Oil and Gas Terms Beginning with “O”

A type of circular seal commonly found in downhole tools and a wide range of surface equipment applications. The specification of O-ring material depends on the conditions for which the seal is intended, such as system operating temperature and pressure. Various backup systems are used to support the O-ring seal in either dynamic or static sealing applications.

Ratio of the volume percent oil to the volume percent brine in an oil mud, in which each is expressed as a percent of the total liquid in the mud. OBR is determined by retort analyses of oil and water content, but the brine content is calculated from the water content by using the chloride and calcium titration data. For example, if a mud contains 60 vol.% oil and 20 vol.% brine, the oil percentage is [60/(60 + 20)] 100 = 75 %, and the brine percentage is [20/(60 +20)] = 25%. That OBR is written as 75/25.

Pertaining to a drilling-grade bentoniteclay with API/ISO specifications. API specifications for this clay are similar to those of OCMA.

Outside or outer diameter. Casing and tubing are commonly described in terms of inside diameter (ID) and outside diameter.

Abbreviation for Offshore Operators Committee, a group of more than 85 oil and gas producers who operate in the Gulf or Mexico. OOC was formed in the 1950s. It interacts with various governmental organizations, such as the Environmental Protection Agency (EPA) and Minerals Management Service (MMS), to keep regulations up to date and economically sound. Many rules about discharge of mud and cuttings into Gulf waters have been developed jointly by OOC and the EPA.

Abbreviation for Occupational Safety and Health Act of the US. OSHA laws protect safety and health of workers and give workers the right to know what materials they are handling. Safe handling of mud chemicals and the general safety of all drilling operations are of prime concern to OSHA, the operator and the mud engineer. Monitoring for toxic hydrogen sulfide [H2S] in air around the rig and testing mud for sulfides are an essential part of rig safety.

The Oslo and Paris Commission, formerly known as PARCOM. The commission is a group of experts who advise North Sea countries on environmental policy and legislation. OSPAR has been influential in establishing North Sea legislation on drilling fluids that has served as the model for other operating areas. OSPAR has published lists of environmentally acceptable and unacceptable products, referred to as the "green," "grey" and "black" lists. The Green or A list consists of products posing relatively little harm to the environment (specifically the marine environment). Examples include inert minerals such as bentonite, inorganic salts that are common constituents of seawater such as sodium and potassium chloride, and simple organic products such as starch and carboxymethylcellulose (CMC). The Grey List consists of products 'requiring strong regulatory control' and includes heavy metals such as zinc, lead and chromium. The Black list covers products considered unsuitable for discharge and includes mercury, cadmium and 'persistent oils and hydrocarbons of a petroleum origin.' The inclusion of hydrocarbons in the black list has been the driving force behind the reduction of oil discharges in the North Sea and elsewhere and has serious implications for the use of oil and synthetic fluids.

Ratio of the volume percent oil to the volume percent water in an oil mud, where each is a percent of the total liquid in the mud. OWR is calculated directly from the retort analysis of an oil mud. For example, if a mud contains 60 vol.% oil and 18 vol.% water, the oil percentage is [60/(60 + 18)]100 = 77% and the water percent is [18/(60 +18)] = 23%. That OWR is written as 77/23.

A technique for inversion, or generating a model that is consistent with the data, of electromagnetic data, including resistivity and magnetotelluric data. The algorithm is named for William of Occam (1300 to 1349), who asserted that scientific hypotheses and reasoning should be as simple as possible. The use of Occam's inversion produces a smooth model that fits a data set within certain tolerances, although a smooth model might not be the best fit to the data.

The relationship between voltage (V), electric current (I) and resistance (R), named for German physicist Georg Simon Ohm (1789 to 1854), commonly expressed as the formula below:V/I = R.

The director of a seismic acquisition field crew who operates the recording equipment.

Typically an assembly of vertically oriented geophones and hydrophones connected by electrical wires and deployed on the seafloor to record and relay data to a seismic recording vessel. Such systems were originally introduced to enable surveying in areas of obstructions (such as production platforms) or shallow water inaccessible to ships towing seismic streamers (floating cables). Recent developments provide four component (4C) seabed systems to record shear wave (S-wave) as well as P-wave energy.

An eight-carbon alcohol. Iso-octanol is used as a defoamer for water muds.

A production or injection well that has a lateral or diagonal displacement with respect to the other wells in an injection pattern. The existence of an off-pattern well affects oil recovery and water/oil ratio.

A production or injection well that has a lateral or diagonal displacement with respect to the other wells in an injection pattern. The existence of an off-pattern well affects oil recovery and water/oil ratio.

In surface seismic acquisition, the horizontal distance from source to receiver. In a vertical seismic profile, offset is the horizontal distance between the source and the wellhead or the surface projection of the receiver in the case of a deviated well. Offset between seismic source and receiver creates a delay, or moveout, in the arrival time of a reflection that can be corrected before stacking and can be used to determine velocity.

A type of vertical seismic profile in which the source is located at an offset from the drilling rig during acquisition. This allows imaging to some distance away from the wellbore.

An existing wellbore close to a proposed well that provides information for planning the proposed well. In planning development wells, there are usually numerous offsets, so a great deal is known about the subsurface geology and pressure regimes. In contrast, rank wildcats have no close offsets, and planning is based on interpretations of seismic data, distant offsets and prior experience. High-quality offset data are coveted by competent well planners to optimize well designs. When lacking offset data, the well planner must be more conservative in designing wells and include more contingencies.

A contract between mineral owner, otherwise known as the lessor and a company or working interest owner, otherwise known as the lessee in which the lessor grants the lessee the right to explore, drill and produce oil, gas and other minerals for a specified primary term and as long thereafter as oil, gas or other minerals are being produced in paying quantities. This lease gives the lessee a working interest. The oil and gas lease is granted in exchange for royalty payments to the lessor.

(noun) A pressure vessel used at the surface to separate a multiphase production stream into its individual components of oil, gas, and water. Separators operate using the principles of gravity settling, centrifugal force, and coalescence, and may be configured as two-phase (gas-liquid) or three-phase (gas-oil-water) units.

The portion of a reservoir where the oil saturation is increased because of the application of an improved oil recovery method.

An invert-emulsionmud, or an emulsion whose continuous phase is oil. In the past, the term referred to an oil mud containing less than about 5 vol.% water. This definition, at the time, distinguished mud with less than 5 vol.% water from invert-emulsion oil muds, which had more than 5 vol.% water. Today, this distinction is not practical because most commercial oil muds can be formulated with more or less than 5 vol.% water using essentially the same types of products.

The volume percent of oil in a mud. The term should not be used to refer to the amount of synthetic fluid.

The surface area above a subsurface oil accumulation is called an oil field.

Oil and dissolved gas volume at reservoir conditions divided by oil volume at standard conditions. Since most measurements of oil and gas production are made at the surface, and since the fluid flow takes place in the formation, volume factors are needed to convert measured surface volumes to reservoir conditions. Oil formation volume factors are almost always greater than 1.0 because the oil in the formation usually contains dissolved gas that comes out of solution in the wellbore with dropping pressure.

A dispersion of oil droplets into an aqueous medium. This describes an emulsionmud, as contrasted with an invert-emulsion mud. The term should not be used to refer to synthetic-in-water emulsion because a synthetic fluid is not a true oil.

A strip-mining process involving the removal and subsequent processing of tarsand from shallow reservoirs containing heavy, viscous oil. The oil is mostly bitumen and does not flow at reservoir conditions. For strip mining to be economic, the reservoir must be shallow and have high oil saturation. Strip mining is energy intensive, both in mining the tar sand and in its subsequent processing. Large amounts of waste material, called gangue, are produced. Companies are usually required to landscape the area when mining is completed.

More generally, a mud system that has any type of nonaqueous fluid as the external phase. This definition would include the newer variety of oil muds that are more exactly defined as synthetic-base muds. Synthetic mud is analogous to oil mud.

The oil content of cuttings, normally expressed as grams of oil adsorbed per kilogram of dry cuttings. The value has been used as the basis for discharge regulations for oil-base muds in several operating areas.

A subsurface oil accumulation. An oil field can consist of one or more oil pools or distinct reservoirs within a single large trap. The term "pool" can create the erroneous impression that oil fields are immense caverns filled with oil, instead of rock filled with small oil-filled pores.

In the context of heavy oil, an oil sand is a porousrocklayer, often considered to be a mixture of sand, clay, water, and bitumen. The term is predominantly used in Canada, where over 170 billion barrels of bitumen are estimated to be held by large oil sand deposits in the Athabasca, Cold Lake and Peace River regions of Alberta. Oils from such sands may have a gravity of less than 10° API. Both in-situ recovery and mining methods are used to extract the resource, with surface mining used for extracting deposits of extra heavy oil at shallow depths of less than 100 m [328 ft].

An expansion in oil volume that can occur when a solvent contacts a reservoir fluid. The swelling is due to the complete or partial dissolution of the solvent molecules into the reservoir fluid. The amount of swelling is dependent on the pressure, temperature, composition and physical properties of the solvent and the reservoir fluid. Reservoir oil swelling can result in improved oil recovery by mobilizing residual oil trapped in inaccessible pore spaces.

A producing well with oil as its primary commercial product. Oil wells almost always produce some gas and frequently produce water. Most oil wells eventually produce mostly gas or water.

An invert-emulsionmud, or an emulsion whose continuous phase is oil. In the past, the term referred to an oil mud containing less than about 5 vol.% water. This definition, at the time, distinguished mud with less than 5 vol.% water from invert-emulsion oil muds, which had more than 5 vol.% water. Today, this distinction is not practical because most commercial oil muds can be formulated with more or less than 5 vol.% water using essentially the same types of products.

A dispersion of oil droplets into an aqueous medium. This describes an emulsion mud, as contrasted with an invert-emulsion mud. The term should not be used to refer to synthetic-in-water emulsion because a synthetic fluid is not a true oil.

A chemical used in preparation and maintenance of an oil- or synthetic-base drilling fluid that forms a water-in-oil emulsion (invert emulsion). An oil-mud emulsifier lowers the interfacial tension between oil and water, which allows stable emulsions with small drops to be formed. Historically, oil-mud emulsifiers have been classified as primary and secondary. Secondary emulsifiers are generally not used alone to make a stable oil mud. Emulsifiers can be calcium fatty-acid soaps made from various fatty acids and lime, or derivatives such as amides, amines, amidoamines and imidazolines made by reactions of fatty acids and various ethanolamine compounds. These emulsifiers surround water droplets, like an encapsulating film, with the fatty acid component extending into the oil phase. Emulsifier molecules that cannot fit around drops form clusters (micelles) in the oil phase or adsorb onto solids. Oil-mud emulsion drops each behave like a small osmotic cell. The emulsifier around the drops acts like a semipermeable membrane through which water can move but ions cannot pass. Thus, oil muds have the special capability (which water muds do not have) to control water transfer to and from the drops simply by adjusting salinity within the water phase of the oil mud.

The quality of a source rock that makes it more likely to generate oil than gas. The nature of the organic matter (kerogen) in source rocks varies from coaly, plant-like material commonly found in terrestrial source rocks to algal or other marine material that makes up marine source rocks. Marine source rocks are commonly oil-prone.

A bounding surface in a reservoir above which predominantly oil occurs and below which predominantly water occurs. Although oil and water are immiscible, the contact between oil and water is commonly a transition zone and there is usually irreducible water adsorbed by the grains in the rock and immovable oil that cannot be produced. The oil-water contact is not always a flat horizontal surface, but instead might be tilted or irregular.

The surface of contact between a water layer and an oil layer.

Pertaining to the preference of a solid to be in contact with an oil phase rather than a water or gas phase. Oil-wet rocks preferentially imbibe oil. Generally, polar compounds or asphaltenes deposited from the crude oil onto mineral surfaces cause the oil-wet condition. Similar compounds in oil-base mud also can cause a previously water-wet rock to become partially or totally oil-wet.

Ratio of the volume percent oil to the volume percent brine in an oil mud, in which each is expressed as a percent of the total liquid in the mud. OBR is determined by retort analyses of oil and water content, but the brine content is calculated from the water content by using the chloride and calcium titration data. For example, if a mud contains 60 vol.% oil and 20 vol.% brine, the oil percentage is [60/(60 + 20)] 100 = 75 %, and the brine percentage is [20/(60 +20)] = 25%. That OBR is written as 75/25.

Ratio of the volume percent oil to the volume percent water in an oil mud, where each is a percent of the total liquid in the mud. OWR is calculated directly from the retort analysis of an oil mud. For example, if a mud contains 60 vol.% oil and 18 vol.% water, the oil percentage is [60/(60 + 18)]100 = 77% and the water percent is [18/(60 +18)] = 23%. That OWR is written as 77/23.

An electrochemical source that provides electrical power to a downhole or surface tool used for determining the location or assisting in the extraction of hydrocarbons. Unlike a conventional battery, an oilfield battery must safely provide power sufficient to enable a tool to perform as specified, even on extended jobs. Designed with rugged external packaging to fit within a confined tool space, it must tolerate extreme temperatures while withstanding high shock and vibration. Its internal chemistry must also accommodate a long shelf life.

The group of hydrocarbon compounds that has one or more double or triple bonds between carbon atoms in the linear chain. Ethylene, C2H2, is the smallest olefin. Synthetic olefinic hydrocarbons are made by polymerization of ethylene under catalytic conditions. They are used in synthetic-base mud and as lubricants for water muds.

A low-molecular-weight polymer typically with two to five monomer units.

(noun) A single seismic trace recording the amplitude of reflected seismic energy as a function of two-way travel time at one surface location, providing a vertical profile of acoustic impedance contrasts in the subsurface. One-dimensional seismic data is the fundamental element from which multi-dimensional seismic datasets are constructed.

The time measured from a check-shot survey or vertical seismic profile (VSP), which is the time energy takes to travel from an energy source at the surface of the Earth to a receiver at a depth of interest.

The termination of shallowly dipping, younger strata against more steeply dipping, older strata, or the termination of low-angle reflections in seismic data against steeper reflections. Onlap is a particular pattern of reflections in seismic data that, according to principles of sequence stratigraphy, occurs during periods of transgression.

The uncased portion of a well. All wells, at least when first drilled, have openhole sections that the well planner must contend with. Prior to running casing, the well planner must consider how the drilled rock will react to drilling fluids, pressures and mechanical actions over time. The strength of the formation must also be considered. A weak formation is likely to fracture, causing a loss of drilling mud to the formation and, in extreme cases, a loss of hydrostatic head and potential well control problems. An extremely high-pressure formation, even if not flowing, may have wellbore stability problems. Once problems become difficult to manage, casing must be set and cemented in place to isolate the formation from the rest of the wellbore. While most completions are cased, some are open, especially in horizontal or extended-reach wells where it may not be possible to cement casing efficiently.

The theoretical flow capacity of gas wells if the bottomhole pressure could be reduced to atmospheric pressure. Test procedures to determine the AOF (Atmospheric Open-Flow) potential were often prescribed by law and enforced by state agencies. The open-flow potential capacity of the well was then used to determine the maximum rate that the gas well would be allowed to produce into a pipeline. During that period when there was an excessive amount of gas available for sale, this approach allowed an orderly method of allocating production rights to operators. The methodology is still used in some areas.The test required use of multiple rates, usually three or four, with measurement or calculation of the bottomhole pressure at the end of each flow period. The values of the difference between the square of the average reservoir pressure and the square of the bottomhole pressure were computed for each flow rate. These values were then plotted on the y-axis of a log-log plot versus the rate on the x-axis. The value of the open-flow potential is obtained by extrapolating the best straight line to the value of the average reservoir pressure squared minus atmospheric pressure squared, and then reading the corresponding rate off the x-axis.

A well completion that has no casing or liner set across the reservoirformation, allowing the produced fluids to flow directly into the wellbore. This type of completion suffers the major disadvantage that the sandface is unsupported and may collapse. Also, without any casing or liner installed, selective treatments or remedial work within the reservoir section are more difficult.

A type of sand-control completion in which the gravel pack screen is packed off in an openhole section with no casing or liner to support the producing formation. The openhole interval is often prepared by underreaming a section of reservoir below the last casing shoe. When the treatment is to be applied on an existing well, a section of casing may be milled out.

A type of packer designed for use in openhole applications such as drillstem testing. Openhole packers are typically configured with one large element that can be deformed easily to contact the uneven formation surface, yet retain strength and sufficient integrity to withstand the anticipated differential pressures.

A drillstem test performed in an openhole section of the wellbore. The test once was a popular method of assessing the productivity of exploration wells without the need to runcasing or liner across the reservoir interval. Openhole testing now is less common because of the risks and limits associated with fluid flow from an unsupported formation.

A device used in stage cementing to open the stage collar or diverting valve through which the second or subsequent cement stage is placed. The opening bomb is dropped down the casing string to land in a seat within the stage collar. Applying pump pressure then activates a sliding collar that opens access ports, enabling circulation across the upper casing interval.

An agreement between parties who own a working interest in a well that sets out responsibilities and duties of the operator and nonoperators, including drilling the test well and subsequent wells, and sharing of expenses and accounting methods.

The lowermost gas-lift valve in a gas-lift completion through which the lift gas is injected during normal production. During startup, the upper gas-lift valves open in sequence, from the top down, to enable the tubing fluids to be displaced. At predetermined pressures, each of the upper valves closes to eventually route all lift gas through the operating valve, which is placed at the optimal depth for the reservoir and completion conditions.

Ownership by a lessee, company or working interest owner, which is burdened with the costs of leasing the acreage and drilling and operating a well.

The owner of the right to drill or produce a well, or the entity contractually charged with drilling of a test well and production of subsequent wells.

A measure of the amount of light reflected by a fluid from an optical probe. It is the same as the relative refractive index of light between the probe and the fluid, being close to 1 in gas, 1.35 in water and 1.5 in oil.

A small sensor in a production logging tool that distinguishes the type of fluid in its vicinity as it is moved up and down a production well. The optical probe is sensitive to the optical index of the fluid in front of the probe. It can distinguish easily between gas and liquid, and less easily between oil and water. Light is sent from a source down a fiber-optic cable to the tip of the probe, and reflected back to a sensor, which converts it into an electrical signal. Gas, with a low optical index, reflects a significant amount of light and gives a large signal, while liquids give low signals.As with other local probes, the output is set to be nearly digital, so that the percentage of the time that the probe sees gas is a direct measure of gas holdup in front of the probe. By using several probes, it is possible to obtain the average holdup and an image of the flow structure across the well.

A type of organic fluid, such as acetic or formic acid, used in oil and gas well-stimulation treatments. The use of inhibited HCl at elevated temperatures is limited to relatively short exposure times because of the risk of increased corrosion. Inhibited organic acids are much less reactive with metals than are HCl or mixtures of HCl and HF. For this reason, organic acids are commonly used successfully at high bottomhole temperatures or when long contact times between acid and pipe are needed, such as during perforating. Organic acids also are used to protect exotic alloys, such as aluminum or chrome-plated parts.

A type of damage in which heavy hydrocarbons precipitate when temperature or pressure is reduced. These deposits are commonly located in the tubing, gravel pack and perforations, or inside the formation. The injection of cold treating fluids promotes the formation of organic deposits.Organic deposits such as paraffins or asphaltenes are resolubilized using aromatic organic solvents such as toluene or xylene. Small amounts of alcohol help to further dissolve asphaltenes.

Pertaining to a material that associates with organic and oily surfaces and liquids and rejects aqueous systems. An example of organophilic behavior in drilling fluids is the organophilic coating applied to clays and lignites to make them dispersible in oil-base muds.

Clay minerals whose surfaces have been coated with a chemical to make them oil-dispersible. Bentonite and hectorite (plate-like clays) and attapulgite and sepiolite (rod-shaped clays) are treated with oil-wetting agents during manufacturing and are used as oil-mud additives. Quaternary fatty-acid amine is applied to the clay. Amine may be applied to dry clay during grinding or it can be applied to clay dispersed in water. The latter process is much more expensive, requiring filtering, drying and other manufacturing steps. Organophilic bentonite and hectorite, "bentones," are used in oil muds to build rheology for cuttings lifting and solids suspension. They also contribute to low-permeabilityfilter cake. Organophilic attapulgite and sepiolite are used in oil muds strictly to build gel structure, which may not be long lasting due to shear degradation as the mud is pumped through the bit.

A lignite that has been coated with a chemical that renders it dispersible in oil. The treatment is usually accomplished with a type of quaternary amine compound.

An instrument that records the flow rate of gas through a pipeline. The flow rate is calculated from the pressure differential created by the fluid passing through an orifice of a particular size and other parameters such as static pressure, temperature, density of the fluid and size of the pipe.

A type of valve, typically found on small-diameter pipework, that incorporates an orifice or flow-restriction device to control fluid flow.

A major episode of plate tectonic activity in which lithospheric plates collide and produce mountain belts, in some cases including the formation of subduction zones and igneous activity. Thrust faults and folds are typical geological structures seen in areas of orogeny.

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.

The pressure that must be applied to the low-salinity side of an osmotic system to prevent water movement into the high-salinity side by osmosis. Conversely, it is the suction pressure that a high-salinity system exerts on the low-salinity system across a semipermeable membrane in an osmotic system.

A body of rock exposed at the surface of the Earth. Construction of highways and other man-made facilities and resultant removal of soil and rock has created spectacular outcrops in some regions.

Outside or outer diameter. Casing and tubing are commonly described in terms of inside diameter (ID) and outside diameter.

The maximum distortion permitted on the cross-sectional profile of a coiled tubing string. The mechanical performance of oval tubing deteriorates as the degree of ovality increases. The most critical effect is the ability of the tube to resist collapse under differential pressure. String ovality limits are generally determined by the maximum diameter that can pass through the primary pressure-control equipment. In high-pressure operations, the ovality limits will generally be reduced to maintain an adequate safety margin against string collapse.

The amount of pressure (or force per unit area) in the wellbore that exceeds the pressure of fluids in the formation. This excess pressure is needed to prevent reservoir fluids (oil, gas, water) from entering the wellbore. However, excessive overbalance can dramatically slow the drilling process by effectively strengthening the near-wellbore rock and limiting removal of drilled cuttings under the bit. In addition, high overbalance pressures coupled with poor mud properties can cause differential sticking problems. Because reservoir pressures vary from one formation to another, while the mud is relatively constant density, overbalance varies from one zone to another.

(noun) The total thickness of rock and sediment overlying a formation of interest, from the surface (or mudline in offshore environments) to the top of that formation. Overburden stress, generated by the cumulative weight of these overlying materials, is a primary component of the in-situ stress state and affects formation compaction, fracture gradients, and wellbore stability.

A specially prepared fluid used to displace matrix acid treatments away from the wellbore at the conclusion of a stimulation treatment. The overflush is typically formulated from a weak acid solution or brine to maintain a low pH environment in the near-wellbore formation that prevents the precipitation of reaction products as the treatment fluids are flowed back.The overflush is normally a weak acid or brine pumped behind the main treating fluid (mixture of hydrofluoric [HF] and hydrochloric [HCl] or organic acids).The overflush has several purposes:· displacement of the nonreacted mud acid into the formation.· displacement of mud-acid reaction by-products such as amorphous silica. The minimum overflush volume should have at least 3 ft [1 m] of radial penetration inside the formation to displace potential problems away from the critical matrix.· removal of potential oil-wet relative-permeability problems caused by some corrosion inhibitors.

Pertaining to a hydrocarbon source rock that has generated as much hydrocarbon as possible and is becoming thermally altered.

Subsurface pressure that is abnormally high, exceeding hydrostatic pressure at a given depth. The term geopressure is commonly, and incorrectly, used synonymously. Abnormally high pore pressure can occur in areas where burial of fluid-filled sediments is so rapid that pore fluids cannot escape, so the pressure of the pore fluids increases as overburden increases. Drilling into overpressured strata can be hazardous because overpressured fluids escape rapidly, so careful preparation is made in areas of known overpressure.

A percentage share of production, or the value derived from production, which is free of all costs of drilling and producing, and is created by the lessee or working interest owner and paid by the lessee or working interest owner.

Ownership in a percentage of production or production revenues, free of the cost of production, created by the lessee, company and/or working interest owner and paid by the lessee, company and/or working interest owner out of revenue from the well.

A downhole tool used in fishing operations to engage on the outside surface of a tube or tool. A grapple, or similar slip mechanism, on the overshot grips the fish, allowing application of tensile force and jarring action. If the fish cannot be removed, a release system within the overshot allows the overshot to be disengaged and retrieved.

A thrust fault having a relatively large lateral displacement.

A condition in downhole pumping operations that occurs when the stroke length at the subsurface sucker-rod pump is longer than the surface stroke length (polished rod). This phenomenon is caused by the elongation of the rod string because of dynamic loads imposed by the pumping cycle.In this situation, when the upstroke begins at the surface, the downhole pump maybe still be moving downward; when the downstroke begins, the downhole pump is still moving upward.

A model for converting relative elemental yields from a pulsed neutron spectroscopy log to absolute weight concentrations using the assumption that the sum of all oxides in the rockmatrix is 1. The model is based on the observation that, with few exceptions, sedimentary minerals are oxides, so that the sum of the dry weight percent of all oxides must be 100%. The weight percent of an oxide can be calculated from the dry weight percent of the cation by knowing the chemical formula (for example, SiO2 from Si).The absolute dry weight percent, W, of element i is given by Wi = F * Yi / Si where F is the unknown normalization factor, Yi is the measured spectral gamma ray yield and Si is the tool sensitivity to that element, measured in the laboratory. The dry weight percent of the oxide is then Oi = F* Xi * Yi / Si where Xi is the oxide association factor, given by the chemical formula. Since the sum of all Oi equals 1, it is possible to calculate F and determine each Wi .

A phenomenon exploited for the purpose of detecting and quantifying the flow of water in or around a borehole based on oxygen activation. Oxygen (16O) can be activated by high-energy neutrons to produce an isotope of nitrogen (16N), which decays back to oxygen with a half-life of 7.1 seconds and emits an easily detected gamma ray of 6.13 MeV. The neutrons are supplied by the generator of a pulsed-neutron spectroscopy tool. The gamma rays are counted in detectors placed above the tool, for upward flow, or below it for downward flow.Various techniques have been developed to analyze the count rates in terms of water velocity, flow rate and distance from tool. The earliest methods were based of the ratio of the counts from two detectors. Stationary oxygen and other background signals are eliminated by calibration in a known zone of zero flow, by counting only in a window near 6.13 MeV, or by optimizing the detector spacings. More recently, impulse-activation techniques have been introduced. While they require the tool to be stationary, they are generally more accurate.

A chemical agent used in some brines and completion fluids to reduce corrosion resulting from, or exacerbated by, dissolved oxygen. Oxygen scavengers capture the dissolved oxygen in a harmless chemical reaction that renders the oxygen unavailable for corrosive reactions. The use of oxygen scavengers is more critical in applications in which fluids are to be circulated in the wellbore. Additional oxygen may be dissolved during agitation at surface.