Oil and Gas Terms Beginning with “I”

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

An outdated distinction between two types of oil muds. In the past, invert-emulsion oil muds were those with more than 5 vol.% emulsified water, and oil-base muds were those with less than 5 vol.% water. Today, this distinction is not pertinent because the general term oil mud covers all water concentrations.

Inductive-source induced polarization.

A particular type of induction log designed to read an intermediate distance into the formation while maintaining good vertical resolution. The medium-induction array of eight coils (IM) is produced by three transmitters and five receivers running at 20 kHz. A small fourth transmitter coil was added in tools built since 1968. The midpoint of the integrated radial geometrical factor is 30 in. [76 cm] in radius. The vertical resolution is about 4 ft [1.2 m] but varies with conditions. The IM is combined with a deep-induction log on the same sonde to produce a dual induction log.

A synthetic hydrocarbon liquid made by the polymerization of ethylene, H2C=CH2. IOs are one of several synthetic fluids that have recently been used as base for synthetic-base muds and in other applications where refined oils might otherwise be used except for HSE concerns. IOs are linear structures that have their olefin double bond in the center of the chain length. They are made by isomerization of linear alphaolefins (LAO), which have their double bond at the end of the chain. Because the olefin bond is in the central area of the chain, the physical properties of IOs are different (for example, they are generally lower viscosity) compared with the LAOs from which they are made.

Abbreviation for the Institute of Petroleum, a standardization body for the petroleum industry in Europe. Several industry-standard drilling-fluid tests are adopted from IP and ASTM procedures.

A simple alcohol (C3H7OH) used as a solvent in some mud analyses. A 50/50 xylene/IPA mixture was used in the past as an emulsionbreaker for oil mud but has been replaced with propylene glycolnormal propyl ether (PNP).

A mathematical tool used in production engineering to assess well performance by plotting the well production rate against the flowing bottomhole pressure (BHP). The data required to create the IPR are obtained by measuring the production rates under various drawdown pressures. The reservoir fluid composition and behavior of the fluid phases under flowing conditions determine the shape of the curve.

Abbreviation for the International Organization for Standardization, a developer and publisher of international standards.ISO is a network of national-standards institutes from 157 countries, with one member per country and a Central Secretariat that coordinates the system from offices in Geneva, Switzerland.(Recognizing that the acronym for International Organization for Standardization would vary widely from one member country to the next, ISO founders decided to standardize the organization acronym. ISO is derived from the Greek isos, meaning equal.)API and ISO joint committees are developing standards for worldwide use.

A remotely controlled piece of equipment that makes up or breaks down pipe using a rotary table and torque wrenches. Also called a Hydraulic Wrench.

A gas defined by the fundamental equation of state, pV = nRT, where pressure, p, times volume, V, equals moles of gas, n, times gas constant, R, times temperature, T. The units are arbitrary and are accommodated by the value of the gas constant R, which is different for every set of units.

Pertaining to one of three main classes of rocks (igneous, metamorphic and sedimentary). Igneous rocks crystallize from molten rock, or magma, with interlocking mineral crystals. Igneous rocks that crystallize slowly, typically below the surface of the Earth, are plutonic igneous rocks and have large crystals (large enough to see with the naked eye). Volcanic igneous rocks crystallize quickly at the Earth's surface and have small crystals (usually too small to see without magnification). Common examples include granite (plutonic) and rhyolite (volcanic), diorite (plutonic) and andesite (volcanic), and gabbro (plutonic) and basalt (volcanic). Igneous rocks typically comprise the minerals quartz, mica, feldspar, amphibole, pyroxene and olivine.

[K1-1.5Al4(Si7-6.5Al1-1.5O20)(OH)4]A group of clay minerals formed during the alteration of silicate minerals such as mica and feldspar and commonly found in marine shales.

A dense mineral whose specific gravity is 4.67 g/cm3, composed of FeO·TiO2. Ilmenite is used as a weighting agent for cement and mud.

In remote sensing, to record and interpret electromagnetic energy from the surfaces of planets or satellites using photographic displays.

A virtual well used to mathematically create the effect of a flow barrier. The pressure transient behavior both at the well and in the reservoir is identical for the following two cases: 1) a well near a barrier represented by a plane normal to the bedding, or 2) a well producing or injecting at the same rate as the tested well. In the second case, the effect is of a barrier bisecting the space between the two wells.

The process of absorbing a wetting phase into a porousrock. Imbibition is important in a waterdrivereservoir because it can advance or hinder water movement, affecting areal sweep. Spontaneous imbibition refers to the process of absorption with no pressure driving the phase into the rock.It is possible for the same rock to imbibe both water and oil, with water imbibing at low in situwater saturation, displacing excess oil from the surface of the rock grains, and oil imbibing at low in-situ oil saturation, displacing excess water. An imbibition test is a comparison of the imbibition potential of water and oil into a rock. The wettability of the rock is determined by which phase imbibes more.

Pertaining to a hydrocarbon source rock that has not fully entered optimal conditions for generation.

Pertaining to a condition in which two fluids are incapable of forming molecularly distributed mixtures or attaining homogeneity at that scale. The fluids separate into two phases with an interface between them. For example, oil and water are immiscible.

In electromagnetics or electrical circuit theory, the ratio of voltage to current when these are represented by phasor quantities in alternating current circuits. (A phasor is a complex number that represents the amplitude and phase of a quantity that varies sinusoidally in time.) Electrical impedance, also symbolized by Z, is a complex number that has the same units (ohms) as resistivity.

A device that responds to fluid flow and is used as the sensor in a flowmeter. In a spinner flowmeter, the term refers to the spinner, or in some cases to one of the blades of the spinner. An impeller is also used in a torque flowmeter.

Pertaining to a rock that is incapable of transmitting fluids because of low permeability. Shale has a high porosity, but its pores are small and disconnected, so it is relatively impermeable. Impermeable rocks are desirable sealing rocks or cap rocks for reservoirs because hydrocarbons cannot pass through them readily.

A single, impenetrable barrier to fluid flow in a reservoir that causes a change of a factor of two in the slope of buildup or drawdown curves. These are often observed in a normal test if the barrier is close (a few hundred feet or less) to the tested well. Most tests are not long enough to detect the presence of distant barriers. Two perpendicular barriers cause a change in the slope of a factor of four, and so forth, and models exist for a variety of geometries, typically for up to four barriers.

Materials to provide cathodic protection. Impressed current anodes are relatively inert to corrosion and require an external power source to generate the electric current that will bring cathodic protection to the structure.Impressed current systems are used mainly when the current required for cathodic protection is large.

A method for recovering additional oil beyond fluid expansion, rockcompressibility, gravitational drainage, pressure decline and natural waterdrive or gasdrive. This term is used in both a restricted sense and a more general sense. In its restricted sense, it is a process, such as waterflooding or gasflooding, that adds energy to a reservoir to stimulate oil production and increase recovery factor. In its more general sense, it is any activity that increases oil production and increases the recovery factor. This sense can also include, for example, enhanced oil recovery methods, infill drilling, hydraulic fracturing, and drilling horizontal and multilateral wells.

A type of oxygen activation technique for measuring water flow in which a short neutron burst is followed by a long observation period, during which the activated flowing oxygen is recognized at the detector by its signature. Stationary oxygen gives a gradually decaying signal, whereas flowing oxygen can be distinguished by a peak at a time after the neutron pulse that is related to its velocity. To cover a wide range in water velocity, several detectors at different spacings are needed. The flow volume can be estimated from the area under the peak. While recordings are typically made with the tool stationary, continuous logs are also possible.In an alternative type of impulse method, the neutrons are emitted in regular bursts until the count rate from the activated oxygen reaches a constant level. Then, after terminating the bursts, the time for the count rate to decrease by one half is measured. This time can be related to the water velocity.

Seismic data whose energy source is impulsive and of short duration, as with an air gun, rather than vibratory, as with a vibrator.

In the original location or position, such as a large outcrop that has not been disturbed by faults or landslides. Tests can be performed in situ in a reservoir to determine its pressure and temperature.

A seismic line within a 3D survey parallel to the direction in which the data were acquired. In marineseismic data, the in-line direction is that in which the recording vessel tows the streamers.

A method of thermal recovery in which fire is generated inside the reservoir by injecting a gas containing oxygen, such as air. A special heater in the well ignites the oil in the reservoir and starts a fire.The heat generated by burning the heavy hydrocarbons in place produces hydrocarbon cracking, vaporization of light hydrocarbons and reservoir water in addition to the deposition of heavier hydrocarbons known as coke. As the fire moves, the burning front pushes ahead a mixture of hot combustion gases, steam and hot water, which in turn reduces oil viscosity and displaces oil toward production wells.Additionally, the light hydrocarbons and the steam move ahead of the burning front, condensing into liquids, which adds the advantages of miscible displacement and hot waterflooding.In situ combustion is also known as fire flooding or fireflood.

Analysis performed by downhole tools to determine physical and chemical properties of fluids. Typical analyses that can be performed downhole include basic density and viscosity measurements at sampling pressure and temperature.

Downhole measurement of fluid viscosity, typically performed either with logging tools based on nuclear magnetic resonance (NMR) or with sampling tools such as formation testers.

The deviation from vertical, irrespective of compass direction, expressed in degrees. Inclination is measured initially with a pendulum mechanism, and confirmed with MWD accelerometers or gyroscopes. For most vertical wellbores, inclination is the only measurement of the path of the wellbore. For intentionally deviated wellbores, or wells close to legal boundaries, directional information is usually also measured.

An instrument used to measure the dip of the Earth's magnetic field.

Pertaining to strata that are relatively ductile and tend to flow under stress rather than deform by brittle faulting or fracturing. The bed thickness of incompetent beds tends to change during deformation.

An approach to stochastic imaging or simulation of a reservoir that is nonparametric. The Gaussian approach is a simpler approach that includes a normal score transform of the data to produce a new variable that is univariate and normally distributed.

Models of reservoirs built using indicator (nonparametric) methods.

The principle of an activation log, which is a log of elemental concentrations derived from the characteristic energy levels of gamma rays emitted by a nucleus that has been activated by neutron bombardment. The term is often used to refer specifically to the pulsed neutron spectroscopy measurement.

A type of damage in which foreign particles injected during normal well operations, such as drilling, completion, workover, stimulation or enhanced recovery, block the near-wellbore formation, reducing well productivity.Potentially damaging particles in drilling fluids include clays, cuttings, weighting agents and fluid-loss control materials. In workover and stimulation fluids, suspended solids include bacteria and polymer residues. Foreign plugging particles can also be introduced as a result of poor water-handling practices. These foreign particles include debris from tanks and tubing.

An electromagnetic method that uses electrodes with time-varying currents and voltages to map the variation of electrical permittivity (dielectric constant) in the Earth at low frequencies. Induced polarization is observed when a steady current through two electrodes in the Earth is shut off: the voltage does not return to zero instantaneously, but rather decays slowly, indicating that charge has been stored in the rocks. This charge, which accumulates mainly at interfaces between clay minerals, is responsible for the IP effect. This effect can be measured in either the time domain by observing the rate of decay of voltage or in the frequency domain by measuring phase shifts between sinusoidal currents and voltages. It is often used in exploration for minerals and can sometimes distinguish different types of mineralization. The IP method can probe to subsurface depths of thousands of meters.

What Is an Induction Log? An induction log is a wireline log that measures the electrical conductivity and resistivity of subsurface formations by transmitting alternating electromagnetic fields into the rock and detecting the secondary currents those fields induce, enabling petrophysicists to distinguish hydrocarbon-saturated reservoirs from water-saturated rock without requiring physical contact between the logging tool and the borehole wall. Key Takeaways Induction logging operates on Faraday's law: an alternating current in a transmitter coil generates a time-varying magnetic field that induces eddy current loops in the formation; the magnitude of these currents is proportional to formation conductivity, which the receiver coil converts into a resistivity measurement in ohm-meters. The technique performs best in non-conductive borehole environments, including oil-base mud (OBM), synthetic-base mud (SBM), and air-drilled holes, where electrode-based tools such as laterologs cannot establish adequate electrical contact with the formation. H.G. Doll of Schlumberger introduced the first practical induction logging technique in 1949, solving a fundamental limitation of pre-existing electrical logging methods that required conductive mud to function. Modern array induction tools, including Schlumberger's Array Induction Tool (AIT) and Baker Hughes's High-Definition Induction Log (HDIL), deploy multiple transmitter-receiver coil arrays processed simultaneously in software to deliver invasion-corrected true formation resistivity (Rt) at multiple depths of investigation from 10 inches (25 cm) to 90 inches (229 cm). True resistivity (Rt) from the induction log is the primary input to Archie's equation for water saturation calculation, making it the single most consequential petrophysical measurement in conventional reservoir evaluation. How the Induction Log Works The fundamental operating principle derives directly from electromagnetic induction theory. A transmitter coil in the tool carries a sinusoidal alternating current at a frequency typically between 10 and 40 kilohertz. This current generates an oscillating primary magnetic field that propagates radially outward into the formation. According to Faraday's law of induction, the time-varying magnetic field induces small eddy current loops in the formation, oriented concentrically around the tool axis. The magnitude of these eddy currents depends on the electrical conductivity of the formation rock and its contained fluids. The eddy currents in turn generate their own secondary magnetic field, which the receiver coil detects. Because the secondary signal is proportional to formation conductivity, the tool electronics convert it to conductivity in millisiemens per meter (mS/m) and invert to resistivity in ohm-meters (ohm-m) for log presentation. The classic two-coil system, one transmitter and one receiver, has a fundamental limitation: it also detects signal from the conductive borehole fluid and from formation layers well above and below the depth of interest. To address this, Doll derived a theoretical geometric factor that describes the contribution of each volume element of formation to the total receiver signal. The practical solution was to add focusing coils: secondary transmitter and receiver coils wound in reverse polarity at specific spacing intervals to cancel the unwanted borehole and shoulder-bed contributions. The most widely used classic focused induction tool, the 6FF40, used a six-coil array with the primary transmitter-to-receiver spacing of 40 inches (102 cm). This tool dominated the logging industry for decades and became the de facto standard induction measurement referenced in countless petrophysical databases. A critical physical limitation of induction logging is the skin effect. At very low formation resistivity values, typically below 1 ohm-m, the induced eddy currents are so strong that they themselves attenuate the primary electromagnetic field before it penetrates the full intended depth of investigation. This skin-effect attenuation causes the raw induction reading to underestimate true conductivity and, therefore, overestimate resistivity in highly conductive formations. Virtually all modern induction tools apply a skin-effect correction in real time using the measured signal phase and amplitude relationships. The correction factor is small at resistivities above 5 ohm-m but can reach 20 to 30 percent in shales or brine-saturated sands with conductivities above 1,000 mS/m. Operating frequency choice involves a trade-off. Lower frequencies, around 10 kHz, reduce skin-effect attenuation and achieve deeper radial penetration but degrade vertical resolution because the wavelength is longer. Higher frequencies, approaching 200 kHz in some modern tools, improve vertical resolution and are less sensitive to borehole effects but are more susceptible to skin effect in conductive formations. Array induction tools resolve this by using multiple frequencies or multiple coil spacings processed jointly so that the interpreter receives a family of resistivity curves, each representing a different radial depth of investigation from the same logging run. Induction Log Across International Jurisdictions Resistivity logging via induction methods is mandatory or effectively required by regulatory or industry frameworks in every major hydrocarbon-producing region. The specific tool technology and data-submission requirements differ by jurisdiction. Canada (Alberta): The Alberta Energy Regulator (AER) requires that all wells drilled to a producing horizon include a resistivity log as part of the well-completion data submitted under Directive 062, Well Abandonment. AER Directive 020 (Well Abandonment) and Directive 065 (Resources Applications for Conventional Oil and Gas Reservoirs) reference resistivity logging as part of standard formation evaluation for pool delineation and volumetric reserve estimation. In the Duvernay, Cardium, Viking, and Montney plays, dual induction with a shallow laterolog (ILd, ILm, Rxo) or array induction logs are the standard petrophysical suite. Petrophysical interpretation results, including Sw from Archie's equation using ILd-derived Rt, are filed with AER as part of pool registration packages. The AER Integrated Decision Approach (IDA) system stores digital LAS-format wireline log data including induction curves. United States: The Bureau of Safety and Environmental Enforcement (BSEE) requires resistivity logging on all offshore wells on the US Outer Continental Shelf under 30 CFR Part 250, Subpart D. On federal onshore lands, the Bureau of Land Management (BLM) imposes equivalent logging requirements through approved drilling permits. In the major unconventional plays, Permian Basin operators run array induction tools in every horizontal lateral; the AIT or HDIL deep resistivity is the primary Rt input for log-based EUR calculations used in both reserve bookings and completions optimization for fracture stage placement. The Society of Petroleum Engineers (SPE) and Society of Petrophysicists and Well Log Analysts (SPWLA) publish industry standards for log quality control, and induction tool calibration checks at surface conditions are required before and after each logging run. Australia: The National Offshore Petroleum Safety and Environmental Management Authority (NOPSEMA) requires well completion reports that include formation evaluation data under the Offshore Petroleum and Greenhouse Gas Storage (Resource Management and Administration) Regulations 2011. NOPSEMA's well reporting guidelines specify that resistivity logs, including induction or laterolog data appropriate to borehole conditions, must be included in the well completion report submitted within 90 days of well completion. The Geoscience Australia National Offshore Petroleum Information Centre (NOPIMS) stores digital well log data from Commonwealth-jurisdiction offshore wells; induction log LAS files are part of mandatory data submissions. Onshore petroleum regulators in Western Australia (DEMIRS) and the Northern Territory (DPIR) follow equivalent disclosure requirements for unconventional and conventional exploration wells. Norway and the North Sea: Sodir (formerly the Norwegian Petroleum Directorate, NPD) maintains the Diskos National Data Repository, which collects digital well log data from all wells drilled on the Norwegian Continental Shelf under regulations implementing the Petroleum Act of 1996. Induction and laterolog resistivity data are among the mandatory log types submitted to Diskos at well completion. The NORSOK D-010 Well Integrity in Drilling and Well Operations standard and Equinor's internal petrophysical standards specify dual or array induction as the primary deep resistivity measurement in oil-base mud environments, which are common in high-pressure, high-temperature (HPHT) wells on the Norwegian shelf. Equinor's petrophysical data management group reviews induction log quality metrics, including horn effects at bed boundaries and borehole size corrections, before accepting logs into their reservoir databases. Saudi Arabia and the Middle East: Saudi Aramco's standard logging suite, documented in the company's proprietary Well Engineering Manual, includes dual induction as a mandatory log in all exploration and appraisal wells and in development wells where non-conductive mud is used. ADNOC (Abu Dhabi National Oil Company) and the Kuwait Oil Company (KOC) follow equivalent requirements through their own well engineering frameworks. In carbonate reservoirs of the Arabian Platform, induction logging interpretation is complicated by highly saline formation water (resistivity Rw as low as 0.01 ohm-m at reservoir temperature), which means that even hydrocarbon-bearing zones may show relatively low true resistivity. Petrophysicists at Saudi Aramco use multi-salinity Archie models and resistivity-porosity crossplots to identify productive zones that would appear water-saturated on a naive resistivity cutoff. Fast Facts Schlumberger's 6FF40 induction tool, introduced commercially in the early 1950s, was used on more wells worldwide than any other resistivity logging tool in history before being superseded by array induction technology in the late 1980s. Today, array induction tools generate 5 to 8 simultaneous resistivity curves per logging run, providing invasion profiles that were impossible to derive from a single-curve induction measurement. The global wireline logging market, of which resistivity logging is the largest segment, generates approximately USD $6 billion in annual revenue.

A combination of a 6FF40induction log with an electrode measurement such as a 16-in. [40-cm] short normal. In some cases other induction arrays, such as the 5FF27, may have been used. The use of both measurements in the induction electrical survey (IES) gave a qualitative indication of invasion.

A neutron interaction in which part of the kinetic energy lost by a neutron in a nuclear collision excites the nucleus. The excited nucleus will usually emit characteristic gamma rays upon de-excitation. Inelastic neutron scattering is possible only if the neutron energy exceeds a characteristic threshold for the element. Inelastic neutron scattering is the principle behind the carbon-oxygen log, which is used to determine water saturation behind casing.

The extra resistance of a porous medium to fluid flow, beyond that predicted by Darcy's law, caused by local accelerations within the tortuous pore volume. The inertial resistance is proportional to the fluid density times the flow rate. The inertial resistance is significant with gas because flow rates can be high. In laboratory measurements of permeability, inertial resistance can be important for high-permeability samples where high flow rates are needed to have significant pressure gradients. The effect is corrected by using the Forchheimer equation and making measurements at several flow rates (often through an unsteady state test).

The addition of wells in a field that decreases average well spacing. This practice both accelerates expected recovery and increases estimated ultimate recovery in heterogeneous reservoirs by improving the continuity between injectors and producers. As well spacing is decreased, the shifting well patterns alter the formation-fluid flow paths and increase sweep to areas where greater hydrocarbon saturations exist.

A reservoir with no apparent outer boundary limit affecting fluid flow during a test period. Unless an outer boundary is close to the wellbore, such as a nearby fault, it usually takes a day or more for outer boundaries to affect well-test results. Since most tests are of relatively short duration, outer boundaries usually do not affect test results.

A planar crack penetrated by a well or propagated from a well by hydraulic fracturing with zero pressure drop in the fracture during production.

A type of packer that uses an inflatable bladder to expand the packer element against the casing or wellbore. In preparation for setting the packer, a drop ball or series of tubing movements are generally required, with the hydraulic pressure required to inflate the packer provided by carefully applying surface pump pressure. Inflatable packers are capable of relatively large expansion ratios, an important factor in through-tubing work where the tubing size or completion components can impose a significant size restriction on devices designed to set in the casing or liner below the tubing.

A mathematical tool used in production engineering to assess well performance by plotting the well production rate against the flowing bottomhole pressure (BHP). The data required to create the IPR are obtained by measuring the production rates under various drawdown pressures. The reservoir fluid composition and behavior of the fluid phases under flowing conditions determine the shape of the curve.

The study, collection and management of information, especially with respect to computer technology. Information theory is an important component in the construction and efficient usage of databases.

To prevent, arrest or slow down any action. For example, one can inhibit a corrosion process, such as by coatingdrillpipe with amine films to arrest pipe corrosion in air. In drilling fluids, the terms inhibit, inhibition and inhibitive mud system refer to arresting or slowing the hydration, swelling and disintegration of clays and shales.

An acid treatment fluid that has been mixed with chemical additives to control the corrosive effect on the mixing and pumping equipment, as well as on any wellbore tubulars and completion equipment that the fluid may contact. Almost all acid treatments require the addition of an inhibitor to protect against undesirable reactions.

A mud that slows or stops hydration, swelling and disintegration of shales. A variety of mud types have been labeled as "inhibitive muds." The degree of inhibition is not quantitative, but qualitatively they range from highly inhibitive (balanced-activity oil muds), moderately inhibitive (potassium muds and silicate muds), fairly inhibitive (calcium-based fluids) to slightly inhibitive (lignosulfonate, lignite water muds) to non-inhibitive (freshwater, nontreated muds).

A chemical agent added to a fluid system to retard or prevent an undesirable reaction that occurs within the fluid or with the materials present in the surrounding environment. A range of inhibitors is commonly used in the production and servicing of oil and gas wells, such as corrosion inhibitors used in acidizing treatments to prevent damage to wellbore components and inhibitors used during production to control the effect of hydrogen sulfide [H2S].

A short flow period at the beginning of a drillstem test. This period is followed immediately by a longer shut-in period to allow the pressure to closely approach initial reservoir pressure. The initial flow period is commonly 5 to 10 minutes, and the initial shut-in period is commonly 30 minutes to one hour. When plotted on a pressure buildup plot, extrapolation of the best straight line gives what is usually accepted as the best obtainable value of initial formation pressure.

The reservoir pressure measured in a discovery well, usually referred to as pi. This value is necessary for many reservoir engineering calculations, such as reserve determination.

The comparatively short shut-in period following the initial flow period of a drillstem test. This period is followed immediately by much longer flow and shut-in periods to allow the pressure to closely approach initial reservoir pressure. The initial flow period is commonly 5 to 10 minutes, and the initial shut-in period is commonly 30 minutes to one hour. When plotted on a pressure buildup plot, extrapolation of the best straight line gives what is usually accepted as the best obtainable value of initial formation pressure.

Natural gas injected into a formation to maintain or restore reservoir pressure. Other reasons for gas injection are gas-lift operations, cycling in gas-condensate reservoirs or storing gas.

A small-diameter conduit that is run alongside productiontubulars to enable injection of inhibitors or similar treatments during production. Conditions such as high hydrogen sulfide [H2S] concentrations or severe scale deposition can be counteracted by injection of treatment chemicals and inhibitors during production.

A downhole completion component that enables injection of treatment chemicals or inhibitors into the production conduit. The injection mandrel is equipped with a port- and check-valve system to direct fluid pumped down the annulus or injection line into the production conduit.

The particular arrangement of production and injection wells. The injection pattern for an individual field or part of a field is based on the location of existing wells, reservoir size and shape, cost of new wells and the recovery increase associated with various injection patterns. The flood pattern can be altered during the life of a field to change the direction of flow in a reservoir with the intent of contacting unswept oil. It is common to reduce the pattern size by infill drilling, which improves oil recovery by increasing reservoir continuity between injectors and producers. Common injection patterns are direct line drive, staggered line drive, two-spot, three-spot, four-spot, five-spot, seven-spot and nine-spot. Normally, the two-spot and three-spot patterns are used for pilot testing purposes. The patterns are called normal or regular when they include only one production well per pattern. Patterns are described as inverted when they include only one injection well per pattern.

The pressure needed to inject fluid into the formation to pressurize or displace hydrocarbons.

Any pump used to inject fluid into the reservoir or production system. Injection pumps vary in volume and pressure capacity, from the large injection pumps used in water-injection wells, to much smaller low-volume injection pumps used in continuous scale-inhibitor treatments.

A procedure conducted to establish the rate and pressure at which fluids can be pumped into the treatment target without fracturing the formation. Most stimulation treatments and remedial repairs, such as squeezecementing, are performed following an injection test to help determine the key treatment parameters and operating limits.

Water injected into the reservoir to pressurize and displace hydrocarbons to producing wells. Injection water is also used in water-storage operations in offshore and remote locations with economic and environmental constraints.

A well in which fluids are injected rather than produced, the primary objective typically being to maintain reservoirpressure. Two main types of injection are common: gas and water. Separated gas from production wells or possibly imported gas may be reinjected into the upper gas section of the reservoir. Water-injection wells are common offshore, where filtered and treated seawater is injected into a lower water-bearing section of the reservoir.

The testing of wells in which fluid is being injected into the reservoir. The most common type of test is a falloff test, in which injection is halted and the pressure decline is measured as a function of time. The most common situation is a waterflood. In many reservoirs, the formation pressure is high enough to maintain a full column of fluid in the reservoir, and the pressure can be monitored at the surface. The bottomhole pressure is then calculated by adding the weight of the fluid column to the surface pressure. Gas-injection wells, although less common, lend themselves to similar testing. The rise in fluid pressure as a function of time while injection is taking place could theoretically be used also, but this type of approach is rarely used. The equations and theory for these tests are an exact mirror image of those for buildup and drawdown testing. Calculated results for these wells are usually good because the formations are commonly liquid-filled. Frequently water-injection wells are inadvertently fractured at some time in their life and consequently have a negative skin effect.

An in-situ recording in which a material with high neutron-capture cross section is injected into the flowstream of a production or injection well to determine fluid paths and velocities. The material used is normally borax or water with high salinity, both of which cause a significant increase in the capture cross section measured by a pulsed-neutron log. In the most common application, the material is injected across the producing intervals of a production well. By comparing pulsed neutron logs recorded before and after injection, the injectivity, and hence productivity, of each interval can be estimated. Any cement channels or leaks will also be observed.

A procedure conducted to establish the rate and pressure at which fluids can be pumped into the treatment target without fracturing the formation. Most stimulation treatments and remedial repairs, such as squeezecementing, are performed following an injection test to help determine the key treatment parameters and operating limits. Likewise, injection tests are also conducted when pumping secondary recovery fluids such as water, nitrogen [N2], carbon dioxide [CO2], natural gas and steam.

One of the principal equipment components of a coiled tubing unit. The injector head incorporates special profiled chain assemblies to grip the coiled tubing string and a hydraulic drive system that provides the tractive effort for running and retrieving the string from the wellbore. The base of the injector head is secured to the wellheadpressure-control equipment by the stripper assembly mounting system. The gooseneck mounted on top of the injector head feeds the tubing string from the reel around a controlled radius into the injector head.

A section on a log print that gives the scales of the curves displayed and the depth scale. There usually is an insert at the beginning and end of each interval surveyed.

A valve in the drillstring that may be used to prevent the well from flowing uncontrollably up the drillstring.

Inside or inner diameter. Casing, tubing and drillpipe are commonly described in terms of inside diameter and outside diameter (OD).

A downhole tool on which downhole gauges or instruments that are to be temporarily left in the wellbore are attached. The instrument hanger is run into the wellbore on slickline and set in a completion nipple at the required depth.

A device made of rubber or polyurethane that has electronic devices. An instrumented pig is run through a pipeline to record irregularities that could represent corrosion. An instrumented pig is also called a smart pig.

A well equipped with monitoring equipment and completion components that can be adjusted to optimize production, either automatically or with some operator intervention.

A downhole tool used with a jar to increase the impact force imparted as the jar is fired. Similar in function to an accelerator, intensifiers typically use compressed gas rather than a mechanical spring to store the energy released during operation.

A property of the interface between two immiscible phases. When the phases are both liquid, it is termed interfacial tension; when one of the phases is air, it is termed surface tension. Interfacial tension is the Gibbs free energy per unit area of interface at fixed temperature and pressure. Interfacial tension occurs because a molecule near an interface has different molecular interactions than an equivalent molecule within the bulk fluid. Surfactant molecules preferentially position themselves at the interface and thereby lower the interfacial tension.

The pressure variation with time recorded in observation wells resulting from changes in rates in production or injection wells. In commercially viable reservoirs, it usually takes considerable time for production at one well to measurably affect the pressure at an adjacent well. Consequently, interference testing has been uncommon because of the cost and the difficulty in maintaining fixed flow rates over an extended time period. With the increasing number of permanent gauge installations, interference testing may become more common than in the past.

Referring to any particle in the size range 250 to 2000 microns.

A casing string that is generally set in place after the surface casing and before the production casing. The intermediate casing string provides protection against caving of weak or abnormally pressured formations and enables the use of drilling fluids of different density necessary for the control of lower formations.

A length of pipe used below the surface casing string, but before the productioncasing is run, to isolate one or more zones of the openhole to enable deepening of the well. There may be several intermediate casing strings. Depending on well conditions, these strings may have higher pressure integrity than the prior casing strings, especially when abnormally pressured formations are expected during the drilling of the next openhole section.

An artificial-lift method, used in relatively low-productivity wells, in which the gas-lift system is operated on an intermittent basis to enable the buildup of liquids in the wellbore.

The excess material formed on the internal surface of a coiled tubing string by the longitudinal weld during manufacture. The internal flash can be removed in some larger sizes of string to make a fullbore string less susceptible to localized corrosion that can occur in the area of the longitudinal weld.

The discontinuous phase of an emulsion, the dispersed droplets of emulsified fluid.

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

The assignment of values to points intermediate to two data points. Linear interpolation plots values along a straight line between the values of the two nearest data points. Other interpolation techniques involve other functions. Interpolation is used to fill in missing data in well logs and other data sets. It may also be used to assign values to grid elements in maps.

In geophysics, analysis of data to generate reasonable models and predictions about the properties and structures of the subsurface. Interpretation of seismic data is the primary concern of geophysicists.

The gas stored in the pore space of a reservoirrock. Measurement of interstitial gas and adsorbed gas, which is the gas accumulated on the surface of another solid material, such as a grain of reservoir rock, allows calculation of gas in place in a reservoir.

Water that occurs naturally within the pores of rock. Water from fluids introduced to a formation through drilling or other interference, such as mud and seawater, does not constitute interstitial water. Interstitial water, or formation water, might not have been the water present when the rock originally formed. In contrast, connate water is the water trapped in the pores of a rock during its formation, also called fossil water.

A technique for improving the accuracy of injection profiles measured using the velocity-shot method. In the normal velocity-shot method, the flow velocity is determined by the time of flight of a radioactive slug between two detectors or between ejector and detector. Since the distance involved may be several feet, the vertical resolution is low. The interval method consists of making overlapping measurements from which a higher resolution injection profile may be determined.

The elapsed time between two seismic events.

The amount of time for a wave to travel a certain distance, proportional to the reciprocal of velocity, typically measured in microseconds per foot by an acoustic log and symbolized by t or DT. P-wave interval transit times for common sedimentaryrock types range from 43 (dolostone) to 160 (unconsolidated shales) microseconds per foot, and can be distinguished from measurements of steel casing, which has a consistent transit time of 57 microseconds per foot.

The velocity, typically P-wave velocity, of a specific layer or layers of rock, symbolized by vint and commonly calculated from acoustic logs or from the change in stacking velocity between seismic events on a common midpointgather.

The volume close to the borehole wall in which some or all of the moveable fluids have been displaced by mudfiltrate. It consists of the flushed zone and the transition zone or annulus. In simple models, the invaded zone and the flushed zone are considered synonymous.

Pertaining to the influence of invasion on the response of logging measurements. For example, an invasion correction is the correction made to a deep-reading measurement such as an induction log for the effect of the invaded zone. The correction is based on a suitable invasion model, such as a step profile or transition zone model.

(adjective) Describing a mathematical or analytical process in which observed data (such as seismic records, well test pressures, or production rates) are used to determine the underlying model parameters that produced those observations. Inverse methods are fundamental to seismic inversion, history matching, and reservoir characterisation.

Generally, a finite impulse response (FIR) filter that has been designed to transform the usually irregular vertical response functions of raw measurements into a smooth, well-behaved response function such as a Gaussian response or a Kaiser window function. The criteria for designing inversefilters can include vertical response, depth of investigation and near-field (cave effect) response. Inverse filters have been used for many years to improve the response of induction arrays.

The problem of determining the value or spatial variation of a physical property or feature by comparing measurements to the predictions of a model. For example, seismic traveltimes from a source to a receiver can be used to build a model of seismic velocity in the Earth, or earthquakearrival times can be used to determine the timing and focus (location) of an earthquake. A typical inverse problem in electromagnetics is to determine the variation of electrical conductivity in the Earth from measurements of induced electric and magnetic fields. A forward problem, in contrast, involves taking an assumed model and calculating what the observed values should be, such as the predicting seismic traveltimes between a source and a receiver given a velocity model.

A mathematical process by which data are used to generate a model that is consistent with the data, the process of solving the inverse problem. In seismology, surface seismic data, vertical seismic profiles and well log data can be used to perform inversion, the result of which is a model of Earth layers and their thickness, density and P- and S-wave velocities. Successful seismic inversion usually requires a high signal-to-noise ratio and a large bandwidth.

An emulsion in which oil is the continuous or external phase and water is the internal phase. Invert emulsion usually refers to an oil-base mud, and the terms are considered synonyms. Invert-emulsion muds can be run with 5 to 50% water in the liquid phase, although there are systems that are 100% oil.

An outdated distinction between two types of oil muds. In the past, invert-emulsion oil muds were those with more than 5 vol.% emulsified water, and oil-base muds were those with less than 5 vol.% water. Today, this distinction is not pertinent because the general term oil mud covers all water concentrations.

An outdated distinction between two types of oil muds. In the past, invert-emulsion oil muds were those with more than 5 vol.% emulsified water, and oil-base muds were those with less than 5 vol.% water. Today, this distinction is not pertinent because the general term oil mud covers all water concentrations.

An injection pattern in which four production wells are located at the corners of a square and the injector well sits in the center.

A process of exchanging one ion for another ion on a charged, solid substrate, such as a natural clay, zeolite or resin. Cation exchange on clay minerals occurs in muds and during mud testing. In muds, various positive ions (for example, Ca+2) on clay surfaces can be replaced by other positive ions (for example, Na+) that occur in higher concentration or have a higher affinity for the substrate. In water muds, cation exchange is an ongoing and dynamic process. In mud testing, the methylene blue dye is a cationic dye that goes onto clays in the mud sample being tested to show the extent of cation-exchange capacity. Ion exchange is the basis for clay peptization, which can improve a poorly performing clay.

A group of minerals and inorganic compounds made up of iron that is in +2 (ferrous) and +3 (ferric) valence states and oxygen in the -2 valence state, such as ferrous oxide, FeO, and ferric oxide, Fe2O3. Fe3O4 is a mixture of ferric oxide and ferrous oxide that commonly occurs in a fine-grained, magnetic crystalline form. Hematite, Fe2O3, the most common iron oxide, exists in several crystalline forms. Other forms of hematite are too abrasive to use as weighting material in drilling fluids.

A group of compounds containing iron in +2 (ferrous) and +3 (ferric) valence states and sulfur in -2 (sulfide) valence states. Examples are ferric sulfide [Fe2S3], ferrous sulfide [FeS] and iron disulfide (FeS2, which is found in the minerals marcasite and pyrite). Fe2S3 and FeS2 are insoluble, but FeS dissolves in alkaline muds to form a gelatinous solid, Fe(OH)2, and soluble S-2 ions. FeS is the primary component in the mineralpyrrhotite, one of many possible sources of sulfides in water mud. Pyrrhotite might also coexist with BaSO4 in some barite ores. Even in trace amounts, FeS in a barite supply could generate enough sulfides in a weighted mud system to cause stress-corrosion cracking of drillpipe.Reference:Binder GG, Carlton LA and Garrett RL: "Evaluating Barite as a Source of Soluble Carbonate and Sulfide Contamination in Drilling Fluids," Journal of Petroleum Technology 33, no. 12 (December 1981): 2371-2376.Garrett RL: "Quality Requirements for Industrial Minerals Used in Drilling Fluids," Mining Engineering 39, no. 11 (November 1987): 1011-1016.

Aerobic bacteria that convert iron from the ferrous [Fe+2] to the ferric [Fe+3] state and produce ferric hydroxide [Fe(OH)3], which is a highly insoluble by-product that will damage the formation. Iron-oxidizing bacteria also produce some corrosion, but they are considered harmful mainly because they cover sulfate-reducing bacteria colonies and protect them from attack with bactericides.

The lowest water saturation, Swi, that can be achieved in a core plug by displacing the water by oil or gas. The state is usually achieved by flowing oil or gas through a water-saturated sample, or spinning it in a centrifuge to displace the water with oil or gas. The term is somewhat imprecise because the irreducible water saturation is dependent on the final drive pressure (when flowing oil or gas) or the maximum speed of rotation (in a centrifuge). The related term connate water saturation is the lowest water saturation found in situ.

A contour connecting points of equal true vertical thickness of strata, formations, reservoirs or other rock units. A map that displays isochores is an isochore map. The terms isopach and isopach map are incorrectly used interchangeably to describe isochores and isochore maps. Isopachs and isochores are equivalent only if the rock layer is horizontal.

A line joining points of equal time or age, such as a reflection in a seismicprofile or contours in an isochron map.

A contourmap showing the traveltimes to one particular seismic event or reflection.

A type of deliverability test conducted in gas wells. This test is used to generate a stabilized gas deliverability curve (IPR) without actually flowing the well for the time required to achieve stabilized conditions (pseudosteady state). This type of test is especially useful for low-permeability reservoirs.In an isochronal test, the well flows at a constant rate and then is shut in, allowing the pressure to build up to the average reservoir pressure. The same procedure typically is repeated four times. It is called isochronal because the flow periods are of the same length. A stabilized point (pseudosteady state) is usually obtained at the end of the test.

The pore volume not connected to the pore network. Isolated porosity can be significant in volcanic rocks and some carbonates, for example as vugular, moldic and intraparticle porosity.

A synthetic hydrocarbon liquid made by the polymerization of ethylene, H2C=CH2. IOs are one of several synthetic fluids that have recently been used as base for synthetic-base muds and in other applications where refined oils might otherwise be used except for HSE concerns. IOs are linear structures that have their olefin double bond in the center of the chain length. They are made by isomerization of linear alphaolefins (LAO), which have their double bond at the end of the chain. Because the olefin bond is in the central area of the chain, the physical properties of IOs are different (for example, they are generally lower viscosity) compared with the LAOs from which they are made.

A contour that connects points of equal thickness. Commonly, the isopachs, or contours that make up an isopach map, display the stratigraphic thickness of a rock unit as opposed to the true vertical thickness. Isopachs are true stratigraphic thicknesses; i.e., perpendicular to bedding surfaces.

A simple alcohol (C3H7OH) used as a solvent in some mud analyses. A 50/50 xylene/IPA mixture was used in the past as an emulsionbreaker for oil mud but has been replaced with propylene glycol normal propyl ether (PNP).

The state of gravitational equilibrium between the lithosphere and the asthenosphere of the Earth such that lithospheric plates "float" at a given elevation depending on their thickness. The balance between the elevation of the lithospheric plates and the asthenosphere is achieved by the flowage of the denser asthenosphere. Various hypotheses about isostasy take into account density (Pratt hypothesis), thickness (Airy hypothesis), and pressure variations to explain topographic variations among lithospheric plates. The current model consists of several layers of different density.

A correction for variations in the density or thickness of the Earth's crust. Isostatic corrections are commonly applied to gravity data and are made according to a specific model for isostasy.

A type of formation whose rock properties are the same in all directions. Although this never actually occurs, fluid flow in rocks approximates this situation closely enough to consider certain formations isotropic.

Permeability that is the same in all directions. This never really occurs, but permeability along various directions of a formation is often close enough for calculation purposes.

A quality of directional uniformity in material such that physical properties do not vary in different directions. In rocks, changes in physical properties in different directions, such as the alignment of mineral grains or the seismicvelocity measured parallel or perpendicular to bedding surfaces, are forms of anisotropy. (Compare with homogeneity.)

The use of repeated forward modeling of a logging tool response to produce modeled logs that very closely match the measured logs. The final model is then the log analyst?s best estimate of the formation properties. Iterative forward modeling is a hand-operated inversion. The technique is used mainly for laterologs and induction logs when the formation or the environment are complex, so that the environmental effects cannot be separated and treated individually by automatic inversion. Iterative forward modeling allows the log analyst to use local knowledge and petrophysics to select between the many possible solutions that are mathematically correct. These cases occur most often in horizontal wells, or vertical wells with the combined effects of invasion and large resistivity contrast between beds.

Mathematical techniques that require an algorithm or equation to be repeated until a condition is met. These methods usually require the convergence of a result to a value. Computers are excellent tools for performing iterative techniques quickly and efficiently.