Reservoir Glossary

on 16 November 2024. Posted in Reservoir Engineering

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Acid Gas - A gas that can form acidic solutions when mixed with water. The most common acid gases are hydrogen sulfide ($H_2 S$) and carbon dioxide ($CO_2$) gases. Both gases cause corrosion, hydrogen sulfide is extremely poisonous. Hydrogen sulfide and carbon dioxide gases are obtained after a sweetening process applied to a sour gas.
Associated Gas - A natural gas found in contact with or dissolved in crude oil in the reservoir.
Average Temperature of a Gas Column - The mean kinetic energy of the gas molecules within a specific vertical column of gas, often measured in an atmosphere. It is determined by taking the temperature of the gas at different heights in the column and calculating a weighted or simple average, depending on the distribution of mass and temperature across the column.

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Behind-pipe Reserves - Expected to be recovered from zones in existing wells, which will require additional completion work or future recompletion prior to the start of production.
Bubble Point Pressure - A reservoir refers to the pressure at which the first bubble of gas comes out of solution in the reservoir fluid as the pressure decreases. The bubble point pressure is a necessary property in reservoir engineering that influences how oil and gas are produced and managed from a reservoir.

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Capillary Pressure - The pressure difference between two fluid phases in a porous medium that arises due to the interfacial tension between the fluid phases and the surface tension between fluids and the porous medium.
Chemical Flooding - A general term for injection processes that use special chemical solutions. Micellar, alkaline and soap-like substances are used to reduce surface tension between oil and water in the reservoir, whereas polymers such as polyacrylamide or polysaccharide are employed to improve sweep efficiency. The chemical solutions are pumped through specially distributed injection wells to mobilize oil left behind after primary or secondary recovery. Chemical flooding is a major component of enhanced oil recoveryprocesses and can be subdivided into micellar-polymer flooding and alkaline flooding.
Coalbed Methane - Natural gas contained in coal deposits, whether or not stored in gaseous phase. Coalbed methane, though usually mostly methane, may be produced with variable amounts of inert or even non-inert gases.
Compressibility Drive in Gas Reservoirs - In gas reservoirs, compressibility drive is the natural energy mechanism by which gas expands as the pressure within the reservoir declines, helping to push gas toward the production wells. This is primarily due to the high compressibilityof gases compared to liquids.
Condensing Drive - A gasflood process in which an injection gas enriched with components of intermediate molecular weight, for example butane, is injected into a reservoir to achieve multiple-contact miscibility. Upon contact with the oil, intermediate molecular-weight hydrocarbons transfer from the injected gas phase into the oil phase, a process in which those components are said to condense into the oil. Formation of miscibility may require several contacts between fresh enriched gas and the oil containing condensed components.
Contingent Resources - Those quantities of petroleum which are estimated, on a given date, to be potentially recoverable from known accumulations but which are not currently considered to be commercially recoverable.
Conventional Gas - A natural gas occurring in a normal porous and per-meable reservoir rock, either in the gaseous phase or dissolved in crude oil, and which technically can be produced by normal production practices.
Crude Oil - A naturally occurring, unrefined petroleum product found deep underground, formed from the remains of ancient marine organisms such as plants, algae, and bacteria.

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Deterministic Estimate - The method of estimation of reserves or resources is called deterministic if a single best estimate is made based on known geological, engineering, and economic data.
Developed Reserves - Expected to be recovered from existing wells including reserves behind pipe. Improved recovery reserves are considered developed only after the necessary equipment has been installed, or when the costs to do so are relatively minor. Developed reserves may be sub-categorized as producing or non-producing.
Dry Gas (Lean Gas) - Natural gas, mainly from gas fields rather than oil fields, which does not contain appreciable quantities of the heavier hydrocarbons, such as propane and butane, which condense easily. Methane and ethane are principal components of dry gases.
Dry Gas Reservoir - A type of natural gas reservoir where the predominant hydrocarbon present is methane (CH4) with very little or no condensate (liquid hydrocarbons) or water. This type of reservoir is distinct from a "wet gas" reservoir, which contains significant quantities of liquid hydrocarbons like condensates or natural gas liquids (NGLs) along with methane.

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Effective Pore Volume - The total volume of the interconnected pore spaces within a reservoir rock that can actually store and transmit fluids. It is used in reservoir engineering as it helps in estimating the amount of recoverable hydrocarbons or water present in the reservoir. The effective pore volume is essential for estimating the reserves and planning the production strategy of a reservoir. It provides insight into how much fluid (oil, gas, or water) the reservoir can hold and how much can be potentially extracted.

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Formation Temperature for a Given Gradient - The temperature of a subsurface geological formation at a specific depth. It is influenced by the geothermal gradient, which describes how temperature increases with depth beneath the Earth's surface. The formation temperature can be calculated if the geothermal gradient and the surface temperature are known.
Formation Volume Factor - Used in reservoir engineering and petroleum production. It is used to relate the volume of oil and gas in the reservoir to the volume at surface conditions. There are different formation volume factors for oil and gas, but the concept is similar for both.
Free Gas - The gaseous phase present in a reservoir or other contained area. Gas may be found either dissolved in reservoir fluids or as free gas that tends to form a gas cap beneath the top seal on the reservoir trap. Both free gas and dissolved gas play important roles in the reservoir-drive mechanism.

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Gas Adsorbed in Coal Bed Methane Reservoirs - In coal bed methane (CBM) reservoirs, the gas stored is primarily methane. Unlike conventional g as reservoirs where gas is stored in pore spaces, in CBM reservoirs, methane is primarily adsorbed onto the surface of the coal within micropores.
Gas Bubble Radius - Used in petroleum engineering and reservoir engineering, particularly in the context of gas reservoirs and gas condensate reservoirs. It is the radius of a bubble of gas that forms in a liquid phase within the reservoir when the pressure drops below the bubble point pressure.
Gas Cap - The gas that accumulates in the upper portions of a reservoir where the pressure, temperature and fluid characteristics are conducive to free gas. The energy provided by the expansion of the gas cap provides the primary drive mechanism for oil recovery in such circumstances.
Gas Cap Drive - A type of reservoir-drive mechanism in which the energy for the transport and production of reservoir fluids is provided by the expansion of gas either in the gas cap or inside the oil phase.
Gas Cap Gas - A free natural gas which overlies and is in contact with crude oil in the reservoir.
Gas Cap Ratio - A reservoir is the ratio of the volume of free gas in the gas cap to the volume of oil in the oil zone within a petroleum reservoir. It is a parameter in reservoir engineering as it helps to determine the overall recovery strategy and production plan. The gas cap acts as a natural pressure maintenance mechanism for the reservoir. As oil is produced and pressure drops, the gas in the gas cap expands and helps to maintain reservoir pressure, which can enhance oil recovery.
Gas Cap Shrinkage - The reduction in the volume of the gas cap in a reservoir due to various factors such as production activities and changes in reservoir pressure. The gas cap is a layer of natural gas that sits above the oil zone in a petroleum reservoir. It serves as a source of drive energy to help push oil towards the production wells.
Gas Condensate Reservoir - A type of hydrocarbon reservoir that primarily contains natural gas with a significant amount of condensable hydrocarbons. These hydrocarbons, known as condensates, exist in a gaseous state in the reservoir under high pressure and temperatureconditions.
Gasdrive - A primary recovery mechanism for oil wells containing dissolved and free gas, whereby the energy of the expanding gas is used to drive the oil from the reservoir formation into the wellbore.
Gas in Place - The total quantity of natural gas contained in a subsurface reservoir. It includes all the gas within the rock formations of the reservoir, regardless of whether it is technically or economically recoverable. Understanding and accurately estimating the reservoir gas in place is vital for efficient reservoir management and the planning of gas field development.
Gas in Solution - Gas that is dissolved in a liquid, such as water or oil.
Gas Produced by Gas Expansion - The process in which natural gas is extracted from a gas reservoir due to the expansion of the gas. This typically occurs when the pressure in the reservoir is decreased, either by natural depletion or through production activities. The efficiency and sustainability of this process depend on the reservoir characteristics and the management strategies employed to optimize gas recovery.
Geothermal Gradient - The rate at which the Earth's temperature increases with depth below its surface. On average, the temperature increases by about 25°C for every kilometer of depth.
Gibbs Phase Rule - A fundamental principle in thermodynamics that provides a relationship between the number of phases, components, and degrees of freedom in a system at equilibrium. The rule is crucial in understanding phase diagrams and predicting the behavior of systems in different conditions. It helps identify how many phases can coexist in equilibrium and what variables can be independently adjusted without disturbing that equilibrium.
Gravity Drainage - The least common primary recovery mechanism in which the force of gravity pushes hydrocarbons out of the reservoir, into the wellbore and up to surface. Gravity force is always present in the reservoir, but its effect is greater in thick gas-condensate reservoirs and in shallow, highly permeable, steeply dipping reservoirs.

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Hydrocarbons - Organic compounds composed exclusively of hydrogen and carbon atoms. They are the simplest type of organic molecules and serve as the primary building blocks for many other organic compounds. Hydrocarbons are typically classified into several categories based on their structure and bonding.
Hydrocarbon Pore Volume Occupied by the Gas Cap - The volume of the reservoir's pore space that is filled with gas in a gas cap, as opposed to oil or water. This measurement is important in reservoir engineering as it helps in understanding the distribution of fluids within the reservoir and in estimating recoverable hydrocarbons.
Hydrogen Sulfide - An objectionable impurity present in some natural gas and crude oils and formed during the refining of sulfur-containing oils. It is removed from products by various treating methods at the refinery.

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Improved Recovery (Enhanced Recovery) - The extraction of additional petroleum, beyond primary recovery, from naturally occurring reservoirs by supplementing the natural forces in the reservoir. It includes water-flooding, secondary processes, tertiary processes and any other means of supplementing natural reservoir recovery processes.
Ineffective Porosity - The portion of the porosity within a reservoir rock that does not contribute to the storage and flow of fluids, such as oil, gas, or water. The following types of porosity do not contribute to the effective porosity, which is the porosity that actually contributes to the reservoir's ability to store and transmit fluids. Understanding the distinction between effective and ineffective porosity is crucial for accurate reservoir characterization and for making informed decisions regarding reservoir management and production strategies.
Infill Recovery - 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 flowpaths and increase sweep to areas where greater hydrocarbon saturations exist.

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Lost Oil in Migration - The portion of petroleum that, after being generated in a source rock, migrates toward a reservoir but is lost along the way. This loss can occur due to various geological and physical processes. These losses affect the amount of oil that ultimately accumulates in the reservoir, impacting the potential yield of an oil field.

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Miscible Displacement - A general term for injection processes that introduce miscible gases into the reservoir. A miscible displacement process maintains reservoir pressure and improves oil displacement because the interfacial tension between oil and water is reduced. The effect of gas injection is similar to that of a solution gasdrive. Miscible displacement is a major branch of enhanced oil recovery processes. The fluid most commonly used for miscible displacement is carbon dioxide because it reduces the oil viscosity and is less expensive than liquefied petroleum gas. Miscible displacement is also called miscible gasdrive, miscible drive or miscible flood.
Mixed Deposits - A blend of organic and inorganic compounds such as scales, silts or clays. Migrating fines that become oil-wet often become targets for organic deposits, thereby creating a mixed deposit. Mixed deposits are considered a type of damage. Treating this type of deposit requires a dual-solvent system composed of an aromatic hydrocarbon and an acid.
Mixed-metal Hydroxide - A compound containing hydroxide anions in association with two or more metal cations. MMH particles are extremely small and carry multiple positive charges. They can associate with bentonite to form a strong complex that exhibits highly shear-thinning properties, with high and fragile gel strengths, high yield point (YP), and low plastic viscosity (PV).

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Natural Bitumen (Crude Bitumen) - The portion of petroleum that exists in the semi-solid or solid phase in natural deposits. In its natural state it usually contains sulphur, metals and other non-hydrocarbons. Natural bitumen has a viscosity greater than 10,000 centipoises measured at original temperature in the deposit and atmospheric pressure, on a gas free basis. In its natural viscous state, it is not normally recoverable at commercial rate through a well. Natural Bitumen generally requires upgrading prior to normal refining.
Natural Gas - The portion of petroleum that exists either in the gaseous phase or is in solution in crude oil in natural underground reservoirs, and which is gaseous at atmospheric conditions of pressure and temperature. Natural Gas may include amounts of non-hydrocarbons.
Natural Gas Liquids to Gas Ratio - Ratio of natural gas liquids to gas (in barrels/million cubic feet) in an oil field, calculated using measured natural gas liquids and gas volumes at stated conditions.
Nitrogen Injection - A process whereby nitrogen gas is injected into an oil reservoir to increase the oil recovery factor. Below the minimum miscibility pressure (MMP), this is an immiscible process in which recovery is increased by oil swelling, viscosity reduction and limited crude-oil vaporization. Above the MMP, nitrogen injection is a miscible vaporizing drive. Miscibility of nitrogen can be achieved only with light oils that are at high pressures; therefore, the miscible method is suitable only in deep reservoirs.
Non-Hydrocarbon Gas - In the event that non-hydrocarbon gases are present, the reported volumes should reflect the condition of the gas at the point of sale. Correspondingly, the accounts will reflect the value of the gas product at the point of sale. Hence, if gas sold as produced includes a proportion of carbon dioxide, for example, the reserves and production should also include that $CO_2$. In the case of the $CO_2$ being extracted before sale and the sales gas containing only hydrocarbon gases, the reserves and production should reflect only the hydrocarbon gases that will be sold.
Non-Associated Gas - A natural gas found in a natural reservoir that does not contain crude oil.
Non-Conventional Gas - A natural gas found in unusual underground situations such as very impermeable reservoirs, hydrates, and coal deposits.
Non-producing Reserves - Reserves subcategorized as non-producing include shut-in and behind-pipe reserves. Shut-in reserves are expected to be recovered from (1) completion intervals which are open at the time of the estimate, but which have not started producing, (2) wells which were shut-in for market conditions or pipeline connections, or (3) wells not capable of production for mechanical reasons. Behind-pipe reserves are expected to be recovered from zones in existing wells, which will require additional completion work or future recompletion prior to the start of production.

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Oil in Place - The total quantity of crude oil that is originally found in a subsurface reservoir. It encompasses all the oil contained in the rock formations within a specific reservoir, regardless of whether it can be technically or economically recovered. Determining the reservoir oil in place is crucial for understanding the potential productivity of an oil field and involves a combination of geological, petrophysical, and engineering data.
Osmosis - The movement of water from one aqueous system to another through a semipermeable membrane. Osmotic movement is driven by activity differences between the two systems and can be considered as a vapor-phase transfer. An oil mud acts as an osmotic system. Emulsion film surrounding each brine droplet in an oil mud acts as semipermeable osmotic membrane and allows water molecules to pass back and forth, but restricts ions and larger molecules. Clays in shale formations also are aqueous systems that interact by osmosis with oil-mud droplets.

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Primary Recovery - The first stage of hydrocarbon production, in which natural reservoir energy, such as gasdrive, waterdrive or gravity drainage, displaces hydrocarbons from the reservoir, into the wellbore and up to surface. Initially, the reservoir pressure is considerably higher than the bottomhole pressure inside the wellbore. This high natural differential pressure drives hydrocarbons toward the well and up to surface. However, as the reservoir pressure declines because of production, so does the differential pressure.
Probabilistic Estimate - The method of estimation is called probabilistic when the known geological, engineering, and economic data are used to generate a range of estimates and their associated probabilities.
Producing Reserves - Reserves subcategorized as producing are expected to be recovered from intervals which are open and producing at the time of the estimate. Improved recovery is considered producing only after the improved recovery project is in operation.
Petroleum - A semi-solid product, ranging from white to yellow in color, composed of heavy residual oils and paraffin wax produced by filtration of cylinder stocks. Has varied pharmaceutical and industrial uses.
Prospective Resources - Those quantities of petroleum which are estimated, on a given date, to be potentially recoverable from undiscovered accumulations.

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Reserves - Those quantities of hydrocarbons which are anticipated to be commercially recovered from known accumulations from a given date forward.
Reservoir - A subsurface rock formation containing one or more individual and separate natural accumulations of moveable petroleum that is confined by impermeable rock and is characterized by a single-pressure system.
Reservoir Drive Mechanisms - Natural forces in the reservoir that displace hydrocarbons out of the reservoir into the wellbore and up to surface. Reservoir-drive mechanisms include gasdrive (gas cap or solution gasdrive), waterdrive (bottomwater drive or edgewater drive), combination drive, and gravity drainage. Waterdrive is the most efficient drive mechanism, followed by gasdrive and gravity drainage. Reservoir-drive mechanisms are also called natural drives.
Reservoir Fluid Permeability - A measure of the ability of a porous material (such as rock in a reservoir) to allow fluids (such as oil, gas, or water) to pass through it. It is used in petroleum engineering and hydrogeology for understanding and predicting fluid flow in reservoirs. The higher the permeability, the more easily fluids can flow through the rock.

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Saturated Black Oil Reservoir - A type of hydrocarbon reservoir where the oil is in equilibrium with the gas phase at the reservoir conditions (temperature and pressure).
Secondary Recovery - The second stage of hydrocarbon production during which an external fluid such as water or gas is injected into the reservoir through injection wells located in rock that has fluid communication with production wells. The purpose of secondary recovery is to maintain reservoir pressure and to displace hydrocarbons toward the wellbore. The most common secondary recovery techniques are gas injection and waterflooding.
Shale - A fine-grained, fissile, detrital sedimentary rock formed by consolidation of clay- and silt-sized particles into thin, relatively impermeable layers. It is the most abundant sedimentary rock. Shale can include relatively large amounts of organic material compared with other rock types and thus has potential to become a rich hydrocarbon source rock, even though a typical shale contains just 1% organic matter. Its typical fine grain size and lack of permeability, a consequence of the alignment of its platy or flaky grains, allow shale to form a good cap rock for hydrocarbon traps.
Shale Oil - The liquid obtained from the destructive distillation of oil shale. Further processing is required to convert it into products similar to petroleum oils.
Sloughing Shale - A fine-grained, impermeable, sedimentary rock composed of clays and other minerals, usually with a high percentage of quartz. Shale is the most common, and certainly the most troublesome, rock type that must be drilled in order to reach oil and gas deposits. The characteristic that makes shales most troublesome to drillers is its water sensitivity, due in part to its clay content and the i onic composition of the clay. For this reason, oil-base drilling fluids are the mud of choice to drill the most water-sensitive shales.
Solution Gas - A natural gas which is dissolved in crude oil in the reservoir at the prevailing reservoir conditions of pressure and temperature.
Solution Gasdrive - A type of reservoir-drive mechanism in which the energy for the transport and production of reservoir fluids is provided by the gas dissolved in the liquid. As reservoir fluids enter the wellbore, changing pressure conditions cause the gas to break from solution to create a commingled flow of gas and liquid that aids production.
Sour Gas - A natural gas that contains sulphur, sulphur compounds and/or carbon dioxide in quantities that may require removal for effective use.
Surfactant-alternating Gas - An enhanced oil recovery process in which alternating slugs of a surfactant solution and gas are injected into a reservoir. The injected surfactant and gas mix and generate foam that reduces the gas mobility, especially in previously swept or high-permeability regions of the reservoir. This improves sweep efficiency by mitigating gravity override and viscous fingering during gas injection. The presence of the surfactant in the injectant can also improve recovery by reducing interfacial tension between reservoir oil and the injection phases.
Sweet Crude - Crude petroleum containing little sulfur, with no offensive odor.
Sweet Gas - A gas containing no corrosive components such as hydrogen sulfide and mercaptans, or in such small quantities that no processing is necessary for their removal in order that the gas may be used directly as a non-corrosive domestic heating fuel.

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Tertiary Recovery - The third stage of hydrocarbon production, comprising recovery methods that follow waterflooding or pressure maintenance. The principal tertiary recovery techniques used are thermal methods, gas injection and chemical flooding. The term is sometimes used as a synonym for enhanced oil recovery (EOR), but because EOR methods today may be applied at any stage of reservoir development, the term tertiary recovery is less commonly used than in the past.
Two-phase Formation Volume Factor - The relationship between the volume of a hydrocarbon fluid at reservoir conditions and the volume it occupies at surface conditions. It helps in understanding how much of the reservoir's hydrocarbon can be produced. It is used for calculating the reserves of gas and oil in the reservoir and for designing production facilities.

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Unproved Reserves - It is based on geologic and/or engineering data similar to that used in estimates of proved reserves; but technical, contractual, economic, or regulatory uncertainties preclude such reserves being classified as proved. Unproved reserves may be further classified as probable reserves and possible reserves. Unproved reserves may be estimated assuming future economic conditions different from those prevailing at the time of the estimate. The effect of possible future improvements in economic conditions and technological developments can be expressed by allocating appropriate quantities of reserves to the probable and possible classifications.
Unrecoverable Resources - The portion of discovered or undiscovered petroleum-initially-in-place quantities not currently considered to be recoverable. A portion of these quantities may become recoverable in the future as commercial circumstances change, technological developments occur, or addition data is acquired.
Underground Fluid Withdrawal - The process of extracting fluids like oil, gas, or water from underground reservoirs, such as oil fields, gas reservoirs, or aquifers. This withdrawal can lead to changes in reservoir pressure, fluid distribution, and potentially cause subsidence or other geological impacts.
Undersaturated Black Oil Reservoir - A type of oil reservoir where the pressure within the reservoir is higher than the bubble point pressure of the oil. The bubble point is the pressure at which gas begins to come out of solution and form bubbles within the oil. This type of reservoir is contrasted with a saturated reservoir, where the pressure is at or below the bubble point, and gas is already present as a separate phase within the reservoir.
Undersaturated Volatile Oil Reservoir - A type of oil reservoir that contains oil with high gas content but has not yet reached its bubble point pressure.

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Vaporizing Drive - A gasflood process in which a lean gas, for example methane, nitrogen or carbon dioxide, is injected into a reservoir to achieve multiple-contact miscibility. Upon contact with the oil, light and intermediate molecular-weight hydrocarbons transfer from the oil into the gas phase, thus vaporizing into the gas. Formation of miscibility may require several contacts between gas containing vaporized components and fresh reservoir oil. If the injected gas becomes sufficiently enriched with these components that miscibility results with the oil, then the lean gas and oil have multiple-contact miscibility.

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Water Content of Sour Gas - This is natural gas containing significant amounts of hydrogen sulfide $H_2 S$, can be influenced by various factors such as temperature, pressure, and the composition of the gas. The water content in sour gas is typically higher than in sweet gas due to the presence of $H_2 S$, which can dissolve more water.
Waterdrive - A reservoir-drive mechanism whereby the oil is driven through the reservoir by an active aquifer. As the reservoir depletes, the water moving in from the aquifer below displaces the oil until the aquifer energy is expended or the well eventually produces too much water to be viable.
Water Expansion Term in Gas Reservoir - The effect of water influx on the overall material balance of a gas reservoir. As the pressure in a reservoir drops (due to production of hydrocarbons like gas or oil), the water in the formation expands, and if there's an aquifer connected to the reservoir, water may flow into the reservoir to balance the pressure drop.
Wet Gas - Mainly natural gas produced along with crude petroleum in oil fields. In addition to methane, ethane, propane and butane, wet gas contains some higher hydrocarbons such as pentane and hexane which, with propane and butane, are easily recoverable as liquids.
Wet Gas Reservoir - A type of natural gas reservoir where the produced gas contains significant amounts of hydrocarbonsin liquid form at surface conditions, such as natural gas liquids (NGLs) like ethane ($C_2H_6$), propane ($C_3H_8$), butane ($C_4H_{10}$), and pentane ($C_5H_{12}$). These liquids can be separated from the gas once it is brought to the surface.

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