks.

The increase in the volume of rocks as a result of deformation, such as when fractures develop.

A possible explanation for volume changes in rocks due to strain, such as microfracturing or cracking, and the accompanying change in the ratio of P- to S-wavevelocity. Support for dilatancy theory comes in the form of porosity increases from 20 to 40% that have been measured in laboratory experiments using rock samples.

A rarefaction, or decrease in pressure and density of a medium as molecules are displaced by a P-wave. As P-waves pass through the Earth, the Earth undergoes compression and expansion. These changes in volume contribute to the positive and negative amplitudes of a seismictrace.

A hydrocarbon fluid that is used to dilute heavy oil and reduce its viscosity for easier transportation. Generally a distillation tower cut such as naphtha is used as for heavy oil dilution and transportation. The added diluent may be recovered at the destination using distillation and the diluent may be subsequently pumped back for blending.

The process of adding fresh mud (or liquid phase) in order to reduce the solids content and maintain the properties of the drilling fluid in the active system.

A type of local seismicevent that, in contrast to a bright spot, shows weak rather than strong amplitude. The weak amplitude might correlate with hydrocarbons that reduce the contrast in acousticimpedance between the reservoir and the overlying rock, or might be related to a stratigraphic change that reduces acoustic impedance.

The angle between a planar feature, such as a sedimentarybed or a fault, and a horizontal plane. True dip is the angle a plane makes with a horizontal plane, the angle being measured in a direction perpendicular to the strike of the plane.Apparent dip is the angle measured in any direction other than perpendicular to the strike of the plane. Given the apparent dip and the strike, or two apparent dips, the true dip can be computed.

An algorithm for correcting the effects of dip or boreholedeviation on the response of a logging measurement. These effects are significant for deep-reading logs such as induction and electrode devices. The standard processing used to produce these logs assumes a vertical well with horizontal formation layers. In the presence of a relative dip between the borehole and formation layers, the logs may read incorrectly. For older logs such as the dual induction, a set of inversefilters can be designed to correct for dip effect up to about 60. For modern array logs, iterative forward modeling with a one-dimensional layered earth model can correct up to about 85.

A fault whose primary movement is in the dip direction. Dip faults are also referred to as dip-slip faults.

The procedure in seismic processing that compensates for the effects of a dipping reflector. DMO processing was developed in the early 1980s.

A small antenna used in electromagnetic surveying that can be represented mathematically as a dipole.

A layer of rock or sediment that is not horizontal.

The application of nuclear magnetic resonance (NMR) logging to the determination of hydrocarbon type (gas, light oil, medium oil, heavy oil), using only NMR data. Three techniques are most commonly used: differential spectrum, shifted spectrum and enhanced diffusion.

The instrument used to measure viscosity and gel strength of drilling mud. The direct-indicating viscometer is a rotational cylinder and bob instrument, also known as a V-G meter. Two speeds of rotation, 300 and 600 rpm, are available in all instruments, but some are 6- or variable-speed. It is called "direct-indicating" because at a given speed, the dial reading is a true centipoise viscosity. For example, at 300 rpm, the dial reading (511 sec-1) is a true viscosity. Bingham plasticrheological parameters are easily calculated from direct-indicating viscometer readings: PV (in units of cp) = 600 dial - 300 dial and YP (in units of lb/100 ft2) = 300 dial - PV. Gel strength is also directly read as dial readings in oilfield units of lb/100 ft2.

The instrument used to measure viscosity and gel strength of drilling mud. The direct-indicating viscometer is a rotational cylinder and bob instrument, also known as a V-G meter. Two speeds of rotation, 300 and 600 rpm, are available in all instruments, but some are 6- or variable-speed. It is called "direct-indicating" because at a given speed, the dial reading is a true centipoise viscosity. For example, at 300 rpm, the dial reading (511 sec-1) is a true viscosity. Bingham plasticrheological parameters are easily calculated from direct-indicating viscometer readings: PV (in units of cp) = 600 dial - 300 dial and YP (in units of lb/100 ft2) = 300 dial - PV. Gel strength is also directly read as dial readings in oilfield units of lb/100 ft2.

An individual trained in the science and art of intentionally drilling a well along a predetermined path in three-dimensional space, usually involving deviating the well from vertical and directing it in a specific compass direction or heading. The directional driller considers such parameters as rotary speed, weight on bit, control drilling and when to stop drilling and take surveys of the wellpath, and works closely with the toolpusher.

The intentional deviation of a wellbore from the path it would naturally take. This is accomplished through the use of whipstocks, bottomhole assembly (BHA) configurations, instruments to measure the path of the wellbore in three-dimensional space, data links to communicate measurements taken downhole to the surface, mud motors and special BHA components and drill bits, including rotary steerable systems, and drill bits. The directional driller also exploits drilling parameters such as weight on bit and rotary speed to deflect the bit away from the axis of the existing wellbore. In some cases, such as drilling steeply dipping formations or unpredictable deviation in conventional drilling operations, directional-drilling techniques may be employed to ensure that the hole is drilled vertically. While many techniques can accomplish this, the general concept is simple: point the bit in the direction that one wants to drill. The most common way is through the use of a bend near the bit in a downhole steerable mud motor. The bend points the bit in a direction different from the axis of the wellbore when the entire drillstring is not rotating. By pumping mud through the mud motor, the bit turns while the drillstring does not rotate, allowing the bit to drill in the direction it points. When a particular wellbore direction is achieved, that direction may be maintained by rotating the entire drillstring (including the bent section) so that the bit does not drill in a single direction off the wellbore axis, but instead sweeps around and its net direction coincides with the existing wellbore. Rotary steerable tools allow steering while rotating, usually with higher rates of penetration and ultimately smoother boreholes.Directional drilling is common in shale reservoirs because it allows drillers to place the borehole in contact with the most productive reservoirrock.

Permeability that varies with direction of flow through the porous medium. Lateral permeability contrast can be particularly important in fractured formations, where effective permeability in the direction of the fractures may be many times greater than the matrix permeability. If the permeability in one direction is significantly more than in the other, the flow pattern may more closely approximate linear flow than radial flow. This can be detected from well-test data.Likewise, laminations in most clastic formations cause the permeability normal to the bedding plane to be less than the lateral permeability parallel to bedding. This is called vertical to horizontal permeability anisotropy.

Variograms and semivariograms that have a directional component in addition to the normal distance component. Directional variograms and semivariograms are commonly used where geological features are heterogeneous. For example, fluvial environments dominated by valleys, channels and point bars are likely to have directional components that are detectable and that will influence the behavior of fluids in these formations. Geostatistical models that use directional variograms can be expected to be more reliable in these circumstances.

A wellbore that requires the use of special tools or techniques to ensure that the wellbore path hits a particular subsurface target, typically located away from (as opposed to directly under) the surface location of the well.

The property of some seismic sources whereby the amplitude, frequency, velocity or other property of the resulting seismic waves varies with direction. A directional charge, such as a length of primer cord or a linear array of charges, can be used when directivity is desirable. Directivity is also a property of geophone arrays, air guns, explosives or vibrators, which can be positioned to reduce horizontal traveling noise such as ground roll. Receivers in the form of groups in which the individual geophones or hydrophones are separated from each other in linear (1D) or areal (2D) arrays are directional, and are designed to suppress signal arriving nearly horizontally and to pass nearly vertical arrivals with minimum attenuation or distortion. Directivity is often present, but the difficulty in accounting for it during seismic processing makes it undesirable in most cases.

Describing sedimentaryrock that contains clay minerals. Even small amounts of clay minerals in pores can drastically reduce porosity and permeability. Dirty and clean are qualitative, descriptive terms to describe the relative amount of clay minerals in a rock.

A common coating problem in which the protective coating detaches from the pipeline.

A geologic surface that separates younger strata from older strata and represents a time of nondeposition, possibly combined with erosion. Some disconformities are highly irregular whereas others have no relief and can be difficult to distinguish within a series of parallel strata.

A subsurface boundary or interface at which a physical quantity, such as the velocity of transmission of seismic waves, changes abruptly. The velocity of P-waves increases dramatically (from about 6.5 to 8.0 km/s) at the Mohorovicic discontinuity between the Earth's crust and mantle.

Pertaining to structures in which the shapes of adjacent layers differ or do not conform to one another. Folds of rock layers that have different mechanical properties or competence tend to be disharmonic, with a change in fold shape, symmetry or wavelength from one layer to the next.

A chemical that aids in breaking up solids or liquids as fine particles or droplets into another medium. This term is often applied incorrectly to clay deflocculants. Clay dispersants are various sodium phosphates and sodium carbonates aided by heat, mechanical shearing and time. Powdered polymers are dispersed by precoating the particles with a type of glycol to prevent formation of "fish-eye" globules. For dispersing (emulsification) of oil into water (or water into oils), surfactants selected on the basis of hydrophile-lipophile balance (HLB) number can be used. For foam drilling fluids, synthetic detergents and soaps are used, along with polymers, to disperse foam bubbles into the air or gas.

(noun) A multiphase flow regime in a pipe or wellbore characterised by small gas bubbles distributed uniformly throughout a continuous liquid phase at relatively high liquid velocities, where the buoyancy forces are insufficient to cause significant gas-phase segregation or coalescence.

Clay that is scattered throughout the pore space. There are three general types: pore lining, pore filling and pore bridging. The terms dispersed clay and dispersed shale tend to be used synonymously.

Spatial separation of components within a fluid. This separation is often driven by diffusion, mixing or differential flow. In an oil field components might be separated because of heterogeneity of permeability, or simply because of different paths taken by the fluid through the pore structure. Hydrodynamic dispersion includes both of these mechanical effects and molecular diffusion. The components of an enhanced oil recovery formulation can also be dispersed within a porousrock via differential adsorption properties (chromatographic effects).

The act of forcing a cementslurry that has been pumped into a casing string or drillstring to exit the bottom of the casing or drillstring by pumping another fluid behind it. Cement displacement is similar to definition 5 above, with the noted exception that the cement slurry would not normally be pumped out the top of the annulus, but would instead be placed in a particular location in the annulus. This location might be the entire annulus on a short casing string, or filling only a bottom portion of the casing on longer casing strings.

The fraction of oil that has been recovered from a zone swept by a waterflood or other displacement process. Displacement efficiency equation:

The fluid, usually drilling mud, used to force a cementslurry out of the casing string and into the annulus.

The interface between an injectant and the fluid it is displacing.

A well, often a depleted oil or gas well, into which waste fluids can be injected for safe disposal. Disposal wells typically are subject to regulatory requirements to avoid the contamination of freshwater aquifers.

In water analysis, the soluble components in a sample or the residue left after evaporation of a sample. Dissolved solids are reported as ppm or mg/L. Dissolved solids are included in retort solids and can be calculated from chemical analysis results by assuming that all dissolved solids are either NaCl or CaCl2, or a mixture of the two.

A technique for cleaning core samples in which the water fraction is removed by distillation and the oil fraction is extracted using solvents. Cleaning is done with either the Soxhlet or, when fluid saturation measurements are required, the Dean-Stark apparatus. Different solvents are used depending on the type of fluids and rock, the most common ones being toluene, methanol, xylene and chloroform. Several solvents may be used in sequence.

A change in a waveform that is generally undesirable, such as in seismic waves.

A record of the change in temperature along a well, normally recorded by a fiber-optic cable. The distributed temperature is measured by sending a pulse of laser light down the optical fiber. Molecular vibration, which is directly related to temperature, creates weak reflected signals. These signals are detected at the surface and converted to a log of temperature along the well, sampled approximately every 1 m [3.28 ft] with a resolution of 0.1oC. The fiber-optic cable is normally installed at the time of well completion, so that the distributed-temperature log can be recorded at any later time without well intervention.Introduced in the mid-1990s, the technique can also be used to measure flow rates by creating a temperature transient and observing its movement along the well.

The variation in the values of a one-dimensional data set. There are a number of readily recognized, possible distributions known to statistics, each with mathematical definitions. Statisticians may endeavor to find whether a data set is a good fit to any of the recognized distributions. Some examples include:bimodalBoltzmannchi-squaredgeneral normalGaussian or standard normal (bell-shaped curve)normalPoissonstudent's t.

The daily variation in properties of the Earth, such as the temperature or the local geomagnetic field, or the daily change in sunlight. Such variations depend in part on latitude, proximity to the ocean, the effects of solar radiation and tides and other factors.

In Cartesian coordinates, divergence is the sum of the partial derivatives of each component of the vector field with respect to the corresponding spatial coordinate:

A technique used in injection treatments, such as matrixstimulation, to ensure a uniform distribution of treatment fluid across the treatment interval. Injected fluids tend to follow the path of least resistance, possibly resulting in the least permeable areas receiving inadequate treatment. By using some means of diversion, the treatment can be focused on the areas requiring the most treatment. To be effective, the diversion effect should be temporary to enable the full productivity of the well to be restored when the treatment is complete. There are two main categories of diversion: chemical diversion and mechanical diversion.

A device for measuring in-situ the velocity of fluid flow in a production or injection well in which the total fluid flow is diverted to pass over an impeller, or spinner. Various techniques have been used to achieve this, one of the earliest being the packer flowmeter. In a typical modern device, the diverter consists of a fabric in a metal cage that is collapsed to pass through the tubing and other restrictions. Below the tubing, the cage is opened until an inflatable ring seals against the casing wall. At this point, the up-going production fluids are forced through the diverter and over an impeller. This ensures that the total casing flow is measured, but may also create an extra pressure drop and hence a change in multiphaseflow structure.The diverter flowmeter is particularly suitable for low flow rates in vertical or moderately deviated wells. Readings are made with the tool stationary.

An agreement between the operator and net revenue interest (NRI) owner in which the parties specify the fractional type of interest attributed to the NRI owner by the operator after an examination of title.

A safety device used when running and retrieving tools or drill collars with a flush external surface that may easily pass through the rotary table slips. The dog collar is temporarily attached to the assembly between the tool joint and the slips. If the slips fail to hold the tool assembly, the dog collar will prevent the entire assembly from dropping through and being lost in the wellbore.

The steel-sided room adjacent to the rig floor, usually having an access door close to the driller's controls. This general-purpose shelter is a combination tool shed, office, communications center, coffee room, lunchroom and general meeting place for the driller and his crew. It is at the same elevation as the rig floor, usually cantilevered out from the main substructure supporting the rig.

An abrupt turn, bend or change of direction in a survey line, a wellbore, or a piece of equipment. Dog-legs can be described in terms of their length and severity and quantified in degrees or degrees per unit of distance.

The steel-sided room adjacent to the rig floor, usually having an access door close to the driller's controls. This general-purpose shelter is a combination tool shed, office, communications center, coffee room, lunchroom and general meeting place for the driller and his crew. It is at the same elevation as the rig floor, usually cantilevered out from the main substructure supporting the rig.

An abrupt turn, bend or change of direction in a survey line, a wellbore, or a piece of equipment. Dog-legs can be described in terms of their length and severity and quantified in degrees or degrees per unit of distance.

The name given to dolomitized limestone.

The geochemical process in supratidal sabkha areas where magnesium [Mg] ions from the evaporation of seawater replace calcium [Ca] ions in calcite, forming the mineraldolomite. The volume of dolomite is less than that of calcite, so the replacement of calcite by dolomite in a rock increases the pore space in the rock by 13% and forms an important reservoir rock. Dolomitization can occur during deep burial diagenesis.

A rock composed chiefly (> 90%) of dolomite. The rock is sometimes called dolomite, but dolostone is preferable to avoid ambiguity between the mineral and rock names. Replacement dolomite that forms soon after deposition is typically fine-grained and preserves original sedimentary structures. Recrystallization late in diagenesis produces coarser grained dolomite, destroys sedimentary structures and results in higher porosity.

The set of values assigned to the independent variables of a function.

A structure made up of a number of superposed domains, usually of different size or wavelength. These are used in geostatistical work to describe statistical behaviors on small scales (such as porosity in thin sections) to large scales (such as porosity distributions in reservoirs).

(noun) A geological feature in which a rock mass is divided into distinct structural domains, each with its own characteristic deformation style, fabric orientation, or strain pattern. In structural geology, domainal analysis is used to evaluate heterogeneous deformation in folded and faulted terrains.

A type of anticline that is circular or elliptical rather than elongate. The upward migration of salt diapirs can form domes, called salt domes.

Slang term to describe a seismologist performing seismic field work.

To place lubricant on drillpipe, also known as "doping" the pipe.

A low-volume fluid pump with controllable discharge rate used to inject chemical additives to the mixing or pumping system. Dosing pumps frequently are used to inject fluids that may be difficult to mix efficiently in batch-tank systems because of their low volume.

Located down the slope of a dipping plane or surface. In a dipping (not flat-lying) hydrocarbonreservoir that contains gas, oil and water, the gas is updip, the gas-oil contact is downdip from the gas, and the oil-water contact is still farther downdip.

A pressure gauge, typically run on slickline, used to measure and record downhole pressure. Downhole gauges are commonly used in assessing the downhole pressure under various flowing conditions, the basis of pressure transient analysis.

A receiver located in a wellbore, as opposed to a location on the Earth's surface.

A downhole device that isolates wellbore pressure and fluids in the event of an emergency or catastrophic failure of surface equipment. The control systems associated with safety valves are generally set in a fail-safe mode, such that any interruption or malfunction of the system will result in the safety valve closing to render the well safe. Downhole safety valves are fitted in almost all wells and are typically subject to rigorous local or regional legislative requirements.

Mechanical or electronic devices for measuring various properties in the well such as pressure, fluid flow rate from each branch of a multilateral well, temperature, vibration, composition, fluid flow regime, and fluid holdup. These devices can be permanent; in this case, they are left in place for months or even years.

A seismic source located in a wellbore rather than at the Earth's surface.

The termination of more steeply dipping overlying strata against a surface or underlying strata that have lower apparent dips; a term used to describe a particular geometry of reflections in seismic data in sequencestratigraphy.

Pertaining to equipment, facilities or systems that are located in the production train after the surface choke, typically attached or close to the Christmas tree.

A pipeline that receives natural gas or oil from another pipeline at some specific connection point

The portion of movement of a downhole pump at which the rods are going down and the downhole pump is being filled with fluid.

A technique used to estimate the value of a potential field or seismic data at a surface beneath a measured surface. The method is risky because it assumes continuity of the field, so anomalies affect predictions, especially if they occur beneath the measured surface. Noise can be exaggerated and affect calculations adversely.

Pertaining to a technique in which a packerflowmeter is partially inflated and dragged up the hole to give a continuous flow log. This obsolete technique was introduced in the 1960s because the packer flowmeter could make only stationary measurements.

The process of forcing a nonwetting phase into a porousrock. Oil migrates into most reservoirs as the non-wetting phase, so initial charging of the reservoir is a drainage process.

The reservoir area or volume drained by the well. The terms drainage area, reservoir area and drainage volume are often incorrectly used interchangeably. When several wells drain the same reservoir, each drains its own drainage area, a subset of the reservoir area.

The portion of the volume of a reservoir drained by a well. In a reservoir drained by multiple wells, the volume ultimately drained by any given well is proportional to that well's production rate:Vi = Vt x qi/qt,where Vi is the drainage volume of Well i, Vt is the entire drainage volume of the reservoir, qi is the production rate from Well i, and qt is the total production rate from the reservoir.

A hole or short conduit through which fluids can flow. In equipment applications, a drainhole is generally made to avoid the buildup of pressure within a nonpressure area, such as may occur in the event of a leak in a pressure housing within a tool assembly.

A configuration of layers of rock that has the appearance of a fold, but might form simply through sagging or differential compaction of layers around a preexisting structure (such as a reef) or on an uneven surface.

The difference in height between the static level and the dynamic level in a pumping well, expressed as hydrostatic fluid pressure.

The measurement and analysis of pressure data taken after a well is put on production, either initially or following an extended shut-in period. Drawdown data are usually noisy, meaning that the pressure moves up and down as fluid flows past the gauges and minute variations in flow rate take place. This is especially true for new wells, in which well cleanup commonly occurs for days after production has begun. Such data are difficult to interpret, and the noise often obscures regions of interest to the analyst. Transient downhole flow rates measured while flowing can be used to correct pressure variations through convolution or deconvolution calculations that enable diagnosis and interpretation, analogous to that done for the pressure change and derivative.

The machine on the rig consisting of a large-diameter steel spool, brakes, a power source and assorted auxiliary devices. The primary function of the drawworks is to reel out and reel in the drilling line, a large diameter wire rope, in a controlled fashion. The drilling line is reeled over the crown block and traveling block to gain mechanical advantage in a "block and tackle" or "pulley" fashion. This reeling out and in of the drilling line causes the traveling block, and whatever may be hanging underneath it, to be lowered into or raised out of the wellbore. The reeling out of the drilling line is powered by gravity and reeling in by an electric motor or diesel engine.

An accurately machined device that is pulled through the casing, tubulars and completion components to ensure minimum-diameter specifications are within tolerance, as described in definition 2. While this tool is usually of a short length, the well planner may specify a special drift that either has a longer length or a nonstandard outside diameter. The large-diameter casing drifts are frequently known as "rabbits."

A component of a drillstring that provides weight on bit for drilling. Drill collars are thick-walled tubular pieces machined from solid bars of steel, usually plain carbon steel but sometimes of nonmagnetic nickel-copper alloy or other nonmagnetic premium alloys. The bars of steel are drilled from end to end to provide a passage to pumping drilling fluids through the collars. The outside diameter of the steel bars may be machined slightly to ensure roundness, and in some cases may be machined with helical grooves ("spiral collars"). Last, threaded connections, male on one end and female on the other, are cut so multiple collars can be screwed together along with other downhole tools to make a bottomhole assembly (BHA). Gravity acts on the large mass of the collars to provide the downward force needed for the bits to efficiently break rock. To accurately control the amount of force applied to the bit, the driller carefully monitors the surface weight measured while the bit is just off the bottom of the wellbore. Next, the drillstring (and the drill bit), is slowly and carefully lowered until it touches bottom. After that point, as the driller continues to lower the top of the drillstring, more and more weight is applied to the bit, and correspondingly less weight is measured as hanging at the surface. If the surface measurement shows 20,000 pounds [9080 kg] less weight than with the bit off bottom, then there should be 20,000 pounds force on the bit (in a vertical hole). Downhole MWD sensors measure weight-on-bit more accurately and transmit the data to the surface.

A special fluid designed exclusively for drilling through the reservoir section of a wellbore. The reasons for using a specially designed mud are: (1) to drill the reservoir zone successfully, often a long, horizontal drainhole. (2) to minimize damage and maximize production of exposed zones. (3) to facilitate the well completion needed, which can include complicated procedures. A drill-in fluid should resemble a completion fluid. It may be a brine containing only selected solids of appropriate particle size ranges (salt crystals or calcium carbonate) and polymers. Only additives essential for filtration control and cuttings carrying are present in a drill-in fluid.

Formation solids contained in a mud system, generally considered to be detrimental to the drilling operation because they produce high plasticviscosity, yield point and gel strengths and build poor-quality filter cakes. They also occupy space that is needed for barite in high-density muds. Drill solids cause excessive wear in the mud pumps and other rig equipment. Solids control is aimed at economically and efficiently removing drill solids. This implies removal as soon as possible after they enter the mud system, while the particles are at their largest size.

A special fluid designed exclusively for drilling through the reservoir section of a wellbore. The reasons for using a specially designed mud are: (1) to drill the reservoir zone successfully, often a long, horizontal drainhole. (2) to minimize damage and maximize production of exposed zones. (3) to facilitate the well completion needed, which can include complicated procedures. A drill-in fluid should resemble a completion fluid. It may be a brine containing only selected solids of appropriate particle size ranges (salt crystals or calcium carbonate) and polymers. Only additives essential for filtration control and cuttings carrying are present in a drill-in fluid.

A technique using the noise of the drill bit as a source and receivers laid out along the ground to acquire a vertical seismicprofile (VSP). Acquisition and processing of a drill-noise VSP, also called a seismic-while-drilling VSP, are typically a tougher task than for more conventional VSPs. Drill-noise VSPs yield reliable time-depth information and sometimes reflection information, and can be performed while a well is being drilled, so data from a drill-noise VSP can be considered in decisions during drilling operations.

A packer assembly that can be removed from the wellbore only by drilling or milling. Drillable packers, and similar tools such as bridge plugs, are typically made from cast iron, aluminum, plastic or similar brittle materials.

The supervisor of the rig crew. The driller is responsible for the efficient operation of the rigsite as well as the safety of the crew and typically has many years of rigsite experience. Most drillers have worked their way up from other rigsite jobs. While the driller must know how to perform each of the jobs on the rig, his or her role is to supervise the work and control the major rig systems. The driller operates the pumps, drawworks, and rotary table via the drillers console-a control room of gauges, control levers, rheostats, and other pneumatic, hydraulic and electronic instrumentation. The driller also operates the drawworks brake using a long-handled lever. Hence, the driller is sometimes referred to as the person who is "on the brake."

The depth of a well or features within the wellbore as measured while drilling. The measured length of each joint of drillpipe or tubing is added to provide a total depth or measurement to the point of interest. Drillers depth is the first depth measurement of a wellbore and is taken from the rotary table level on the rig floor. In most cases, subsequent depth measurements, such as those made during the well completion phase, are corrected to the wellheaddatum that is based on drillers depth.

A sudden increase in the rate of penetration during drilling. When this increase is significant (two or more times the normal speed, depending on local conditions), it may indicate a formation change, a change in the porepressure of the formation fluids, or both. It is commonly interpreted as an indication of the bit drilling sand (high-speed drilling) rather than shale (low-speed drilling). The fast-drilling formation may or may not contain high-pressure fluids. Therefore, the driller commonly stops drilling and performs a flow check to determine if the formation is flowing. If the well is flowing, or if the results are uncertain, the driller may close the blowout preventers or circulatebottoms-up. Depending on the bit being used and the formations being drilled, a formation, even if sand, may sometimes drill slower rather than faster. This slowing of drilling progress, while technically also a drilling break, is usually referred to as a "reverse drilling break", or simply "reverse break."

The company that owns and operates a drilling rig. The drilling contractor usually charges a fixed daily rate for its hardware (the rig) and software (the people), plus certain extraordinary expenses. Under this arrangement, the cost of the well is largely a function of the time it takes to drill and complete the well. The other primary contracting methods are footage rates (where the contractor receives an agreed upon amount per foot of hole drilled), or turnkey operations, where the contractor may assume substantial risk of the operations and receives a lump sum payment upon supplying a well of a given specification to the operator.

Personnel who operate the drilling rig. The crew typically consists of roustabouts, roughnecks, floor hands, lead tong operators, motormen, derrickmen, assistant drillers, and the driller. Since drilling rigs operate around the clock, there are at least two crews (twelve hour work shifts called tours, more common when operating offshore), or three crews (eight hour tours, more common onshore). In addition, drilling contractors must be able to supply relief crews from time to time when crew members are unavailable. Though less common now than in years past, the drilling contractor may opt to hire only a driller, and the driller in turn is responsible for hiring everyone reporting to him.

A surfactant-type mud additive intended to prevent formation shales and clays from sticking to the drilling assembly and also to prevent gumboshale from agglomerating and plugging the annulus and flowlines. Some DDs are claimed to be mud lubricants that lessen the torque and drag of the drillstring as it is rotated and moved up and down in the hole.

Any of a number of liquid and gaseous fluids and mixtures of fluids and solids (as solid suspensions, mixtures and emulsions of liquids, gases and solids) used in operations to drill boreholes into the earth. Synonymous with "drilling mud" in general usage, although some prefer to reserve the term "drilling fluid" for more sophisticated and well-defined "muds." Classifications of drilling fluids has been attempted in many ways, often producing more confusion than insight. One classification scheme, given here, is based only on the mud composition by singling out the component that clearly defines the function and performance of the fluid: (1) water-base, (2) non-water-base and (3) gaseous (pneumatic). Each category has a variety of subcategories that overlap each other considerably.

The engineering plan for constructing the wellbore. The plan includes well geometries, casing programs, mud considerations, well control concerns, initial bit selections, offset well information, porepressure estimations, economics and special procedures that may be needed during the course of the well. Although drilling procedures are carefully developed, they are subject to change if drilling conditions dictate.

The speed at which the drill bit can break the rock under it and thus deepen the wellbore. This speed is usually reported in units of feet per hour or meters per hour.

A large-diameter pipe that connects the subsea BOP stack to a floating surface rig to take mudreturns to the surface. Without the riser, the mud would simply spill out of the top of the stack onto the seafloor. The riser might be loosely considered a temporary extension of the wellbore to the surface.

Tubular steel conduit fitted with special threaded ends called tool joints. The drillpipe connects the rig surface equipment with the bottomhole assembly and the bit, both to pump drilling fluid to the bit and to be able to raise, lower and rotate the bottomhole assembly and bit.

Pertaining to the use of drillpipe to move wirelinelogging tools up and down a borehole. In difficult conditions--high well deviation, rough hole--wireline logging tools cannot reach the bottom of the hole under their own weight. In drillpipe-conveyed logging operations, the tools are moved mechanically by the drillpipe, while a wireline maintains the electrical connection.

A maritime vessel modified to include a drilling rig and special station-keeping equipment. The vessel is typically capable of operating in deep water. A drillship must stay relatively stationary on location in the water for extended periods of time. This positioning may be accomplished with multiple anchors, dynamic propulsion (thrusters) or a combination of these. Drillships typically carry larger payloads than semisubmersible drilling vessels, but their motion characteristics are usually inferior.

Well tests conducted with the drillstring still in the hole. Often referred to as DST, these tests are usually conducted with a downhole shut-in tool that allows the well to be opened and closed at the bottom of the hole with a surface-actuated valve. One or more pressure gauges are customarily mounted into the DST tool and are read and interpreted after the test is completed. The tool includes a surface-actuated packer that can isolate the formation from the annulus between the drillstring and the casing, thereby forcing any produced fluids to enter only the drillstring. By closing in the well at the bottom, afterflow is minimized and analysis is simplified, especially for formations with low flow rates. The drillstring is sometimes filled with an inert gas, usually nitrogen, for these tests. With low-permeability formations, or where the production is mostly water and the formation pressure is too low to lift water to the surface, surface production may never be observed. In these cases, the volume of fluids produced into the drillstring is calculated and an analysis can be made without obtaining surface production. Occasionally, operators may wish to avoid surface production entirely for safety or environmental reasons, and produce only that amount that can be contained in the drillstring. This is accomplished by closing the surface valve when the bottomhole valve is opened. These tests are called closed-chamber tests.Drillstem tests are typically performed on exploration wells, and are often the key to determining whether a well has found a commercial hydrocarbonreservoir. The formation often is not cased prior to these tests, and the contents of the reservoir are frequently unknown at this point, so obtaining fluid samples is usually a major consideration. Also, pressure is at its highest point, and the reservoir fluids may contain hydrogen sulfide, so these tests can carry considerable risk for rig personnel.The most common test sequence consists of a short flow period, perhaps five or ten minutes, followed by a buildup period of about an hour that is used to determine initial reservoir pressure. This is followed by a flow period of 4 to 24 hours to establish stable flow to the surface, if possible, and followed by the final shut-in or buildup test that is used to determine permeability thickness and flow potential.

The combination of the drillpipe, the bottomhole assembly and any other tools used to make the drill bit turn at the bottom of the wellbore.

The water and heavy hydrocarbons that condense from the gas stream and accumulate in the lower points of the flowlines.

A device used to collect water and heavy hydrocarbons that drop out of a gas stream in a pipeline.

A ball that is dropped or pumped through the wellbore tubulars to activate a downhole tool or device. When the ball is located on a landing seat, hydraulic pressure generally is applied to operate the tool mechanism.

A heavy steel bar that is dropped through the tubing or running string to fire the percussion detonator on a tubing-conveyed perforating (TCP) gun assembly. The drop bar must be capable of falling through the string with sufficient speed to impart the necessary force for detonation. Therefore, this method of firing is best suited to vertical or slightly deviated wellbores where there will be minimal drag or friction effect.

A device, shaped like a short length of pipe, which is used to drop TCP guns in the rathole or sump. It is commonly used to drop guns that are connected to the completion into the sump, thus providing access to the reservoir for subsequent intervention work. It may also be used to break the tool string into fishable sections.

A perforating gun assembly designed to be detached from the tubing or running string after firing. The detached assembly can then drop, or be pushed, to the bottom of the well depending on deviation and production requirements. Drop-off gun assemblies often are used in underbalanced perforating applications, eliminating the need to kill the well to recover the spent gun assembly. In such cases, the wellbore will be designed to accommodate the spent gun assembly without compromising productivity, while recovery of the gun assembly may be planned during subsequent workover operations. The drop-off mechanism may be automatic and actuated at time of firing, or be actuated after firing.

The failure of a channel or geophone to record a shot or shots in a seismicsurvey, which results in a loss of data.

A hygroscopic solid such as silica gel, calcium chloride [CaCl2] or other materials used in dry-bed dehydrators to absorb water and water vapor from a gas stream.

An in situ combustion technique in which only air or oxygen-enriched air mixtures are injected into a formation. A drawback related to dry combustion is the highly corrosive and noxious combustion products that are produced.

A type of in situ combustion in which the burning front moves in the same direction as the injected air. As air is continuously supplied at the injection well, the fire ignited at this location moves toward the production wells.During forward combustion, the temperature behind the burning front is high, indicating a great amount of heat stored in the formationmatrix. The injected gas heats on contact with the matrix and recovers only a small amount of the heat, with considerable losses to the surrounding formations. Another drawback of dry forward combustion is the presence of a highly viscous oil zone surrounding the production well. The fluid in this zone remains at the original reservoir temperature and its forward displacement by the heated oil is normally difficult.

Gas produced from a well that produces little or no condensate or reservoir liquids. The production of liquids from gas wells complicates the design and operation of surface process facilities required to handle and export the produced gas.

A wellbore that has not encountered hydrocarbons in economically producible quantities. Most wells contain salt water in some zones. In addition, the wellbore usually encounters small amounts of crude oil and natural gas. Whether the well is a "duster" depends on many factors of the economic equation, including proximity to transport and processing infrastructures, local market conditions, expected completion costs, tax and investment recovery conditions of the jurisdiction and projected oil and gas prices during the productive life of the well.

A treated oil that contains small amounts of basic sediments and water (BS&W). Dry oil is also called clean oil.

A subsurface rock that lacks contact with aquifers or meteoric water within the Earth.

A device that removes water and water vapor from a gas stream using two or more beds of solid desiccants, such as silica gel or calcium chloride [CaCl2]. Wet gas is passed through the solid material, which absorbs the water, and then dry gas is collected at the top of the device.The main limitation of this device is that the solid desiccant absorbs only limited quantities of water. When the desiccant saturation point is reached, it must be replaced and sometimes water cannot be removed from it.

A wellbore with simultaneous production of hydrocarbons, water or both from more than one producing zone. Although the term refers to cases in which only two separate zones are present, in actuality there may be multiple zones involved. This completion technique avoids backflow from one reservoir zone to another in the wellbore.

The combination of a deep-induction and a medium-induction array on the same sonde. In a typical implementation, the two arrays share the same transmitters but have different receivers. If the dual-induction log is combined with a shallow laterolog or microresistivity log, it is possible to correct for the effect on invasion on the deep log, assuming a step profile.

A model of shaly formations that considers there to be two waters in the pore space: far water, which is the normalformation water; and near water (or clay-bound water) in the electrical double layer near the clay surface. The clay-bound water consists of clay counter-ions and the associated water of hydration. The volume of this layer is determined by its thickness, which is constant at high salinities, and its area, which is proportional to the counter-ion concentration per unit pore volume (Qv). The volume of clay-bound water per unit pore volume, Swb, can therefore be written as: Swb = alpha * vq * Qvwhere vq = 0.28 cm3/meq at 25oC is the factor relating volume to counter-ion concentration at high salinity and is a function only of temperature, and alpha = 1 above a certain salinity, below which it increases with temperature and with decreasing salinity. The fractional volume of the far water is then (1 ? alpha?* vq * Qv).The dual-water concept was developed for the interpretation of resistivity in shaly sands, but is also useful in the interpretation of nuclear and nuclear magnetic resonance logs. In these cases, the parameter most used is the total volume of clay-bound water in the rock, equal to Swb multiplied by the total porosity.

A dual-porosityreservoir in which flow to the well occurs in both primary and secondary porosity systems.

A rock characterized by primary porosity from original deposition and secondary porosity from some other mechanism, and in which all flow to the well effectively occurs in one porosity system, and most of the fluid is stored in the other. Naturally fractured reservoirs and vugular carbonates are classified as dual-porosity reservoirs, as are layered reservoirs with extreme contrasts between high-permeability and low-permeability layers.

A blank gas-lift valve placed in a gas-lift mandrel to isolate the tubing string from the annulus. Gas-lift valves frequently are replaced with dummy valves during intervention work on wells with gas-lift completions.

A wireline or slickline tool used to place small volumes of cementslurry, or similar material, in a wellbore. Typically, the slurry is placed on a plug or similar device that provides a stable platform for the low-volume cement plug.

A type of fluid pump, commonly used on workover rigs, that has two plungers or pistons. As a positive-reciprocating pump, the fluid flow rate is typically calculated from the number of strokes per minute that the pump makes and the displacement volume per stroke. Such a level of accuracy usually is sufficient for general workover purposes.

(noun) An informal industry term for a dry hole — a well that fails to encounter commercially producible quantities of oil or gas and is subsequently plugged and abandoned. The term reflects the disappointment of drilling an unsuccessful exploration or development well.

A time-variant operation performed on seismic data. Normal moveout (NMO) is a dynamic correction.

Equipment used to measure filtration under dynamic conditions. Two commercial dynamic-filtration testers are available, one of which uses a thick-walled cylinder with rock-like characteristics as the filter medium to simulate radial flow into a wellbore. The other tester uses flat porous disks, such as paper or fused ceramic plates, as filter media. In a dynamic test, filter cake is continually eroded and deposited. Data from this test include a steady-state filtration rate measured during the test, and cake thickness, cake quality and return permeability of the filter medium measured at the conclusion of a test. There is no API standardized test equipment or procedure.

A filtration process in which the slurry being filtered is being circulated over the filter cake, so that the cake is simultaneously eroded and deposited. The erosion rate depends on the shear rate of the fluid at the face of the cake. If the shear rate remains constant, cake thickness and filtration rate reach steady state, usually in a matter of hours. When the conditions change, a new steady state will be established.

The level to which the static fluid level drops in the tubing or casing when the well produced under pumping conditions. The dynamic fluid level is also called the pumping fluid level.

The stationing of a vessel, especially a drillship or semisubmersible drilling rig, at a specific location in the sea by the use of computer-controlled propulsion units called thrusters. Though drilling vessels have varying sea and weather state design conditions, most remain relatively stable even under high wind, wave and current loading conditions. Inability to maintain stationkeeping, whether due to excessive natural forces or failure of one or more electromechanical systems, leads to a "drive off" condition that requires emergency procedures to disconnect the riser from the subsea BOP stack, or worse, drop the riser from the vessel altogether.

The ratio of or difference between the highest and the lowest reading, or strongest and weakest signal, that can be recorded or reproduced by an instrument without distortion.

A type of explosive used as a source for seismic energy during data acquisition. Originally, dynamite referred specifically to a nitroglycerin-based explosive formulated in 1866 by Alfred Bernhard Nobel (1833 to 1896), the Swedish inventor who endowed the Nobel prizes. The term is incorrectly used to mean any explosive rather than the original formulation.

An instrument used in sucker-rod pumping to record the variation between the polished rod load and the polished rod displacement.

The record made by the dynamometer. An analysis of this survey may reveal a defective pump, leaky tubing, inadequate balance of the pumping unit, a partially plugged mud anchor, gas locking of the pump or an undersized pumping unit. The dynamometer card is also called a dynagraph.