Oil and Gas Terms Beginning with “E”

The effective density exerted by a circulating fluid against the formation that takes into account the pressure drop in the annulus above the point being considered. The ECD is calculated as: d + P/0.052*D, where d is the mud weight (ppg), P is the pressure drop in the annulus between depth D and surface (psi), and D is the true vertical depth (feet). The ECD is an important parameter in avoiding kicks and losses, particularly in wells that have a narrow window between the fracture gradient and pore-pressure gradient.

A form of azimuthal anisotropy that occurs when fractures or microcracks are not horizontal. Waves that travel parallel to the fractures have a higher velocity than waves traveling perpendicular to fractures.

Ethylenediamine tetraacetic acid, the reagent used to titrate for calcium and magnesium ions (hardness ions) in water samples. It is also known as versenate or titraver.

(noun) Abbreviation for Enhanced Oil Recovery. A collective term for tertiary recovery techniques used to extract crude oil from a reservoir beyond what is achievable through primary depletion and secondary waterflooding. EOR methods include thermal recovery, chemical injection, gas injection, and microbial processes, each designed to alter fluid properties or reservoir conditions to mobilise residual oil.

Abbreviation for Environmental Protection Agency, a branch of the US government that administers laws passed by the US Congress on environmental matters.

A test for oil-base and synthetic-base muds that indicates the emulsion and oil-wetting qualities of the sample. The test is performed by inserting the ES probe into a cup of 120°F [48.9°C] mud and pushing a test button. The ES meter automatically applies an increasing voltage (from 0 to 2000 volts) across an electrode gap in the probe. Maximum voltage that the mud will sustain across the gap before conducting current is displayed as the ES voltage. The modern ES meter has sine-wave circuitry, whereas older meters used square-wave circuits. (The older units should not be used because they do not correctly address the theory described in the reference below.) The ES sine-wave design and meaning of ES readings have been studied and were found to relate to an oil mud's oil-wetting of solids and to stability of the emulsion droplets in a complex fashion not yet understood.Reference:Growcock FB, Ellis CF and Schmidt DD: "Electrical Stability, Emulsion Stability, and Wettability of Invert Oil-Based Muds," SPE Drilling & Completion 9, no. 1 (March 1994): 39-46.

An electric downhole pump used in heavy oilproduction that is designed with vane and fin configurations to accommodate frictional losses and pump efficiencies caused by heavy oil viscosity.

Hinged steel heavy duty clamps with manual operating handles that crew members latch onto a tool joint. Designed to grip a single size tubular and provide the means to hoist and lower the drill string.

A discontinuous parabola kernel that is used in contouring areal density of data points in a crossplot. The kernel function can take many other forms, such as triangular, rectangular or Gaussian. The function determines the shape of the bump or cluster of data under scrutiny. This technology is often used in cluster analysis and statistical graphic techniques.

A dimension in Euclidian space. Euclidian dimensions are all orthogonal to each other (at right angles to each other) and refer to physical space with X, Y and Z components.

The distance between two points in Euclidian space. Euclidian dimensions are all orthogonal to each other (they are all at right angles to each other) and refer to physical space.

The data observed before the start of radial flow (middle-time transient data). When not dominated by wellbore storage, these data may reveal near-wellbore reservoir features including limited entry, hydraulic fracture and dual-porosity or layering effects. Downhole shut-in greatly improves the quality of the early-time transient data.

The sudden release of accumulated stress in the Earth by movement or shaking. Earthquakes are caused by tectonic activity, volcanoes and human activity (such as explosions). Earthquakes occur in the outer 720 km [445 miles] of the Earth, where rocks tend to break rather than flow under stress. The magnitude of earthquakes is determined according to the logarithmic Richter scale. An earthquake of magnitude 4.5 can cause damage, although humans can feel earthquakes as weak as magnitude 2.0. The San Francisco earthquake of 1906 measured 8.25 on the Richter scale, and the largest ever recorded were 8.9 magnitude earthquakes in Colombia and Ecuador (1906) and Japan (1933), and 9.5 in Chile (1960).

Pertaining to cement that is highly sensitive to the concentration of dispersant, often leading to slurry-stability problems. The term is commonly abbreviated ETD.

Pertaining to cement that is highly sensitive to the concentration of dispersant when the slurry is mixed with water containing a high concentration of salt. Overdispersion often leads to slurry-stability problems. The term is commonly abbreviated ETDS.

A device that helps to keep a wirelinelogging tool away from the center of the borehole. Typical devices are a single bow spring mounted on the outside surface of the logging tool or a set of rubber fingers mounted at the bottom. Some measurements, such as induction logs, respond better when the tool is eccentralized, while others, including acoustic logs, are better when centralized.

The term used to describe how off-center a pipe is within another pipe or the openhole. It is usually expressed as a percentage. A pipe would be considered to be fully (100%) eccentric if it were lying against the inside diameter of the enclosing pipe or hole. A pipe would be said to be concentric (0% eccentric) if it were perfectly centered in the outer pipe or hole. Eccentricity becomes important to the well designer in estimating casing wear, wear and tear on the drillstring, and the removal of cuttings from the low side of an inclined hole. In the latter case, if the drillpipe lies on the low side of the hole (100% eccentric), the eccentricity results in low-velocity fluid flow on the low side. Gravity pulls cuttings to the low side of the hole, building a bed of small rock chips on the low side of the hole known as a cuttings bed. This cuttings bed becomes difficult to clean out of the annulus and can lead to significant problems for the drilling operation if the pipe becomes stuck in the cuttings bed.

The time between each echo in a nuclear magnetic resonance (NMR) measurement. The time is also the time to the first echo and is therefore an important parameter in defining the fastest relaxation time that can be measured. In standard measurements, the echo spacing ranges from 0.2 to 1 ms.

An alternating or transient electrical current in a conductive medium in the presence of a time-varying magnetic field. The eddy current generates its own electromagnetic field.

A technique for measuring the effect of pits and holes in the inner wall of a casing on a high-frequency electrical signal induced in the casing. The eddy-current measurement is used in conjunction with a flux-leakage measurement to determine casing corrosion, the latter being sensitive to the defects on both the inner and outer walls.The principle of measurement is similar to the openholeinduction log, but at higher frequencies. A transmitter coil produces a magnetic field that induces eddy currents in the casing wall. These currents generate their own magnetic field that induces a signal in two closely spaced receiver coils. In smooth casing, these signals are the same, but if the inner wall is pitted, the signals are different. Transmitter-receiver combinations are placed on multiple pads applied against the casing at several azimuths to fully cover the casing wall.

A hydrophonearray in a cable that can be attached to a streamer for acquisition of marineseismic data. The eel can be suspended from the streamer so that the eel is close to the seafloor but the streamer remains high enough to avoid obstacles on the seafloor such as reefs or debris from human activity.

A technique for displacing drilling mud from the annulus using a laminar-flow regime.

A method for determining the effective properties of random fields, commonly abbreviated EMT. Originally developed to estimate transport coefficients, this theory is based on the idea of replacing the inhomogeneous medium by an equivalent homogenous medium such that the fluctuations induced by restoring the heterogeneity average to zero.EMT is used to upscale parameters such as permeability for use in coarse-grained reservoir simulation studies. This use is controversial in reservoirs that are not homogeneous.ReferenceKilpatrick S: Percolation and conduction, Reviews of Modern Physics 45 (1973): 574-614.Mansoori J: "A Review of Basic Upscaling Procedures: Advantages and Disadvantages," in Yarus JM and Chambers RL (eds): Stochastic Modeling and Geostatistics, AAPG Computer Applications in Geology, no. 3. AAPG, Tulsa, Oklahoma, USA, 1994.

The ability to preferentially flow or transmit a particular fluid when other immiscible fluids are present in the reservoir (e.g., effective permeability of gas in a gas-water reservoir). The relative saturations of the fluids as well as the nature of the reservoir affect the effective permeability. In contrast, absolute permeability is the measurement of the permeability conducted when a single fluid or phase is present in the rock.

In the original definition of core analysts, the volume of connected pores in a unit volume of rock. Effective porosity in this sense is the total porosity less the isolated porosity. It is the porosity measured by most core analysis techniques that do not involve disaggregating the sample. In these techniques, the porosity is usually measured on totally dried core samples. Drying removes most of the clay-bound water.In log interpretation, effective porosity means the total porosity less the clay-bound water. The definition is based on the analysis of shaly formations, in which the clay-bound water is considered immobile and hence ineffective. Isolated porosity is rare in such formations and is ignored, being included in the effective porosity.Effective porosity on dried core samples is therefore greater than effective porosity from log analysis, and close to the total porosity from log analysis. In humidity-dried cores, part of the clay-bound water is not removed, and the difference is reduced.In some usage, the capillary-bound water is not considered part of the effective porosity. In this case effective porosity is synonymous with free fluid. Effective porosity is measured in volume/volume, percent or porosity units, p.u.

A value that reflects the number of perforations per unit of length (usually feet) that are producing, or injecting, efficiently. Perforation efficiency may be compromised by gun failure or charge misfire, perforation debris, excessive standoff or poor orientation, or by combinations of these. The effective shot density may be used in treatment design models or to calculate likely productivity response.

In the context of spinner flowmeters, the apparent fluid speed measured by the spinner as the tool is moved up and down the well. The effective velocity is the algebraic sum of the actual fluid velocity and the velocity with which the flowmeter is moving, as determined by cable speed. If the tool moves against the flow, the two velocities are added; if moved with the flow, they are subtracted.

(noun) The fraction of the effective (interconnected) pore volume in a reservoir rock that is occupied by water, calculated by excluding the clay-bound water volume. Effective water saturation is a key parameter in log interpretation and reserve estimation, providing a more accurate measure of movable fluids than total water saturation.

The value of wellbore radius that produces equivalent results to those obtained using a skin factor of zero. It is possible to represent departures from ideal (zero skin) behavior by using the skin effect, or alternatively by changing the value of the effective wellbore radius. For a positive skin effect, effective wellbore radius is smaller than actual wellbore radius. For a negative skin effect, such as often obtained after well stimulation, effective wellbore radius is larger than actual well radius.

The vector that best represents the trend of data in multiple dimensions. This is the major component of principal component analysis (PCA). In log analysis, log data plotted in the number of dimensions equal to the number of curves can be subjected to principal component analysis, replotted in PCA space and then subjected to cluster analysis to find usable electrofacies.

Pertaining to a material that can undergo stress, deform, and then recover and return to its original shape after the stress ceases. Once stress exceeds the yield stress or elastic limit of a material, permanent deformation occurs and the material will not return to its original shape once the stress is removed. In some materials, including rocks, elastic behavior depends on the temperature and the duration of the stress as well as its intensity.

A set of constants, also known as elastic moduli, that defines the properties of material that undergoes stress, deforms, and then recovers and returns to its original shape after the stress ceases. The elastic constants include the bulk modulus, Lame constant, Poisson's ratio, shear modulus, and Young's modulus. Elastic constants are important in seismology because the velocity of waves depends on the elastic constants and density of the rock.

A temporary change in shape caused by applied stress. The change in shape is not permanent and the initial shape is completely recovered once the stress is removed.

The yield point, or the point at which a material can no longer deform elastically. When the elastic limit is exceeded by an applied stress, permanent deformation occurs.

A neutron interaction in which the kinetic energy lost by a neutron in a nuclear collision is transferred to the nucleus. The energy of a neutron is reduced more efficiently in collisions with nuclei of similar mass to the neutron, like hydrogen and other elements of low atomic mass. Elastic neutron scattering is one of the main principles behind the neutron porosity log, as well as the pulsed neutron capture log, which is mainly used to determine water saturation behind casing.

A seismic or acousticwave, such as a P-wave.

A type of gas-lift valve that allows a gas-lift port size to be adjusted remotely from surface to positions from fully open to closed. Developed in the last 10 years, these valves offer the possibility of changing gas-injection points without well intervention.

A small sensor in a productionlogging tool that distinguishes between hydrocarbon and water in its vicinity as it is moved up and down a production well. The electric probe was the first type of local probe to be introduced, appearing initially in the early 1990s. Electric probes measure the local and average holdup of water and give an image of the flow structure across the well.Most electric probes emit a high-frequency current and measure the amplitude of the signal, and hence the impedance, of the fluid in a small sphere near the probe. The output is designed to be digital, indicating conductance when the probe is in front of water, and no conductance when it is in front of hydrocarbon. The water must have a certain minimum salinity, below which the device will not work.

An electric downhole pump used in heavy oil production that is designed with vane and fin configurations to accommodate frictional losses and pump efficiencies caused by heavy oil viscosity.

A difference in vertical and horizontal resistivity within a formation and at the scale of the resistivity measurement. Although there are several possible types of anisotropy, the term usually is used when the electrical properties are the same in all horizontal directions, but different in the vertical direction. For horizontal beds, this type of anisotropy is more fully known as transverse isotropy with a vertical axis of symmetry (TIV). The term may also refer to a difference in resistivities measured parallel and perpendicular to bedding.

The ability of a material to support the flow of an electrical current. In linear isotropic materials, the electric current density at any point in space is proportional to the electric field; the constant of proportionality is the electrical conductivity. Conductivity is the inverse of resistivity in isotropic materials, and is measured in siemens per meter or the archaic units of mhos per meter. The electrical conductivity of the Earth can be measured by electromagnetic methods.

(noun) A corrosion monitoring device consisting of a metallic element exposed to the process environment, with electrical resistance measurements taken periodically to quantify metal loss due to corrosion. Unlike weight-loss coupons, electrical coupons provide near-real-time data on corrosion rates without requiring physical retrieval.

With reference to formation evaluation, the layer between a clay particle and the formation water that has a particular distribution of ions. Clays have an excess negative charge on their surface. When in contact with formation water, this charge attracts an excess of positive cations, normally Na+ together with their molecules of hydration water, into a region near the interface. The layer next to the clay surface, the Stern layer, has no anions, and is always present. Outside the Stern layer is the Gouy layer, through which the ion concentration gradually approaches that of free brine. The thickness of this layer increases as brine salinity decreases.

(noun) A downhole or inline sensor that measures the electrical impedance of a multiphase fluid mixture to determine the fraction of each phase (oil, water, gas) present at a specific point. The probe exploits the contrasting electrical properties of hydrocarbons and formation water to provide local holdup measurements in production logging.

A wireline log of formationresistivity produced by a simple, unfocused arrangement of current emitting and measure electrodes. Conventional electrical devices have four electrodes-current emitting (A), current return (B), measure (M) and measure reference (N)-which may be placed in different configurations. The two most common configurations are the normal and lateral. The currents used are low frequency, typically less than 500 Hz. H.G. Doll recorded the first electrical log on September 5, 1927, in the Pechelbronn field, Alsace, France.

The ability of a material to store a charge from an applied electrical field without conducting electricity.

An instrument used in a corrosion testing to determine metal loss. The probe directly measures the increase in resistance of a metal as its cross-sectional area is reduced by corrosion. At suitable times, once the readings are obtained, these numbers are converted into corrosion rates (mpy).An electrical resistance probe is also called an electrical coupon.

The ability of a material to resist or inhibit the flow of an electrical current, measured in ohm-meters. Resistivity is the reciprocal of conductivity.

A test for oil-base and synthetic-base muds that indicates the emulsion and oil-wetting qualities of the sample. The test is performed by inserting the ES probe into a cup of 120°F [48.9°C] mud and pushing a test button. The ES meter automatically applies an increasing voltage (from 0 to 2000 volts) across an electrode gap in the probe. Maximum voltage that the mud will sustain across the gap before conducting current is displayed as the ES voltage. The modern ES meter has sine-wave circuitry, whereas older meters used square-wave circuits. (The older units should not be used because they do not correctly address the theory described in the reference below.) The ES sine-wave design and meaning of ES readings have been studied and were found to relate to an oil mud's oil-wetting of solids and to stability of the emulsion droplets in a complex fashion not yet understood.Reference:Growcock FB, Ellis CF and Schmidt DD: "Electrical Stability, Emulsion Stability, and Wettability of Invert Oil-Based Muds," SPE Drilling & Completion 9, no. 1 (March 1994): 39-46.

A particular combination of a spontaneous potential log and three electrical logs consisting of a 16-in. [40-cm] short normal, a 64-in. [162-cm] long normal and an 18-ft, 8-in. [5.7-m] lateral. With this combination, it is possible to correct for the effects of invasion in many average logging environments. This combination is sometimes called a conventional electrical log or survey, or simply electrical log, and is also referred to as the ES.

A logging tool based on an arrangement of simple metallic electrodes working at low frequency (less than 500 Hz). The term includes conventional electrical logs, laterologs, micrologs and other microresistivity logs. Electrode devices are used for both wireline and measurements-while-drilling logs. In all electrode devices, a current (IO) and a voltage (VO) are measured on the appropriate electrodes or combinations of electrodes. The apparent formationresistivity is then determined by: Ra = K VO / IO, where K is a system constant for the device concerned.

The resistivity measured by an electrode device. The term often is used to distinguish this resistivity from that measured by the measurements-while-drilling propagation resistivity, or the wirelineinduction resistivity. The term may also refer to measurements-while-drilling toroid devices, such as bit resistivity, ring resistivity and button resistivity. These devices do not use electrodes but have a similar response.Electrode resistivity measurements respond to resistivity, not conductivity. They are therefore best at measuring true formation resistivity at high resistivities, high contrasts in resistivity between formation and drilling mud, and for conductive invasion. These are conditions that are not well covered by propagation and induction resistivity.

An electric motor that acts as a brake. Braking is accomplished by reversing the electric fields on the motor, effectively turning it into a generator. The usage of the generated power, either in useful applications or dissipation as heat, restrains the motor-turned-generator and provides a braking action.

The electromagnetic force, in millivolts, generated by an electrolyte flowing through a permeable medium. This potential is an unwanted contribution to the spontaneous potential (SP) log. In principle, there is no flow into a permeable reservoir at the time of logging, since the mudcake has isolated the reservoir from the borehole. However, it is possible for mudcakes and shales to produce an electrokinetic potential at the time of logging. In normal conditions, any potential is small and equal along the borehole, so that the effect on the SP is negligible. A significant electrokinetic potential can be generated in particular conditions, for example, high differential pressure or poor mudcakes.

An in-situ measurement of the inside diameter of a casing or tubing using an electromagnetic technique. As with the electromagnetic thickness measurement, and usually measured at the same time, a coil centered inside the casing generates an alternating magnetic field. Another coil farther up the tool measures the phase shift introduced by the casing. At high frequency, the signal penetrates less than a tenth of a millimeter into the casing, and the phase shift can be related to the casing internal diameter.Unlike a mechanical or ultrasonic caliper, the measurement does not respond to nonmagnetic scale. For the purpose of determining the true internal diameter, this is a disadvantage, but for the purpose of determining corrosion, it is an advantage.

A process to increase thermal energy in a reservoir using electromagnetic means. The two types of electromagnetic heating are conductive and radio frequency (RF). In conductive heating, a current passes from one electrode through the formation to a second electrode. Electrical resistance of the reservoir brine generates thermal energy, heating the reservoir. In RF heating, transmitters in the wellbore generate electromagnetic waves with frequencies in the microwave range that are directed into the formation. The waves interact with water molecules, generating heat in much the same way a microwave oven does. Since both methods rely on water to transfer thermal energy to the reservoir, higher water saturation increases the efficiency of the heat transfer.

A group of techniques in which natural or artificially generated electric or magnetic fields are measured at the Earth's surface or in boreholes in order to map variations in the Earth's electrical properties (resistivity, permeability or permittivity). Most applications of surface electromagnetic methods today are for mineral and groundwater exploration or for shallow environmental mapping. Electromagnetic or electrical logging is, however, the main technique used in oil exploration to measure the amount of hydrocarbons in the pores of underground reservoirs. Inductive electromagnetic (EM) methods include a variety of low frequency (a few Hz to several kHz) techniques deploying large or small wire coils at or near the surface. In older usage, "electromagnetic method" tended to refer only to inductive methods. This term is now commonly used for any method employing electromagnetic fields, including methods that use direct current (electrical or resistivity methods) and induced polarization (IP), methods that use microwave frequencies (ground-penetrating radar), and methods that use natural electromagnetic fields (magnetotelluric methods).

Pertaining to logs that measure the properties of electromagnetic waves as they move through a formation. Measurements-while-drilling (MWD) propagation resistivity logs work between about 100 kHz and 10 MHz. Dielectric propagation logs work between 20 and 200 MHz. Logs made above 200 MHz and into the GHz range are known as electromagnetic propagation logs. Below about 100 kHz, the measurements are based on the properties of standing waves, not of propagation. Induction and laterolog tools work in this range.

A measurement of the high frequency (about 1 GHz) dielectric properties of the formation. In a typical tool, a microwave transmitter is placed a few inches below two receivers separated by 4 cm [1.6 in.]. At this frequency, the response is best explained as the propagation of a wave. Thus the phase shift and attenuation of the wave between the receivers are measured and transformed to give the log measurements of propagation time and attenuation. Because of the short spacings, the measurement has excellent vertical resolution and reads within inches of the borehole wall except at high resistivity. Different transmitter and receiver spacings and orientations are used, leading to different arrays, such as the endfire array and the broadside array.An ideal measurement would give the plane wave properties of the formation. However, the geometry of the measurement precludes this, so that a correction, known as the spreading-loss correction, is needed for the attenuation and to a much smaller extent for the propagation time. The measurement is also affected by the dielectric properties and thickness of the mudcake. Borehole compensation is used to correct for sonde tilt or a rough borehole wall.

An in-situ measurement of the thickness of a casing or tubing string using an electromagnetic technique. The result is presented as a type of casing-inspection log, giving an estimate of metal loss and detecting corrosion. In the usual method, a coil centered inside the casing generates an alternating magnetic field. Another coil farther up the tool measures the phase shift introduced by the casing. This phase shift depends on the casing-wall thickness and internal diameter, as well as the casing conductivity and magnetic permeability. The effects change at different frequencies, so that by varying the frequency, the thickness and internal diameter can be uniquely determined.Electromagnetic thickness can also be measured using other techniques, for example from a casing-potential profile or a flux-leakage measurement.

Referring to a log of the yields of different elements in the formation, as measured by capture gamma ray spectroscopy using a pulsed neutron generator. The log is a type of pulsed neutron spectroscopy log that uses only the capture spectrum. The capture spectrum is formed by many elements, but since the main purpose of the log is to determine lithology, the principal outputs are the relative yields of silicon, calcium, iron, sulfur, titanium and gadolinium. The yields give information only on the relative concentration of these elements. To get absolute elemental concentrations, it is necessary to calibrate to cores, or, more often, use a model such as the oxide-closure model.The depth of investigation of the measurement is several inches into the formation. It can be run in open or cased hole. The absolute elemental concentrations are insensitive to fluids in the borehole and formation.

Any compensating factor used to bring measurements to a common datum or reference plane. In gravity surveying, elevation corrections include the Bouguer and free-air corrections. Seismic data undergo a static correction to reduce the effects of topography and low-velocity zones near the Earth's surface. Well log headers include the elevation of the drilling rig's kelly bushing and, for onshore locations, the height of the location above sea level, so that well log depths can be corrected to sea level.

A hinged mechanism that may be closed around drillpipe or other drillstring components to facilitate lowering them into the wellbore or lifting them out of the wellbore. In the closed position, the elevator arms are latched together to form a load-bearing ring around the component. A shoulder or taper on the component to be lifted is larger in size than the inside diameter of the closed elevator. In the open position, the device splits roughly into two halves and may be swung away from the drillstring component.

The shape of a wavelet produced by reflection of an actual wave train at one interface with a positive reflection coefficient. The embedded wavelet is useful for generating a convolutional model, or the convolution of an embedded wavelet with a reflectivity function and random noise, during seismicprocessing or interpretation.

The process whereby steel components become less resistant to breakage and generally much weaker in tensile strength. While embrittlement has many causes, in the oil field it is usually the result of exposure to gaseous or liquid hydrogen sulfide [H2S]. On a molecular level, hydrogen ions work their way between the grain boundaries of the steel, where hydrogen ions recombine into molecular hydrogen [H2], taking up more space and weakening the bonds between the grains. The formation of molecular hydrogen can cause sudden metal failure due to cracking when the metal is subjected to tensile stress.

Pertaining to analysis that includes equations or formulae that were derived purely from data analysis and were not derived from a theoretical basis. The majority of equations used in reservoir characterization and reservoir engineering are empirical.

A dispersion of droplets of one liquid in another liquid with which it is incompletely miscible. Emulsions can form in heavy oils that contain a significant amount of asphaltenes. The asphaltenes act as surfactants with treatment or formation water. The resulting emulsion droplets have high-energy bonds creating a very tight dispersion of droplets that is not easily separated. These surface-acting forces can create both oil-in-water and/or water-in-oil emulsions. Such emulsions require temperature and chemical treating in surface equipment in order to separate.

A multiphase-flow regime with oil as the continuous phase, in which water exists as small, approximately homogeneously distributed droplets. There may also be a thin film of water on the pipe wall.

A water-base drilling fluid that contains dispersed oil or synthetic hydrocarbon as an internal phase. Early emulsion muds used diesel or crude oil dispersed into alkaline water-base muds. Synthetic liquids are now being substituted for oils in emulsion muds. Water-base muds containing certain synthetic liquids can be discharged in the Gulf of Mexico because they are environmentally safe and pass the EPA static sheen test and mysid shrimp toxicity tests.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.

Describing parallel or subparallel, closely-spaced, overlapping or step-like minor structural features in rock, such as faults and tension fractures, that are oblique to the overall structural trend.

In drilling fluid parlance, the absorption of a polymer film onto cuttings and wellbore walls to form a coat or barrier. The term is usually applied to shale encapsulation by long-chain, acrylamide-acrylate (PHPA) polymers. Negative sites on PHPA may bond to positive sites on the clays in shales, although it is not well-defined how (or how well) encapsulation works. A viscous polymer film, according to some test results, slows diffusion of water molecules into the shale and thus slows hydration and disintegration. This does not prevent wellbore problems but can delay their onset.

A particular arrangement of transmitters and receivers used in the electromagnetic propagation measurement in which the dipoles used as sensors are oriented along the axis of the tool. The orientation is combined with relatively long spacings to give deeper penetration, and hence less effect of mudcake or rugosity.

During a titration procedure in analytical chemistry, the point at which reagent addition should be immediately stopped and the volume of reagent recorded. The endpoint represents the end of a specific chemical reaction, such as precipitation, and may be indicated by a color change, a voltage or pH reading or an inflection point on an graphical plot of the data.

A technique in nuclear magnetic resonance (NMR) logging based on a long echo spacing, specially chosen to enhance the diffusion of formation water. Echo spacings in standard NMR logs are too short to allow any significant diffusion relaxation from water. Long echo spacings, for example 3 s, cause diffusion relaxation to limit the maximum T2 from water. They also limit the maximum T2 from light hydrocarbons. However, there is a certain range of viscosity of medium oil that is less affected. The enhanced diffusion technique therefore permits the identification of some medium oils.

An oil recovery enhancement method using sophisticated techniques that alter the original properties of oil. Once ranked as a third stage of oil recovery that was carried out after secondary recovery, the techniques employed during enhanced oil recovery can actually be initiated at any time during the productive life of an oil reservoir. Its purpose is not only to restore formationpressure, but also to improve oil displacement or fluid flow in the reservoir.The three major types of enhanced oil recovery operations are chemical flooding (alkaline flooding or micellar-polymer flooding), miscible displacement (carbon dioxide [CO2] injection or hydrocarbon injection), and thermal recovery (steamflood or in-situ combustion). The optimal application of each type depends on reservoir temperature, pressure, depth, net pay, permeability, residual oil and water saturations, porosity and fluid properties such as oil API gravity and viscosity.Enhanced oil recovery is also known as improved oil recovery or tertiary recovery and it is abbreviated as EOR.

The gas present in the fluids of a wellbore circulatory system. Many well-intervention operations are conducted with the well live or held on balance. Fluids circulated within the wellbore under these conditions are likely to pick up reservoir fluid and gas. The entrained gas and fluid require special handling and processing before the base fluid can be safely recirculated in the wellbore or prepared for disposal.

The hole created in the internal surface of the casing or liner by the perforating charge or bullet. The entrance hole should be clean, free from burrs and round to create an efficient flow path between the reservoir and wellbore. Depending on gun size and standoff, the entrance hole is typically between 3/8" and 1/2" in diameter. The perforation charge design generally is optimized to provide maximum penetration while achieving a medium-size entrance hole. Special charge designs can be used when optimizing the entrance hole size is important, as in sand-control applications.

The adjustments that must be made to log measurements to bring them back to the standard conditions for which the tool has been characterized. Different measurements require different corrections. For example resistivity measurements usually require correction for the borehole, invasion and shoulder beds, and may also be corrected for apparent dip, anisotropy and surrounding beds in horizontal wells. Density measurements require correction only for borehole size, while neutron porosity measurements require corrections for temperature, pressure and a large number of borehole and formation parameters. Not all corrections are significant in all cases. Corrections can be calculated manually, using charts, or applied through software. Conventionally, corrections are applied sequentially, as for example first borehole then invasion. In some situations, such as the combination of deep invasion and high apparent dip on a resistivity measurement, the corrections are too interdependent for sequential application to be accurate. The solution is iterative forward modeling.

Pertaining to the environment of deposition of sediments by wind, such as the sand dunes in a desert. Because fine-grained sediments such as clays are removed easily from wind-blown deposits, eolian sandstones are typically clean and well-sorted.

A measurement based on the slowing down of neutrons between a source and one or more detectors that measure neutrons at the epithermal level, where their energy is above that of the surrounding matter, between approximately 0.4 and 10 eV. The slowing-down process is dominated by hydrogen, and is characterized by a slowing-down length. By measuring the neutrons at the epithermal level, rather than the thermal level, the response is a purer estimate of hydrogen index, unaffected by thermal absorbers. On the other hand, the count rate is smaller for the same source and source-detectorspacing.Epithermal measurements have been made with both the compensated neutron technique and by using a pad pressed against the borehole wall with detectors focused into the formation.

Abbreviation for equivalents-per-million. This unit of concentration is determined by dividing the number of equivalents of a substance by the weight of the solution that contains the substance, expressing the result as parts per million, and then dividing by the valency. For example, a solution containing one milligram of calcium in one kilogram of solution contains 1 ppm calcium. By dividing the ppm by the valency of calcium, which is 2 in this case, the result is the epm of calcium, 0.5 epm.

The high-pressure piping and valves configured around the stripping rams of a snubbing unit. The equalizing loop enables the wellhead pressure to be applied on both sides of the rams when closed, a process required before the rams can be opened.

A high-pressure valve, generally of small diameter, located on a conduit that runs between the two sides of an isolation valve or blowout preventer ram set. The forces acting on isolation devices such as blowout preventer rams can be extremely high, preventing the rams from being opened, or causing damage to the ram set seals during the opening process. The equalizing valve allows the pressure to be equalized across the ram set or isolation valve, enabling the device to be opened safely.

An equation that specifies fluid density as a function of pressure and temperature. A large body of scientific literature describes these functions for all kinds of hydrocarbons, and also for complex mixes of various hydrocarbons with other hydrocarbons and with other fluids. Once the components of a reservoir fluid are determined, the known hydrocarbon properties become a valuable tool in making further calculations of well-test results and predictions of future well and reservoir behavior.

A technique to map a potential field generated by stationary electrodes by moving an electrode around the survey area.

The effective density exerted by a circulating fluid against the formation that takes into account the pressure drop in the annulus above the point being considered. The ECD is calculated as: d + P/0.052*D, where d is the mud weight (ppg), P is the pressure drop in the annulus between depth D and surface (psi), and D is the true vertical depth (feet). The ECD is an important parameter in avoiding kicks and losses, particularly in wells that have a narrow window between the fracture gradient and pore-pressure gradient.

With reference to the conductivity of brines, the ionic conductivity provided by a unit ion concentration. The conductivity of the brine (Cw, in siemens per meter or S/m) is determined by the equivalent conductance, (B, in S/m per meq/cm3) and the ion concentration (N, in milli-ion equivalent, or more commonly milliequivalent, meq per cm3) as Cw = B * N.

The weight of any cementitious material or blend based on the absolute volume of the cement. The term is normally used to define a sack of cement blend in which part of the cement has been replaced, on an absolute volume basis, by a pozzolanic material such as fly ash.

The effective resistivity of the formation water or the mudfiltrate, as it affects the electrochemical potential. The electrochemical potential is the main source of the spontaneous potential log. The equivalent water resistivity, or equivalent resistivity, is defined as the reciprocal of the activity of a solution, so that the formula for the electrochemical potential, Ec, can be written as: Ec = - K log10 (Rmfe / Rwe)where K is a coefficient, and Rmfe and Rwe are the equivalent water resistivities. The importance of Rmfe and Rwe is that they are equal to the actual water resistivities for NaCl solutions above about 0.1 ohm-m. In salty waters below 0.1ohm-m, the equivalent resistivity is lower by an amount that depends on temperature and salinity. In fresh waters, divalent ions such as Ca++ and Mg++ have a strong effect on Ec. Rwe is then related to Rw by an empirical transform that assumes average concentrations of these ions. For fresh mud filtrates, it has been common practice to set Rmfe = 0.85 * Rmf.

The molecular weight of an element, molecule or ion divided by its valence (or valence change for a redox reaction). For example, the molecular weight of calcium hydroxide, or "slaked lime," [Ca(OH)2] is 72.10. Because the valency of calcium in this case is 2, the equivalent weight of lime is 36.05. Mud analyses give concentrations in various units: ppm, mg/L, wt.% and epm. Mud engineers should recognize the meaning of epm and equivalent weight of a mud chemical.

The wearing away of material, usually rock or steel, by the continuous abrasive action of a solids-laden slurry. For erosion to occur usually requires a high fluid velocity, on the order of hundreds of feet per second, and some solids content, especially sand. Erosion may also occur in gas streams, again assuming the presence of sand particles. It is usually difficult to erode the wellbore wall significantly with drilling mud alone due to its relatively low velocity and high viscosity. There is also a dramatic "self-limiting" effect because even slight enlargement of the original gauge wellbore dramatically decreases fluid velocities.

A type of corrosion produced when easily removed scales (such as iron carbonate) that were initially protecting the metals in the pipe are eroded and the underlying metals are corroded.Erosion-corrosion is a common cause of failure in oilfield equipment. The attack is normally localized at changes of pipe sections, bends or elbows where there is high velocity or turbulent flow.

A type of corrosion produced when easily removed scales (such as iron carbonate) that were initially protecting the metals in the pipe are eroded and the underlying metals are corroded.Erosion-corrosion is a common cause of failure in oilfield equipment. The attack is normally localized at changes of pipe sections, bends or elbows where there is high velocity or turbulent flow.

The difference between a data value and the value predicted by a statistical distribution or other mathematical algorithm. Strictly, normal distributions apply only to random events, but they are often used to describe nonrandom events because their distributions look like approximations to a "bell curve." Under these circumstances, the error is only a measure of deviation from a normal distribution and may not have direct physical significance.

A steel cable attached to the rig derrick or mast near the work platform for the derrickman. This cable is anchored at surface level (on a vessel or the Earth) away from the mast in a loose catenary profile, and fitted with a handle and hand brake that is stored at the top. The escape line provides a rapid escape path for the derrickman should well conditions or massive mechanical failure warrant. In such a case the derrickman would disconnect his safety belt from the rig, rehook it over the escape line if time permitted, firmly grip the tee-bar handle and ride the trolley down the cable while holding on to the handle with his hands. The escape line is also known as the "Geronimo line."

The amount of oil and gas expected to be economically recovered from a reservoir or field by the end of its producing life. Estimated ultimate recovery can be referenced to a well, a field, or a basin.

A semi-enclosed coastal environment of deposition in which a river mouth permits freshwater to contact and mix with seawater.

Global sea level and its variations. Changes in sea level can result from movement of tectonic plates altering the volume of ocean basins, or when changes in climate affect the volume of water stored in glaciers and in polar icecaps. Eustasy affects positions of shorelines and processes of sedimentation, so interpretation of eustasy is an important aspect of sequencestratigraphy.

Global sea level, which changes in response to changes in the volume of ocean water and the volume of ocean basins.

A hole dug to contain brine for disposal by evaporation. Some evaporation pits are lined with plastic or asphalt to keep water from filtering through and contaminating nearby free-water aquifers.

A class of sedimentary minerals and sedimentary rocks that form by precipitation from evaporating aqueous fluid. Common evaporite minerals are halite, gypsum and anhydrite, which can form as seawater evaporates, and the rocks limestone and dolostone. Certain evaporite minerals, particularly halite, can form excellent cap rocks or seals for hydrocarbon traps because they have minimal porosity and they tend to deform plastically (as opposed to brittle fracturing that would facilitate leakage).

An appearance of seismic data as a diffraction, reflection, refraction or other similar feature produced by an arrival of seismic energy. An event can be a single wiggle within a trace, or a consistent lining up of several wiggles over several traces. An event in a seismic section can represent a geologic interface, such as a fault, unconformity or change in lithology.

That part of the effect of gas on the neutron porosity measurement that is not explained by differences in hydrogen index. By using the concept of hydrogen index, the only significant contributor to the neutron porosity in a gas zone is the liquid-filled porosity, since the hydrogen indices of gas and matrix are close to zero. However, the resultant liquid-filled porosity is found to be too low. The error comes from treating the gas-filled porosity as matrix. If this matrix is excavated and replaced with gas, the correct response can be predicted.

The cementslurry remaining in the wellbore following a cement squeeze in which the objective is to squeeze slurry into the perforations and behind the casing or liner. The volume of slurry required to effect a successful squeeze is often difficult to estimate. In most cases, an excess allowance is made since a shortage of slurry would result in failure of the operation. Removal of the excess cement slurry before it sets has been a key objective in the development of modern cement-squeeze techniques.

The speed the drilling fluid attains when accelerated through bit nozzles. The exit velocity is typically in the low-hundreds of feet per second. It has been reported that in certain shaly formations, an impingement velocity on the order of 250 feet per second is required to effectively remove newly created rock chips from the bottom of the hole. This impingement velocity is not, however, the same as the exit velocity, since the high-energy fluid jet loses velocity through viscous losses and conversions from kinetic energy to forms of potential energy occur once the fluid leaves the bit. For this reason, the well designer generally seeks to maximize the fluid velocity (or other measure of jet energy) to achieve maximum cleaning at the bottom of the hole.

A cement system exhibiting a bulk volumetric increase after setting. Expanding cement is commonly used to eliminate or minimize the effects of microannuli.

A device or completion component designed to enable relative movement between two fixed assemblies in the event of thermal expansion or contraction. The forces generated by thermal expansion or contraction can be significant. Expansion joints within the completion assembly prevent any movement or forces being transmitted to fixed components such as packers or tubing hangers.

The results of extrapolating from a known data point to points away from measurements. In geostatistical modeling, generating expectation trends is a fundamental process that requires procedures to estimate geometric changes by using various statistical approaches. Often, trends in surfaces are estimated and used to provide bounds on possible facies extrapolation.

A perforating gun assembly that disintegrates upon firing, thereby reducing the volume and dimensions of retrieved components. Expendable guns are typically used where wellbore restrictions allow only limited access, as in through-tubing applications. The distortion caused to the gun assembly during firing would typically prevent recovery of a conventional gun design through the limited clearances. The expendable gun assembly breaks into small pieces that drop to the bottom of the well, leaving only a relatively small subassembly that is easily recovered to surface.

A temporary plug, inserted in the completion assembly before it is run, to enable pressure testing of the completed string. With the operation complete, the expendable plug can be pumped out of the assembly, thereby avoiding a separate retrieval run.

A computer system that uses a rule-based algorithm to provide expertise on a given subject. Many computer programs have been written for use in the oil field using rule-based approaches to provide expert systems. The rules are taken from an expert working in the field and are written in a way that attempts to reproduce the knowledge and approaches used by that expert to solve a range of actual problems. Most such programs are limited to specific disciplines such as dipmeter interpretation, electrofacies determination, reservoir characterization, blowout prevention or drilling fluid selection. Sometimes expert systems are written in computer languages such as LISP that easily handles rules, but once fully tested, expert systems are usually translated to BASIC, C or FORTRAN to be compiled into efficient applications or programs.

The initial phase in petroleum operations that includes generation of a prospect or play or both, and drilling of an exploration well. Appraisal, development and production phases follow successful exploration.

(noun) A geologically defined concept comprising a specific combination of source rock, reservoir rock, seal, trap, and migration pathway in a particular area, which together create the conditions necessary for hydrocarbon accumulation. An exploration play guides the selection and ranking of drilling prospects within a basin or region.

Surface seismic data acquired using an explosive energy source, such as dynamite.

(noun) A seismic acquisition geometry in which the distance between the source and the farthest receiver (offset) is increased beyond the standard spread length, providing longer-offset data that improves velocity analysis, multiple suppression, and imaging of deeper reflectors.

A form of azimuthal anisotropy that occurs when fractures or microcracks are not horizontal. Waves that travel parallel to the fractures have a higher velocity than waves traveling perpendicular to fractures.

A downhole tool used to cut tubing or similar tubulars that have become stuck in the wellbore. The external cutter slips over the fish or tubing to be cut. Special hardened metal-cutters on the inside of the tool engage on the external surfaces of the fish. External cutters are generally used to remove the topmost, possibly damaged, portion of a fish to enable an overshot, or similar fishing tools, to engage on an undamaged surface.

The continuous phase of an emulsion. The internal phase is the dispersed droplets of emulsified fluid.

A downhole tool used to pull or retrieve temporary plugs or similar equipment. The external pulling tool engages on the external surfaces of the item to be retrieved.

A type of tubing connection in which the external diameter of the tubing joint is larger adjacent to the tubing connection to provide the necessary strength. The internal tubing surface is flush to enable good fluid-flow characteristics.