Oil and Gas Terms Beginning with “W”

(noun) Abbreviation for Water Alternating Gas. An enhanced oil recovery technique in which slugs of water and gas (typically carbon dioxide, nitrogen, or hydrocarbon gas) are alternately injected into a reservoir to improve macroscopic sweep efficiency and reduce gas override or viscous fingering that occurs during continuous gas injection.

To suspend drilling operations while allowing cementslurries to solidify, harden and develop compressive strength. The drilling crew usually uses this time to catch up on maintenance items, to rig down one BOP and rig up another one for the new casing, to get tools and materials ready for the next hole section, and other non-drilling tasks. The WOC time ranges from a few hours to several days, depending on the difficulty and criticality of the cement job in question. WOC time allows cement to develop strength, and avert development of small cracks and other fluid pathways in the cement that might impair zonal isolation.

Abbreviation for water/oil ratio, the ratio of produced water to produced oil.

A zone of the upper mantle in which earthquakes occur when a lithospheric plate is subducted, named in honor of seismologists Kiyoo Wadati and Hugo Benioff. The dip of the Wadati-Benioff zone coincides with the dip of the subducting plate. The Wadati-Benioff zone extends to a depth of about 700 km [435 miles] from the Earth's surface.

A custom-designed machine designed to move a drilling rig substructure, using lift cylinders, rollers, travel cylinders, and walking feet to move the rig from well to well.

In digital signalprocessing, a nonsinusoidal transform by addition and subtraction. The Walsh-Hadamard transform is similar to Fourier series analysis, but uses square waves instead of sinusoidal waves. It is used predominantly in communication theory and, to a lesser extent, in filtering logs with a blocky character.

A law stating that lithologies that conformably overlie one another must have accumulated in adjacent depositional environments. Exceptions occur where there are erosional breaks. This law allows for transformations from the vertical data to a horizontal set and is often used when a vertical sequence of facies has been identified and characterized (for example, with Markov chain analysis) to estimate the horizontal depositional pattern.

A retrievable cylindrical device that protects the internal surfaces of wellhead equipment and the top of the last casing suspended.

A borehole; the hole drilled by the bit. May have casing in it or may be open (uncased). Also called a borehole or hole.

A marineseismic data acquisition method that uses one or more vessels to tow source arrays and streamers to record seismic signals, along with one or more source-only vessels sailing parallel to, but at some specified distance from, the recording vessel(s). The source-only vessels provide offset sources that generate reflections from a wide range of azimuths; these reflections are received by streamers towed by the recording vessel(s).

A distinction among three phase behaviors of oil, water and surfactant systems when they form a microemulsion. The salinity of the brine phase is an important parameter influencing which type of behavior occurs. To test for the type of system, surfactant is added to an oil-water system. In a Winsor Type I system, the surfactant forms an oil-in-water microemulsion in the aqueous phase. This behavior is not favorable to achieve ultralow interfacial tension with surfactants. In a Winsor Type II system, the surfactant forms a water-in-oil emulsion in the oil phase. This behavior leads to surfactant retention in the oil phase and is unfavorable for an enhanced oil recovery (EOR) process. In a Winsor Type III system, the surfactant forms a microemulsion in a separate phase between the oil and aqueous phases. This phase is a continuous layer containing surfactant, water and dissolved hydrocarbons. This situation is ideal to achieve ultralow interfacial tension values and is favorable for EOR.

A cable composed of steel wires twisted around a central core of fibre or steel wire to create a rope of great strength and considerable flexibility.

To suspend operations while a cementslurry to develops sufficient compressive strength to allow drilling or other wellbore activity to continue. The WOC time is generally used to test the surface pressure-control equipment, such as the BOP stack. Attempting to drill out the float or guide shoe before the cement has developed sufficient bond strength may result in backing off a casing joint.

The time allotted for the alignment of protons with the static magnetic field during a nuclear magnetic resonance measurement. The term is used more generally with reference to logging tools, and is synonymous with the more general term polarization time.

A type of vertical seismic profile to accommodate the geometry of a deviated well; sometimes called a vertical incidence VSP. Each receiver is in a different lateral position with the source directly above the receiver for all cases. Such data provide a high-resolution seismic image of the subsurface below the trajectory of the well.

A type of vertical seismic profile in which the source is moved to progressively farther offset at the surface and receivers are held in a fixed location, effectively providing a mini 2D seismic line that can be of higher resolution than surface seismic data and provides more continuous coverage than an offset VSP. 3D walkaways, using a surface grid of source positions, provide 3D images in areas where the surface seismic data do not provide an adequate image due to near-surface effects or surface obstructions. Walkaway VSPs in which the receivers are placed just above the reservoir are gaining acceptance as a tool to quantify seismic attributes and calibrate surface seismic data.

The loss of material on the inside or outside of a casing or tubing due to corrosion. Monitoring wall loss in situ helps determine when the pipe may be at risk for leaking or failure. Wall loss is determined by comparing casing or tubing thickness measured by electromagnetic, acoustic resonance or mechanical methods with either an earlier measurement or an assumed value.

An enlarged region of a wellbore. A washout in an openhole section is larger than the original hole size or size of the drill bit. Washout enlargement can be caused by excessive bitjet velocity, soft or unconsolidated formations, in-siturock stresses, mechanical damage by BHA components, chemical attack and swelling or weakening of shale as it contacts fresh water. Generally speaking, washouts become more severe with time. Appropriate mud types, mud additives and increased mud density can minimize washouts.

A tool-string component used with a burn shoe for washover operations. The wash pipe is a relatively large internal-diameter tubular that can be washed over a fish in preparation for engaging and retrieving the fish.

A hole in a pressure-containing component caused by erosion. A washout is relatively common where a high-velocity stream of dry gas carries abrasive sand. The severity generally decreases with sand content, velocity and liquid content.

A type of milling operation in which the outer surfaces of a plug or similar fish are milled with a circular hollow mill. By including wash pipe in the tool string, the mill face can reach over the body of the fish until it can be pushed to bottom, or until the slips or retaining device can be milled out and the fish retrieved.

In fishing operations, a large-diameter pipe fitted with an internal grappling device and tungsten carbide cutting surfaces on the bottom. The washover pipe can be lowered over a fish in the wellbore and to latch onto and retrieve the fish. Since the washover pipe is relatively thin-walled and large in diameter, and may be prone to sticking itself, the washover operation is usually reserved as a measure of last resort before abandoning the fish altogether.

A process in which dirty water is stripped of its solids and made suitable for recycling into a mud system or disposal into sewer systems or other places. In closed mud systems, water containing colloidal matter can be cleaned and recycled. Efficient agglomeration of colloidal solids is achieved by pH adjustment, small additions of alum or a high-molecular-weight polymer. Agglomerated solids are then filtered or centrifuged from the fluid.

A drilling fluid (mud) in which water or saltwater is the major liquid phase as well as the wetting (external) phase. General categories of water-base muds are fresh water, seawater, salt water, lime, potassium and silicate. Subcategorizes of these abound.

A production impairment that can occur when the formationmatrix in the near-wellbore area becomes water-saturated, thereby decreasing the relative permeability to hydrocarbons. Water block may result from the invasion of water-base drilling or completion fluids or from fingering or coning of formation waters.The most extreme cases of water block occur in low-pressure, low-permeability gas formations, where alcoholic acid systems are recommended because they promote water vaporization in the produced gas.Alcoholic acid formulations are a mixture of acid and alcohol. The acids normally employed are usually either hydrochloric acid [HCl], mud acid [HF-HCl or HF-organic acid (formic or acetic)]. The alcohol is either methyl or isopropyl. Alcohol lowers the surface tension of acid and allows deeper penetration of the acid into the matrix of the rock. Alcohol is somewhat soluble in both acid and water, and penetration of low-surface-tension volatile alcohol into a water block will aid in its removal.

The process of removing colloidal materials from water. A chemical coagulant (for example, alum) or a chemical flocculant (for example, polymer) or both are added to the water. Colloidal particles attach to each other and to the additives and clumps grow to sufficient size that they can be separated from the water by gravity settling, centrifuging, hydrocycloning or filtration. Clarification is a final step in a closed mud system when a clear effluent is needed.

(noun) A production phenomenon in which water from an underlying aquifer migrates upward toward the well perforations due to pressure drawdown, forming a cone-shaped intrusion of water around the wellbore that increases the water cut and reduces oil production rate.

A treatment conducted within a reservoir or perforated interval to reduce water production. Water-control treatments may be necessary when the production efficiency of a well, or the process capability of surface facilities, is compromised by the volume of water produced with the oil or gas. Treatment options include selective isolation of the water-producing perforations or localized treatment of the formationmatrix.

A volume of water placed in a tubing string prior to conducting a drillstem test or opening a well to flow. The water cushion is designed to reduce and control the pressuredrawdown applied to the reservoir when the downhole valve or tester valve is opened to initiate flow.

The ratio of water produced compared to the volume of total liquids produced. The water cut in waterdrive reservoirs can reach very high values.

A primary recovery mechanism in which the pressure from free water is sufficient to move hydrocarbons out of the reservoir, into the wellbore and up to surface.Waterdrive reservoirs can have bottomwater drive or edgewater drive. In a bottomwater-drive reservoir, water is located beneath the oil accumulation, while in an edgewater-drive reservoir, water is located only on the edges of the reservoir.

A method of secondary recovery in which water is injected into the reservoirformation to displace residual oil. The water from injection wells physically sweeps the displaced oil to adjacent production wells. Potential problems associated with waterflood techniques include inefficient recovery due to variable permeability, or similar conditions affecting fluid transport within the reservoir, and early water breakthrough that may cause production and surface processing problems

Water and dissolved gas volume at reservoir conditions divided by water volume at standard conditions. This value can often be neglected, since it is always close to 1.0.

A source of energy for acquisition of marineseismic data that shoots water from a chamber in the tool into a larger body of water, creating cavitation. The cavity is a vacuum and implodes without creating secondary bubbles. This provides a short time signature and higher resolution than an air-gun source.

A fluid with water or brine as droplets dispersed into an external phase of oil.

The replacement of produced fluids by formation water. Most petroleum reservoirs are underlain by water, and water influx into a reservoir almost always takes place at some rate when gas or oil is produced. Whether appreciable water is produced along with gas or oil depends on the proximity of the productive interval to the oil-water contact or gas-water contact and whether the well is coning (vertical well) or cresting (horizontal well).

Jargon applied to a mud additive used to control fluid loss.

A chemical used in preparation and maintenance of an emulsionmud, which is a water mud containing dispersed oil (or a synthetic hydrocarbon). Numerous types of emulsifiers will disperse oil into water muds, including sulfonated hydrocarbons, ethyoxylated nonylphenols, alkali-metal fatty-acid soaps, lignosulfonate, lignite and lignin at high pH. Even clays, starch and carboxymethylcellulose aid emulsion mud stability.Reference:Rogers WF: "Oil-in-Water Emulsion Muds," in Composition and Properties of Oil Well Drilling Fluids, 3rd ed. Houston, Texas, USA: Gulf Publishing Company, 1963.

The volume of produced water associated with oil production. In waterdrive reservoirs, water production can be significantly higher than oil production from a field. Consequently, treatment and disposal of produced water, especially in remote locations, have an important impact on the feasibility of a project.

The fraction of water in a given pore space. It is expressed in volume/volume, percent or saturation units. Unless otherwise stated, water saturation is the fraction of formation water in the undisturbed zone. The saturation is known as the total water saturation if the pore space is the total porosity, and the effective water saturation if the pore space is the effective porosity. If used without qualification, the term usually refers to the effective water saturation.

In a cementslurry, the ratio of water to cement expressed as percent; the number of parts of water used to mix with 100 parts of cement.

A test for water mud or oil mud, generally known as the retort test. Proper procedures for retort tests have been published by API. The test is a distillation of a mud sample that measures condensed oil and water collected from the retort. Data obtained are: (1) vol. % water, (2) vol. % oil and (3) vol. % retort solids. Retort solids is the volume that was not recovered as a liquid. Three sizes of retort apparatus are available: 10-, 20- and 50-cm3 mud sample size. Some designs have a small oven in the carrying case to heat the sample (the preferred method for oil muds) while others use a blade heater that goes into the mud sample. Retorts should be heated to around 700°F [371°C] to be effective.

An enhanced oil recovery process whereby water injection and gas injection are alternately injected for periods of time to provide better sweep efficiency and reduce gas channeling from injector to producer. This process is used mostly in CO2 floods to improve hydrocarbon contact time and sweep efficiency of the CO2.

A drilling fluid (mud) in which water or saltwater is the major liquid phase as well as the wetting (external) phase. General categories of water-base muds are fresh water, seawater, salt water, lime, potassium and silicate. Subcategorizes of these abound.

The marine equivalent of ground roll. Water-bottom roll consists of a pseudo-Rayleigh wave traveling along the interface of the water and the seafloor. As the use of seabed receiver systems increases, noise from water-bottom roll has become more of a concern.

A device for determining the water holdup in a producing well by measuring the capacitance or impedance of the fluid. The term is a misnomer because water cut is not the same as water holdup except in the unlikely case where all phases flow at the same velocity. Since hydrocarbons travel faster than water in a production well, the water holdup is larger than the water cut. However, the water-cut meter was often combined with a flowmeter so that the water cut could be estimated by combining the two measurements.

The resistivity of a sample completely filled with water. Called Ro, it is used in contrast to the resistivity of a sample only partially filled with water, Rt. The ratio Rt / Ro is called the resistivity index, I.

A record of the velocity and direction of water flowing in and around a borehole based on oxygen activation. The log may also include estimates of the flow volume and the distance from tool to flowing water. Water, and occasionally carbon dioxide, is the only source of moving oxygen in and around the borehole. Hence, water flow can be detected by oxygen activation, which, being a nuclear technique, is sensitive to flow inside and outside the casing. The measurement is sensitive to small flows, and can be configured to measure upward or downward flow. It is particularly useful as a leak and channel detector, to identify locations of water entry or exit and as a measurement of water velocity in multiphase flow. Logs may be continuous, but the most accurate measurements are made with the tool stationary.Although first tried in the 1960s, the log was not fully studied and implemented until the late 1970s with a purpose-built experimental tool. Standard pulsed-neutron spectroscopy tools were modified to record the log in the 1980s.

A fluid with water or brine as droplets dispersed into an external phase of oil.

A chemical used in preparation and maintenance of an emulsion mud, which is a water mud containing dispersed oil (or a synthetic hydrocarbon). Numerous types of emulsifiers will disperse oil into water muds, including sulfonated hydrocarbons, ethyoxylated nonylphenols, alkali-metal fatty-acid soaps, lignosulfonate, lignite and lignin at high pH. Even clays, starch and carboxymethylcellulose aid emulsion mud stability.Reference:Rogers WF: "Oil-in-Water Emulsion Muds," in Composition and Properties of Oil Well Drilling Fluids, 3rd ed. Houston, Texas, USA: Gulf Publishing Company, 1963.

In a cementslurry, the ratio of water to cement expressed as percent; the number of parts of water used to mix with 100 parts of cement.

Describing the preference of a solid to be in contact with a water phase rather than an oil or gas phase. Water-wet rocks preferentially imbibe water. Generally, sandstones and carbonates are water-wet before contact with crude oil, but may be altered by components of the crude oil to become oil-wet. Certain minerals, as well as different crystallographic faces of the same mineral, may be variably prone to being oil- or water-wet.

The ratio of produced water to produced oil, abbreviated WOR.

A primary recovery mechanism in which the pressure from free water is sufficient to move hydrocarbons out of the reservoir, into the wellbore and up to surface.Waterdrive reservoirs can have bottomwater drive or edgewater drive. In a bottomwater-drive reservoir, water is located beneath the oil accumulation, while in an edgewater-drive reservoir, water is located only on the edges of the reservoir.

A method of secondary recovery in which water is injected into the reservoirformation to displace residual oil. The water from injection wells physically sweeps the displaced oil to adjacent production wells. Potential problems associated with waterflood techniques include inefficient recovery due to variable permeability, or similar conditions affecting fluid transport within the reservoir, and early water breakthrough that may cause production and surface processing problems.

The first indication of increased crude-oil production as the result of a waterflooding project.

(noun) A secondary oil recovery method in which water is injected into a reservoir through dedicated injection wells to maintain reservoir pressure and physically displace oil toward producing wells. Waterflooding is the most widely used improved recovery technique worldwide, typically recovering an additional 15% to 25% of the original oil in place.

A periodic vibrational disturbance in which energy is propagated through or on the surface of a medium without translation of the material. Waves can be differentiated by their frequency, amplitude, wavelength and speed of propagation.

A mathematical expression to represent wavedisplacement and wave velocity (V) as functions of space (x,y,z) and time (t).

The shape of a wave, typically shown as a graph of amplitude (or other quantity of interest) versus time.

The edge of an advancing wave, which includes adjacent points that have the same phase.

The distance between analogous points in a wave train, measured perpendicular to the wavefront. In seismic data, the wavelength is the seismic velocity divided by frequency. It can be expressed mathematically as:

A one-dimensional pulse, usually the basic response from a single reflector. Its key attributes are its amplitude, frequency and phase. The wavelet originates as a packet of energy from the source point, having a specific origin in time, and is returned to the receivers as a series of events distributed in time and energy. The distribution is a function of velocity and density changes in the subsurface and the relative position of the source and receiver. The energy that returns cannot exceed what was input, so the energy in any received wavelet decays with time as more partitioning takes place at interfaces. Wavelets also decay due to the loss of energy as heat during propagation. This is more extensive at high frequency, so wavelets tend to contain less high-frequency energy relative to low frequencies at longer traveltimes. Some wavelets are known by their shape and spectral content, such as the Ricker wavelet.

A step in seismicprocessing to determine the shape of the wavelet, also known as the embedded wavelet, that would be produced by a wave train impinging upon an interface with a positive reflection coefficient. Wavelets may also be extracted by using a model for the reflections in a seismic trace, such as a synthetic seismogram. A wavelet is generated by deconvolving the trace with the set of reflection coefficients of the synthetic seismogram, a process also known as deterministic wavelet extraction. Wavelets may be extracted without a model for the reflections by generating a power spectrum of the data. By making certain assumptions, such as that the power spectrum contains information about the wavelet (and not the geology) and that the wavelet is of a certain phase (minimum, zero), a wavelet may be generated. This is also called statistical wavelet extraction. A particular processing approach to establishing the embedded wavelet is to compare the processed seismic response with the response measured by a vertical seismic profile (VSP) or generated synthetically through a synthetic seismogram in which the embedded wavelet is known. The wavelet can also be extracted through the autocorrelation of the seismic trace, in which case the phase of the wavelet has to be assumed.

The reciprocal of wavelength, so the number of wave cycles per unit of distance, abbreviated as k.

A piece of steel cable placed inside a logginghead that is designed to break at a predetermined tension. If the logging tool becomes stuck in the borehole, there is a danger that the logging cable will break at surface, since this is the place of maximum tension. It is difficult to fish a long length of tangled cable in the borehole. The weak point is designed to break before the cable, so that the latter can be retrieved, leaving only the logging tool and head in the borehole.

A near-surface, possibly unconsolidated layer of low seismicvelocity. The base of the weathered layer commonly coincides with the water table and a sharp increase in seismic velocity. The weathered layer typically has air-filled pores.

The physical, chemical and biological processes that decompose rock at and below the surface of the Earth through low pressures and temperatures and the presence of air and water. Weathering includes processes such as dissolution, chemical weathering, disintegration and hydration.

A method of compensating for delays in seismicreflection or refraction times induced by low-velocity layers such as the weathered layer near the Earth's surface. It is a type of static correction.

A new, completely inexperienced member of the drilling crew. Such a crewmember is stereotyped as prone to making mistakes and being injured, and typically endures pranks played on him by the drilling crew. While the terms weevil and its close cousin, worm, are used widely, they are labels of inexperience, rather than derogatory terms.

A device or system used to measure, display and record the weight of a tubing string, slickline string or coiled tubing string in the wellbore. The weight indicator is the principal means by which the equipment operator monitors the function of downhole tools and equipment. Factors such as fluid density, which affects buoyancy and wellheadpressure, also impact the forces measured at surface. These factors can influence the apparent string weight significantly.

A mud that contains commercial weighting material such as barite or hematite. The economic difference in weighted and unweighted muds is the cost of replacing weighting material according to the solids control practices used. Solids control techniques, such as dilution or hydrocycloning, that can be economical in unweighted muds are not necessarily economical for weighted muds, although centrifugation (incorrectly called "barite recovery") is typically performed when using weighted muds to control mud viscosity.

A high-specific gravity and finely divided solid material used to increase density of a drilling fluid. (Dissolved salts that increase fluid density, such as calcium bromide in brines, are not called weighting materials.) Barite is the most common, with minimum specific gravity of 4.20 g/cm3. Hematite is a more dense material, with minimum specific gravity of 5.05 g/cm3, per API and ISO specifications. Calcium carbonate, specific gravity 2.7 to 2.8, is considered weighting material but is used more for its acidsolubility than for density. Siderite, specific gravity around 3.8, has been used to densify mud, but can cause problems by dissolving into the mud at high pH. Ilmenite, specific gravity of 4.6 has been used in drilling fluid and cement. Only barite and hematite have API/ISO standards.

That period when drilling debris and fluids are still coming out of the formation and perforations. During this time, the skin effect is changing and any well-test results may reflect temporary obstruction to flow that will not be present in later tests.

The technology focused on maintaining pressure on open formations (that is, exposed to the wellbore) to prevent or direct the flow of formation fluids into the wellbore. This technology encompasses the estimation of formation fluid pressures, the strength of the subsurface formations and the use of casing and mud density to offset those pressures in a predictable fashion. Also included are operational procedures to safely stop a well from flowing should an influx of formation fluid occur. To conduct well-control procedures, large valves are installed at the top of the well to enable wellsite personnel to close the well if necessary.

The well production or injection rate.

The change in pressure at one well caused by production from one or more other wells.

(noun) A continuous record of measurements made as a function of depth in a wellbore, acquired by lowering electronic instruments on a wireline, drillpipe, or coiled tubing through the borehole. Well logs measure physical properties of the formation and borehole fluids, including resistivity, porosity, density, acoustic velocity, and natural radioactivity.

Activities associated with drilling a wellbore to intercept one of more specified locations. The term usually is used in reference to directional or horizontal wells that are oriented to maximize contact with the most productive parts of reservoirs via hydraulic fracturing or to optimize intersection with natural fractures. Geomechanical analysis of natural fractures and stresses and geological analysis of the reservoir are critical to successful well planning. Advanced formation evaluation and drilling technology support the drilling operation in real time.

The description of a proposed wellbore, including the shape, orientation, depth, completion, and evaluation. Well plans might be relatively simple for vertical wellbores. Directional or horizontal wellbores require more detailed planning about where to land the well and begin directional drilling, how long the directional or horizontal section should be, and how to evaluate and complete the well. Shale gas wells, many of which are horizontal wells, require highly detailed well plans to optimize production from reservoirs that are vertically and laterally heterogeneous.

The flow rate at which a well is theoretically capable of producing. This is usually defined by a mathematical formula related to Darcy's law, often at maximum theoreticalpressure drawdown. These theoretical rates were, and still are to some extent, used to set the production quota for an individual well in prorated or unitized production situations.

The volume of produced fluid per unit of time.

The maintenance procedures performed on an oil or gas well after the well has been completed and production from the reservoir has begun. Well service activities are generally conducted to maintain or enhance the well productivity, although some slickline and coiled tubing applications are performed to assess or monitor the performance of the well or reservoir. Slickline, coiled tubing, snubbing and workover rigs or rod units are routinely used in well service activities.

(noun) Any treatment performed on a well to restore or enhance its productivity or injectivity beyond the natural capacity of the formation. Well stimulation techniques include hydraulic fracturing, matrix acidising, acid fracturing, and solvent treatments, each designed to improve flow by removing near-wellbore damage or creating new flow paths.

(noun) A multi-well pressure transient test in which fluid is produced from or injected into one well while pressure changes are monitored at one or more offset observation wells. The test provides information about interwell reservoir properties such as transmissibility, storativity, and directional permeability.

Any restriction to flow from near-well reductions in flow capacity. This damage is thought to result from reductions in near-well permeability caused by perforating debris or from the solids or mud filtrateinvasion caused by the drilling process.

A schematic diagram that identifies the main completion components installed in a wellbore. The information included in the wellbore diagram relates to the principal dimensions of the components and the depth at which the components are located. A current wellbore diagram should be available for any well intervention operation to enable engineers and equipment operators to select the most appropriate equipment and prepare operating procedures that are compatible with any downhole restrictions.

Following a surface shut-in, the flow into a well caused by the compressibility of the fluids in the wellbore. Most of the flow occurs from compression of gas in the wellbore. The practical result is that the sandface flow rate is not zero and, therefore, not constant. This gives rise to one form of the wellbore-storage effect.

Distortions in the reservoir response due to wellbore storage. The characteristic trends are an early unit slope trend with pressure change and the derivative overlain on the log-log plot, followed by a "hump" in the pressure derivative that gradually disappears as reservoir trends become recognizable. Complex behavior in the wellbore, such as wellbore phase distribution, can result in a more complex transient trend. A crucial part of the transient analysis is to distinguish the effects of wellbore storage from the interpretable reservoir response.

The surface termination of a wellbore that incorporates facilities for installing casing hangers during the well construction phase. The wellhead also incorporates a means of hanging the production tubing and installing the Christmas tree and surface flow-control facilities in preparation for the production phase of the well.

The pressure registered in the wellhead of a producing well.

An in situ combustion technique in which water is injected simultaneously or alternately with air into a formation.Wet combustion actually refers to wet forward combustion and was developed to use the great amount of heat that would otherwise be lost in the formation. The injected water recovers the heat from behind the burning front and transfers it to the oil bank ahead. Because of this additional energy, the oil displacement is more efficient and requires less air. In spite of these advantages, a wet combustion process cannot avoid liquid-blocking problems and use of wet combustion is limited by the oil viscosity.Wet combustion is also called in situ steam generation or a combination of forward combustion and waterflooding, which is abbreviated as COFCAW.

Natural gas containing significant heavy hydrocarbons. Propane, butane and other liquid hydrocarbons can be liquefied.

Oil that contains basic sediment and water (BS&W).

The proportion of a wet clay that is clay-bound water. A formation that has 100% clay would have a porosity equal to the wet-clay porosity (WCLP), all of it being clay-bound water, and a volume of dry clay equal to (1 - WCLP). The concept is used to relate the volume of clay-bound water, CBW, to the volume of dry clay, Vdcl, in an actual rock, since the ratio of the two is the same and equal to:CBW / Vdcl = WCLP / (1 - WCLP).

The preference of a solid to contact one liquid or gas, known as the wetting phase, rather than another. The wetting phase will tend to spread on the solid surface and a porous solid will tend to imbibe the wetting phase, in both cases displacing the nonwetting phase. Rocks can be water-wet, oil-wet or intermediate-wet. The intermediate state between water-wet and oil-wet can be caused by a mixed-wet system, in which some surfaces or grains are water-wet and others are oil-wet, or a neutral-wet system, in which the surfaces are not strongly wet by either water or oil. Both water and oil wet most materials in preference to gas, but gas can wet sulfur, graphite and coal.Wettability affects relative permeability, electrical properties, nuclear magnetic resonance relaxation times and saturation profiles in the reservoir. The wetting state impacts waterflooding and aquifer encroachment into a reservoir.Reservoir wetting preference can be determined by measuring the contact angle of crude oil and formation water on silica or calcite crystals or by measuring the characteristics of core plugs in either an Amott imbibition test or a USBM test.

A type of damage in which the formationwettability is modified, generating a change in relative permeability that eventually affects well productivity.Surfactants or other additives in drilling fluids, especially oil-base mud, or other injected fluids can change formation wettability. A naturally water-wet formation could be changed into an oil-wet formation with consequent production impairment caused by reduction of oil relative permeability.Wettability change is normally treated with mutual solvents to remove the rock-oil coating (asphaltene or paraffinprecipitation), followed by a strong water-wet surfactant to reduce the tendency of further hydrocarbon precipitation.

A complete section of a conventionally drilled core. The section may be up to about 2 feet [0.6 m] in length, with typical core diameters lying between 1.75 and 5.25 in. [4.4 and 13.3 cm]. The term full-diameter core is also used, but generally refers to shorter sections of about 6 in. [15 cm]. The advantage of whole core analysis is that it measures properties on a larger scale, closer to that of the reservoir. This is particularly important for heterogeneous formations such as many carbonates or fractured materials.

A dilution process which involves selective dumping of the active system (such as sand traps and "bottoms up" mud) and replacement of the lost volume with fresh mud. This process has proved economical with inhibitive water-base systems and is the only method that actually removes colloidal size particles.

A dilution process which involves selective dumping of the active system (such as sand traps and "bottoms up" mud) and replacement of the lost volume with fresh mud. This process has proved economical with inhibitive water-base systems and is the only method that actually removes colloidal size particles.

A common seismic display that shows trace amplitude versus time as an oscillating line about a null point.

An exploration well. The significance of this type of well to the drilling crew and well planners is that by definition, little if anything about the subsurface geology is known with certainty, especially the pressure regime. This higher degree of uncertainty necessitates that the drilling crews be appropriately skilled, experienced and aware of what various well parameters are telling them about the formations they drill. The crews must operate top-quality equipment, especially the blowout preventers, since a kick could occur at virtually any time. If a wildcat is especially far from another wellbore, it may be described as a "rank wildcat."

A valve located on the side of a Christmas tree or temporary surface flow equipment, such as may be used for a drillstem test. Two wing valves are generally fitted to a Christmas tree. A flowing wing valve is used to control and isolate production, and the kill wing valve fitted on the opposite side of the Christmas tree is available for treatment or well-control purposes. The term wing valve typically is used when referring to the flowing wing.

(noun) A rubber or elastomeric plug pumped inside casing or drillpipe to separate different fluids and wipe the inner wall clean during cementing or displacement operations. Top and bottom wiper plugs are used in primary cementing to prevent contamination between the cement slurry and the displacing or displaced fluids.

An abbreviated recovery and replacement of the drillstring in the wellbore that usually includes the bit and bottomhole assembly passing by all of the openhole, or at least all of the openhole that is thought to be potentially troublesome. This trip varies from the short trip or the round trip only in its function and length. Wiper trips are commonly used when a particular zone is troublesome or if hole-cleaning efficiency is questionable.

A safety device attached to the slickline at surface between the hay pulley and stuffing-box pulley. The wire clamp generally is applied when the slickline is to be stationary for a period of time. This prevents the tool string from dropping down the wellbore if the winch unit fails or the slickline becomes damaged at surface.

A type of screen used in sand control applications to support the gravel pack. The profiled wire is wrapped and welded in place on a perforated liner. Screen is available in a range of sizes and specifications, including outside diameter, material type and the geometry and dimension of the screen slots. The space between each wire wrap must be small enough to retain the gravel placed behind the screen, yet minimize any restriction to production.

A general term used to describe well-intervention operations conducted using single-strand or multistrand wire or cable for intervention in oil or gas wells. Although applied inconsistently, the term commonly is used in association with electric logging and cables incorporating electrical conductors. Similarly, the term slickline is commonly used to differentiate operations performed with single-strand wire or braided lines.

A downhole tool used to cut slickline from a tool string that is stuck or jammed in a wellbore. The wireline cutter is attached to the slickline at surface and dropped down the wellbore. When the cutting tool impacts the tool string, a cutting mechanism cuts the slickline and enables recovery of the line in preparation for further fishing operations.

Test taken with a wireline formation tester. The wireline formation pressure measurement is acquired by inserting a probe into the borehole wall and performing a minidrawdown and buildup by withdrawing a small amount of formation fluid and then waiting for the pressure to build up to the formation pore pressure. This measurement can provide formation pressures along the borehole, thereby giving a measure of pressure with depth or along a horizontal borehole. The trend in formation pressure with depth provides a measure of the formation-fluid density, and a change in this trend may indicate a fluid contact. Abrupt changes in formation pressure measurements with depth indicate differential pressuredepletion and demonstrate barriers to vertical flow. Lateral variation in formation pressure measurements along a horizontal well or in multiple vertical wells indicate reservoirheterogeneity.

(noun) A wireline-conveyed tool that isolates a small area of the borehole wall with a packer or probe, draws formation fluid samples, and measures formation pressure at discrete depth points. The tool provides direct measurements of reservoir pressure, fluid mobility, and fluid type, and can recover representative fluid samples for laboratory analysis.

A fishing tool used for the retrieval of broken or cut slickline from the wellbore. Wireline grabs are intended to catch and engage wireline that has been bunched or nested in the wellbore. For that reason, they are often run after a blind box or similar fullbore tool has been used to nest the wireline.

A continuous measurement of formation properties with electrically powered instruments to infer properties and make decisions about drilling and production operations. The record of the measurements, typically a long strip of paper, is also called a log. Measurements include electrical properties (resistivity and conductivity at various frequencies), sonic properties, active and passive nuclear measurements, dimensional measurements of the wellbore, formation fluid sampling, formation pressure measurement, wireline-conveyed sidewall coring tools, and others. In wireline measurements, the logging tool (or sonde) is lowered into the open wellbore on a multiple conductor, contra-helically armored wireline. Once lowered to the bottom of the interval of interest, the measurements are taken on the way out of the wellbore. This is done in an attempt to maintain tension on the cable (which stretches) as constant as possible for depth correlation purposes. (The exception to this practice is in certain hostile environments in which the tool electronics might not survive the temperatures on bottom for the amount of time it takes to lower the tool and then record measurements while pulling the tool up the hole. In this case, "down log" measurements might actually be conducted on the way into the well, and repeated on the way out if possible.) Most wireline measurements are recorded continuously even though the sonde is moving. Certain fluid sampling and pressure-measuring tools require that the sonde be stopped, increasing the chance that the sonde or the cable might become stuck. Logging while drilling (LWD) tools take measurements in much the same way as wireline-logging tools, except that the measurements are taken by a self-contained tool near the bottom of the bottomhole assembly and are recorded downward (as the well is deepened) rather than upward from the bottom of the hole (as wireline logs are recorded).

A type of safety valve in which the principal components can be run and retrieved by wireline or slickline. The valve assembly is landed in a ported nipple that is equipped with a control line connected to the surface control system. This configuration enables the safety valve to be easily retrieved for repair or maintenance, but the resulting internal bore of the WRSV must be relatively small.

An interactive computer suitable for seismic data processing, interpretation and modeling that is particularly useful for studies of large quantities of seismic data, particularly 3D seismic data.

A generic term used to describe a tubing string used to convey a treatment or for well service activities. Both coiled and jointed tubing strings are referred to as work strings.

A percentage of ownership in an oil and gas lease granting its owner the right to explore, drill and produce oil and gas from a tract of property. Working interest owners are obligated to pay a corresponding percentage of the cost of leasing, drilling, producing and operating a well or unit. After royalties are paid, the working interest also entitles its owner to share in production revenues with other working interest owners, based on the percentage of working interest owned.

The process of performing major maintenance or remedial treatments on an oil or gas well. In many cases, workover implies the removal and replacement of the production tubing string after the well has been killed and a workover rig has been placed on location. Through-tubing workover operations, using coiled tubing, snubbing or slickline equipment, are routinely conducted to complete treatments or well service activities that avoid a full workover where the tubing is removed. This operation saves considerable time and expense.

A well-control fluid, typically a brine, that is used during workover operations. Since the wellbore is in contact with the reservoir during most workover operations, workover fluids should be clean and chemically compatible with the reservoir fluids and formation matrix.

A high-porosity, high-permeability channel that develops when heavy oil is produced simultaneously with sand (during cold heavy oil production with sand, or CHOPS). Wormholes develop in a radial pattern away from the borehole and can extend 150 m [492 ft] from the borehole. The development of wormholes can cause reservoir pressure to fall below the bubblepoint, resulting in dissolved gas coming out of solution and forming foamy oil.

A type of strike-slip fault in which the fault surface is vertical, and the fault blocks move sideways past each other. Given the geological complexity of some deformed rocks, including rocks that have experienced more than one episode of deformation, it can be difficult to distinguish a wrench fault from a strike-slip fault. Also, areas can be deformed more than once or experience ongoing structuring such that fault surfaces can be rotated from their original orientations.