Glossary of Borehole, Boring, and Well Terms

Abandoned A well is abandoned if it is found to be a dry hole, noncommercial, or once it ceases to produce oil and/or natural gas in commercial quantities. The costs associated with abandoning a well or production facility are referred to as the Abandoning Costs, which costs typically cover the plugging of wells; removal of well equipment, production tanks and associated installations; and surface remediation.

Abnormal Pressure The term is usually associated with higher than normal pressure, increased complexity for the well designer and an increased risk of well control problems. Pressure gradients more than around 10 pounds per gallon equivalent fluid density are considered abnormal.

Acidic Water Acidic water has a pH of less than 7, (which is neutral) and alkaline water has a pH of more than 7. Acid water has more free hydrogen ions (H+) than hydroxyl ions (OH-). Most wetlands have acidic water because of the decaying organic material of wetland vegetation.

Acidizing The pumping of acid into the wellbore to remove near-well formation damage and other damaging substances. This procedure commonly enhances production by increasing the effective well radius.

Acre-foot A measure of water volume, principally used in the western states of the US. An acre-foot (acre-ft) is the volume of water required to cover 1 acre of land (43,560 square feet) to a depth of 1 foot. Equal to 325,851 gallons or 1,233 cubic meters. As a very rough estimate, an acre-foot is often used as a measure of how much water a (western US) family of four might use in one year.

Activated Carbon Material used in water conditioning. It is very porous and acts as an absorbent for organic matter and some dissolved gases. Homeowners with carbon filters should pay attention to service and maintenance instructions.

Aeration Process of bringing air into contact with water to remove or reduce unwanted dissolved gases and/or to oxidize dissolved compounds. For example, aeration devices can be effective for removing radon from water.

Air Stripping The process of removing contaminants from solution in water to solution in air. Air stripping towers are vertical cylindrical air stripping devices are often used in ground water remediation at sites where gasoline has contaminated ground water.

Alignment A measure of the vertical straightness of a well. It is the horizontal distance between the well's actual centerline and the true vertical centerline from the top of the hole. Well alignment is particularly important for line-shaft turbine pumps that have the pump motor at the surface.

Alkaline Water Water with a pH greater than 7. In typical water analysis alkalinity is represented by carbonates and bicarbonates. See also pH, acid water.

Alkalinity The capacity of water for neutralizing an acid solution.

Allowable The amount of oil and/or natural gas a well or a leasehold is permitted to produce under pro ration orders of a state regulatory body.

Alluvium Sedimentary deposits of silt, sand, gravel, that have been transported and then deposited by running water, usually a stream or river. Modern alluvial deposits are found in streambeds, river valleys, flood plains, deltas and estuaries Many ancient geological formations are made up of alluvial sediments. Alluvial aquifers are important water sources. See also sand & gravel aquifers, stratified drift.

Amplitude Anomaly An abrupt increase in seismic amplitude that can indicate the presence of hydrocarbons. Amplitude anomalies that indicate the presence of hydrocarbons can result from sudden changes in acoustic impedance, such as when a gas sand underlies a shale, and in that case, the term is used synonymously with hydrocarbon indicator.

Anaerobic A condition of oxygen deficiency found in some saturated soils. Changes of oxygen levels in soils and rock sediments can have important effects on ground water chemistry.

Annulus The space between a drilled hole and the well casing. Sealing the annulus can reduce the chances of surface contaminants reaching groundwater.

Anticlinal Trap A type of structural hydrocarbon trap whose closure is controlled by the presence of an anticline.

Anticline An arch-shaped fold in rock in which rock layers are upwardly convex. Anticlines form many excellent hydrocarbon traps, particularly in folds with reservoir-quality rocks in their core and impermeable seals in the outer layers of the fold. A syncline is the opposite type of fold.

API Gravity A specific gravity scale developed by the American Petroleum Institute (API) for measuring the relative density of various petroleum liquids, expressed in degrees. API gravity is gradated in degrees on a hydrometer instrument and was designed so that most values would fall between 10 and 70 API gravity.

Aquiclude A saturated rock formation or layer of geologic sediments with low permeability. Aquicludes do not yield significant amounts of water to wells but may be important as water storage zones that release water to more permeable formations.

Aquifer Recharge The process/processes by which water from precipitation (or some other part of the hydrologic system) reaches and hence increments stores of ground water.

Aquifer A geological formation or structure that has the capability to store and/or transmit water to wells and springs. Use of the term aquifer is usually restricted to those water-bearing formations capable of yielding water in sufficient quantity to constitute a usable supply source.

Aquifer Test Hydraulic test of an aquifer based on calculations using data from measurements of ground water level response (drawdown and recovery) to controlled pumping. (Occasionally tests may add water to a well). Aquifer tests typically allow hydrologists to predict the amount of water in an aquifer and the rates at which it may be safely withdrawn.

Aquitard A geologic formation having very low permeability through which water cannot move.

Area of Influence The land area overlying the extent of a pumping well's cone of depression.

Artesian Water Ground water that is under pressure when tapped by a well and is able to rise above the level at which it is first encountered. It may or may not flow out at ground level. The pressure in such an aquifer commonly is called artesian pressure, and the formation containing artesian water is an artesian aquifer or confined aquifer.

Artesian Aquifer Artesian aquifers (confined aquifers) occur where overlying impermeable rock layers "trap" ground water under pressure. Depending on geology and topography, a single aquifer may be artesian (confined) in one place and unconfined in another.

Artesian Well Wells (bore holes) that penetrate artesian aquifers. Water will rise up the well casing to the pressure level of the aquifer. Artesian flow describes the natural flow to the surface of water from confined aquifers. In some parts of the US any well drilled into bedrock is (incorrectly) called an artesian well.

Artificial Recharge A process where water is put back into ground water storage by use of engineering devices such as spreading basins or recharge wells.

Associated Gas Natural gas that is produced from the same reservoir along with crude oil, either as free gas or in solution.

Auger A machine having a rotating helical shaft for boring into the earth.

Back Off To unscrew drillstring components downhole. The drillstring, including drillpipe and the bottomhole assembly, are coupled by various thread forms known as connections or tool joints. Often when a drillstring becomes stuck it is necessary to back off the string as deep as possible to recover as much of the drillstring as possible. To facilitate the fishing or recovery operation, the back off is usually accomplished by applying reverse torque and detonating an explosive charge inside a selected threaded connection.

Bailer A cylindrical, bucket-like piece of equipment used to evacuate its liquid content in, or remove mud and rock cuttings from, the hole or wellbore.

Bedrock The solid, but often fractured and fissured, rock formations that occur beneath soils, unconsolidated sediment deposits or weathered materials. Exposed bare rock is bedrock at the surface. Sediments or weathered material overlying bedrock is sometimes called regolith or overburden.

Bentonite A colloidal clay of volcanic origin used as the main ingredient in drilling fluid (drilling mud) used in rotary well drilling processes and also used as a grouting medium to seal well casing in the drilled hole.

Bicarbonate Alkilinity in water is usually composed of bicarbonate and is reported as mg/L CaCO3.

Bit The cutting tool used in well drilling. Drill bits vary in complexity from the simple chisel used in cable tool drilling to tri-cone bits used in mud rotary operations.

Blind Ram A thick, heavy steel component of a conventional ram blowout preventer. The blind ram has no space for pipe and is instead blanked off to be able to close over a well that does not contain a drillstring. It may be loosely thought of as the sliding gate on a gate valve.

Blowout An uncontrolled flow of reservoir fluids into the borehole, and sometimes catastrophically to the surface. A blowout may consist of saltwater, oil, natural gas or a mixture of these. Blowouts occur in all types of exploration and production operations, not just during drilling operations.

Blowout Preventor A large valve at the top of a well that may be closed if the drilling crew loses control of formation fluids. By closing this valve (usually operated remotely by hydraulic actuators), the drilling crew usually regains control of the reservoir, and procedures can then be initiated to increase the mud weight until it is possible to open the BOP and retain pressure control of the formation. A BOP comes in a variety of styles, sizes and pressure ratings. Since a BOP is critically important to the safety of the crew, the rig and the hole or wellbore itself, the BOP is inspected, tested and refurbished at regular intervals.

Borehole The hole itself, including the openhole or uncased portion of the well. Borehole may refer to the inside diameter of the hole wall, the rock face that bounds the drilled hole.

Bottomhole Assembly The lower portion of the drillstring, consisting of (from the bottom up) the bit, bit sub, a mud motor (in certain cases), stabilizers, drill collars, heavyweight drillpipe, jarring devices (jar) and crossovers for various thread forms. The bottomhole assembly can also include directional drilling and measuring equipment, measurements-while drilling tools, logging-while-drilling tools and other specialized devices.

Bottomhole Location The actual location of a hole or wellbore at its deepest point.

Bottomhole Pressure The pressure at or near the depth of the producing formation.

Break Circulation To establish circulation of drilling fluids after a period of static conditions. Circulation may resume after a short break, such as taking a survey or making a mousehole connection, or after a prolonged interruption, such as after a round trip.

Break Out To unscrew drillstring components, which are coupled by various thread forms known as connections, including tool joints and other threaded connections.

Bridge A wellbore obstruction caused by a buildup of material such as scale, wellbore fill or cuttings that can restrict borehole access or, in severe cases, eventually close the borehole.

Bridge-off The accumulation or buildup of material, such as sand or scale, within a wellbore, insofar as the flow of fluids or passage of tools or downhole equipment is severely obstructed. In extreme cases, the wellbore can become completely plugged or bridged-off, requiring some remedial action before normal production can be resumed.

Bridge Plug A downhole tool that is located and set to isolate the lower part of the wellbore. Bridge plugs may be permanent or retrievable, enabling the lower wellbore to be permanently sealed from production or temporarily isolated from a treatment conducted on an upper zone.

Bright Spot A seismic amplitude anomaly or high amplitude that can indicate the presence of hydrocarbons. Bright spots result from large changes in acoustic impedance and tuning effects, such as when a gas sand underlies a shale, but can also be caused by phenomena other than the presence of hydrocarbons, such as a change in lithology. The term is often used synonymously with hydrocarbon indicator.

Bullhead To forcibly pump fluids into a formation, usually formation fluids that have entered the wellbore during a well control event.

Bump the Plug To observe the increase in pump pressure indicating that the top cement plug has been placed on the bottom plug or landing collar. Bumping the plug concludes the cementing operation.

Cable Tool Drilling Drilling by cable tool (percussion drilling) is achieved by the breaking and crushing action of heavy drilling tools suspended on a cable which are repeatedly lifed and dropped.

Cap the Well To regain control of a blowout well by installing and closing a valve on the wellhead.

Cased Hole A wellbore lined with a string of casing or liner. Although the term can apply to any hole section, it is often used to describe techniques and practices applied after a casing or liner has been set across the reservoir zone, such as cased-hole logging or cased-hole testing.

Casing A cylindrical device (steel or plastic) that is installed in a well to maintain the well opening and to provide a seal. In most states casing is required for at least the first 20 or 40 feet of water wells. Well drillers typically install well casing in 20 foot lengths.

Casing Joint A length of steel pipe, generally around 40 feet long with a threaded connection at each end. Casing joints are assembled to form a casing string of the correct length and specification for the hole in which it is installed.

Casing Patch A downhole assembly or tool system used in the remedial repair of casing damage, corrosion or leaks. In some cases, such as in depleted wells, a casing patch may be an economic means of safely deepening a well.

Casing Point The location (or depth) at which intermediate casing is run to protect the openhole and control unstable formations or over pressure zones deeper in the hole. In the case of completion, the point at which a decision must be made to either complete the well, as a well capable of producing oil and/or natural gas in commercial quantities, or to plug and abandon the well as a dry hole or noncommercial.

Casing Shoe The bottom of the casing string, including the cement around it, or the equipment run at the bottom of the casing string.

A representation of the integrity of the cement job, especially whether the cement is adhering solidly to the outside of the casing.

Cement Plug A balanced plug of cement slurry placed in the hole or wellbore. Cement plugs are used for a variety of applications including hydraulic isolation, provision of a secure platform, and in window-milling operations for sidetracking a new hole or wellbore.

Channeling The condition in which cement flows in a channel only on some sides of the casing or borehole annulus, and thus does not provide adequate hydraulic isolation in all radial azimuths.

Christmas Tree An assembly of valves, spools, pressure gauges and chokes fitted to the wellhead of a completed well to control production. Christmas trees are available in a wide range of sizes and configurations, such as low or high-pressure capacity and single or multiplecompletion capacity.

Circulate To pump drilling fluid through the whole active fluid system. The round trip made by drilling fluid; down through the drillstring, up on the outside of the drillstring (between the drillpipe and the walls of the hole), through the mud system and then back down the drillstring again.

Completion A generic term used to describe the assembly of downhole tubulars and equipment required to enable safe and efficient production from an oil and/or natural gas well.

Condensate A hydrocarbon that is in the gaseous phase at reservoir conditions but condense into liquid as it travels up the wellbore and reaches separator conditions.

Cone of Depression A shape in the form of an inverted cone that develops in the water table (or potentiometric surface) as a result of pumping from a well. In practice the shape of the 'cone' resulting from pumping from a well is often not symmetrical.

Confined Aquifer An aquifer, overlain by an impermeable layer, in which the water is under pressure greater than that of the atmosphere.

Core A continuous cylinder of rock, usually from 5 to 10 centimeters in diameter, cut from the bottom of a borehole as a sample of an underground formation.

Cuttings Small pieces of rock that break away due to the action of the bit. Cuttings are screened out of the drilling mud system at the 'shale shakers' and are monitored for composition, size, shape, color, texture, hydrocarbon content and other properties.

Darcy's Law A equation that states that flow through porous media is directly proportional to hydraulic head and inversely proportional to the length of flow. Henri Darcy was a French engineer who worked at the Dijon water works in the mid 19th century. His 'law' is the basis for much of the science of ground water hydrology and one of the most important basic equations used in hydrogeologic calculations.

Deviation The angle at which a hole or wellbore diverges from vertical. Wells can deviate from vertical because of the dips in the beds being drilled through. Wells can also be deliberately deviated with the use of a whipstock or other steering mechanism. Wells are often deviated or turned to a horizontal direction to increase exposure to producing zones, intersect a larger number of fractures, or to follow a complex structure.

Directional Drilling The intentional 'deviation' of a hole from the path it would naturally take. This is accomplished with the use of whipstocks, bottomhole assembly configurations, instruments to measure the path of the hole in three-dimensional space, data links to communicate measurements taken downhole to the surface, mud motors and special bottomhole assembly components and drill bits. In some cases, such as drilling steeply dipping formations or unpredictable deviation in conventional drilling operations, directional drilling techniques may be employed to ensure that the hole is drilled vertically.

Downhole A term to describe tools, equipment, and instruments used in the hole or wellbore, also, conditions or techniques applying to the hole or wellbore.

Drawdown The change of ground water level caused by pumping measured as the difference between the static water level and the water level at a particular well location after a specific period of pumping.

Drill Collar A component of a drillstring that provides weight on the bit for drilling.

Drilling Mud A special mixture of clay, water, and chemical additives pumped downhole through the drillpipe and drill bit. The mud cools the rapidly rotating bit; lubricates the drillpipe as it turns in the hole; carries rock cuttings to the surface; and serves as a plaster to prevent the wall of the hole from crumbling or collapsing. Drilling mud also provides the weight or hydrostatic head to prevent extraneous fluids from entering the hole and to control downhole pressures that may be met with.

Drillpipe Steel pipe screwed together and used to carry and rotate the drilling tools in a well, and to permit the circulation of drilling fluid. Drillpipe comes in lengths of approximately 30 feet. As the well is drilled deeper, they must constantly disconnect the drillstring, thread on another 30 foot section and then drill deeper.

Dry and Abandoned A well which is drilled and did not encounter oil and/or natural gas in commercial quantities and is subsequently abandoned.

Dry Gas Natural gas from a well that is free of liquid hydrocarbons, or gas that has been treated to remove all liquids; pipeline gas.

Dry Hole A well that has not encountered hydrocarbons in economically producible quantities.

Dual Completion A single wellbore having tubulars and equipment that enable production from two segregated zones. In most cases, two tubing strings will be used to provide the necessary level of control and safety for the fluids from both zones. However, in some simple dual completions, the second or upper zone is produced up the tubing-casing annulus.

Dual Purpose Wells Wells designed with the capability of pumping water underground during artificial recharge and to the surface from the aquifer during recovery.

Dual Rotary Drilling The distinguishing feature of the dual rotary rig is a lower rotary drive which is used to independently advance the casing. The rotary top drive handles the inner drill string, which can be tooled with a down-the-hole (DTH) hammer, drag bit, or rolling cone bit. Cuttings are typically removed using air from either the on-board compressor or an auxiliary compressor. Rotary drilling method, employed in conjunction with a cyclone collection system, ensures an accurate sample of the formation being drilled. A key benefit of casing rotation is a straight hole, even when drilling in cobbles and boulders. A straight hole helps to minimize sidewall friction, reduce stress on casing joints and welds and enable greater casing depths.

Electrical Log An electrical survey of an uncased hole which reflects the degree of resistance of the rock strata to electric current. From the results of the survey, geologist are able to determine the nature of the rock penetrated in the hole and some indications of its permeability.

Field The area encompassing a group of producing oil and/or gas wells, generally from the same pool.

Fish Anything left in a hole or wellbore. It does not matter whether 'the fish' consists of junk metal, a hand tool, a length of drillpipe or drill collars, or an expensive mud motor and directional drilling package. Once the component is lost, it is properly referred to as simply 'the fish.' Typically, anything put into the hole is accurately measured and sketched, so that appropriate fishing tools can be selected if the item must be fished out of the hole.

Fishing Tool A general term for special mechanical devices used to aid the recovery of equipment lost downhole. These devices generally fall into four classes: diagnostic, inside grappling, outside grappling, and force intensifiers or jars. Diagnostic devices may range from a simple impression block made in a soft metal, usually lead, that is dropped rapidly onto the top of the fish so that upon inspection at the surface, the crew may be able to custom design a tool to facilitate attachment to and removal of the fish. Other diagnostic tools may include electronic instruments and even downhole sonic or visual-bandwidth cameras. Inside grappling devices, usually called spears, generally have a tapered and threaded profile, enabling the crew to first guide the tool into the top of the fish, and then thread the fishing tool into the top of the fish so that recovery may be attempted. Outside grappling devices, usually called 'overshots' (see below), are fitted with threads or another shape that 'swallows' the fish and does not release it as it is pulled out of the hole. Overshots are also fitted with a crude drilling surface at the bottom, so that the overshot may be lightly drilled over the fish, sometimes to remove rock or metallic junk that may be part of the sticking mechanism. Jars are mechanical downhole hammers, which enable the crew to deliver high-impact loads to the fish, far more than what could be applied in a quasi-static pull from the surface.

Flowing Well A well capable of producing oil by its own energy, without the aid of a pump or other means.

Fluid Level The depth, or distance from surface, that the fluid in a well incapable of natural flow will reach under static conditions.

Formation A general term for the rock around the borehole. In the context of formation evaluation, the term refers to the volume of rock seen by a measurement made in the borehole, as in a log or a well test.

Formation Damage A general term to describe the reduction in permeability to the nearwellbore area of a reservoir formation. There are several recognized damage mechanisms, such as the invasion of incompatible fluids swelling the formation clays, or fine solids from dirty fluids plugging the formation matrix.

Gas Cap The natural gas that accumulates in the upper portions of a reservoir where the pressure, temperature and fluid characteristics are conducive to free natural gas.

Gas Lift An artificial-lift method in which gas is injected into the production tubing to reduce the hydrostatic pressure of the fluid column. The resulting reduction in bottomhole pressure allows the reservoir liquids to enter the wellbore at a higher flow rate. The injection gas is typically conveyed down the tubing-casing annulus and enters the production train through a series of 'gas-lift valves.' The gas-lift valve position, operating pressures and gas injection rate are determined by specific well conditions.

Gravel Pack A sand-control method used to prevent production of formation sand. In gravel pack operations, a steel screen is placed in the wellbore and the surrounding annulus packed with prepared gravel of a specific size designed to prevent the passage of formation sand. The primary objective is to stabilize the formation while causing minimal impairment to well productivity.

Gross Pay The total thickness of a reservoir rock, including the impervious layers which do not contain hydrocarbons.

Grout A fluid sealing mixture usually comprising bentonite and or cement that is used to seal well casing. Once emplaced, grout forms an impermeable seal.

Hydraulic Conductivity A property of an aquifer (or part of an aquifer) that is measure of the ability of the rocks/ sediments to allow water to flow under specific hydraulic gradients.

Hydraulic Gradient A measurement used in ground water science to calculate directions and rates of ground water flow. The hydraulic gradient is the slope of the water table in unconfined aquifers or the pressure surface in confined aquifers. It may be measured from the point of recharge to the point of discharge or between any two places within a ground water system. The hydraulic gradient is a ratio of the vertical difference between two places on the water table and their horizontal distance apart.

Hydrobabble Description of mistaken concepts in hydrology usually expressed as 'facts' by people who do not have the scientific background to understand cause-and effect in the hydrologic system.

Hydrofracture A technique of increasing flow to a wells in bedrock by using high-pressure forces down a well to open fractures. The technique is typically used to increase flow in very low yielding wells.

Hydrogeology The study of geology from the perspective of its role and influence in hydrology. On the other hand, geohydrology is a term used to describe the study of hydrology from the perspective of the influences of geology. In practice both terms are used interchangeably. Ground water science is the common field of study.

Impermeable Layer A layer of a geologic formation (consolidated or non-consolidated) which does not allow water to pass through. Most clays are considered impermeable even although flow may in fact occur at extremely slow rates.

Induced Infiltration Pumping from wells adjacent to rivers of lakes that results in river/lake bed flow to the adjacent aquifer.

Injection Well A well constructed for the purpose of injecting directly into the ground. Usually used to describe wells used for injecting treated (or untreated) wastewater. Wastewater is generally forced (pumped) into the well for dispersal or storage into a designated aquifer. Injection wells are generally drilled into rock formations that don't deliver drinking water, unused aquifers, or below freshwater levels.

Jar A mechanical device used downhole to deliver an impact load to another downhole component, especially when that component is stuck.

Joint A length of pipe, usually referring to drillpipe, casing or tubing.

Junk Anything in the hole or wellbore that is not supposed to be there. The term is usually reserved for small pieces of steel such as hand tools, small parts, bit nozzles, pieces of bits or other downhole tools, and remnants of milling operations.

Kelly A long square or hexagonal steel bar with a hole drilled through the middle for a fluid path. The kelly is used to transmit rotary motion from the rotary table or kelly bushing to the drillstring, while allowing the drillstring to be lowered or raised during rotation. The kelly goes through the kelly bushing, which is driven by the rotary table. The kelly bushing has an inside profile matching the kelly's outside profile (either square or hexagonal), but with slightly larger dimensions so that the kelly can freely move up and down inside.

Kelly Bushing An adapter that serves to connect the rotary table to the kelly. The kelly bushing has an inside diameter profile that matches that of the kelly, usually square or hexagonal. It is connected to the rotary table by four large steel pins that fit into mating holes in the rotary table. The rotary motion from the rotary table is transmitted to the bushing through the pins, and then to the kelly itself through the square or hexagonal flat surfaces between the kelly and the kelly bushing. The kelly then turns the entire drillstring because it is screwed into the top of the drillstring itself. Depth measurements are commonly referenced to the KB, such as 8,327 feet KB, meaning 8,327 feet below the kelly bushing.

Kick A flow of reservoir fluids into the hole during drilling operations. The 'kick' is physically caused by the pressure in the hole being less than that of the formation fluids, thus causing flow.

Liner Another name for casing, particularly intermediate casing.

Log Detailed depth related records of certain significant details of an oil or gas well; usually obtained by lowering measurement instruments into a well.

Lost Circulation A lack of mud returning to the surface after being pumped down a hole. Lost circulation occurs when the drill bit encounters natural fissures, fractures or caverns, and mud flows into the newly available space. Lost circulation may also be caused by applying more mud pressure (that is, drilling overbalanced) on the formation than it is strong enough to withstand, thereby opening up a fracture into which mud flows

Measured Depth The length of the borehole, as if determined by a measuring stick. This measurement differs from the 'true vertical depth' of the well in all but vertical wells.

Monitoring Well A well constructed or used for the purposes of water level or water quality data collection. Monitoring wells are often installed to provide an early warning of contamination occurring down gradient from a landfill or industrial facility.

Mousehole A shallow borehole under the rig floor, usually lined with pipe, in which joints of drillpipe are temporarily placed.

Mud Log A progressive analysis of the borehole cuttings washed up from the hole by the drilling mud, plus a record of the variations in drilling rate, mud pumping pressure, depths of formation changes and an analysis of the mud for oil and natural gas traces.

Mud Weight The mass per unit volume of a drilling fluid, synonymous with mud density. Mud weight controls hydrostatic pressure in a hole and prevents unwanted flow into the hole. The weight of the mud also prevents collapse of an openhole.

Net Pay The aggregate thickness of only those parts of the reservoir which contain and produce hydrocarbons.

Observation Well A well constructed in a specific location for the purpose of observing(measuring) changes in water level. An existing well perhaps drilled for a different purpose may also be used to observe water level changes. Observation wells are typically used for short duration data collection such as before, during and after an aquifer test. Wells that are used to collect data on a long term basis are usually referred to as monitoring wells.

Offset Well An existing wellbore close to a proposed well that provides information for planning the proposed well.

Openhole The uncased portion of a borehole.

Overshot 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.

Packer A downhole device used in almost every completion to isolate the annulus from the production conduit, enabling controlled production, injection or treatment. A typical packer assembly incorporates a means of securing the packer against the casing or liner wall, such as a slip arrangement, and a means of creating a reliable hydraulic seal to isolate the annulus, typically by means of an expandable elastomeric element. Packers are classified by application, setting method and possible retrievability. They may be run on wireline, pipe or coiled tubing.

Pay A reservoir or portion of a reservoir that contains economically producible hydrocarbons. The term derives from the fact that it is capable of 'paying' an income. Pay is also called 'pay sand' or 'pay zone.' The overall interval in which pay sections occur is the gross pay; the smaller portions of the gross pay that meet local criteria for pay (such as minimum porosity, permeability and hydrocarbon saturation) are net pay.

Perforate To create holes in the production casing or liner to achieve efficient communication between the reservoir and the wellbore. The characteristics and placement of the communication paths (perforations) can have significant influence on the productivity of the well. Therefore, a robust design and execution process should be followed to ensure efficient creation of the appropriate number, size and orientation of perforations. A perforating gun assembly with the appropriate configuration of shaped explosive charges and the means to verify or correlate the correct perforating depth can be deployed on wireline, tubing or coiled tubing.

Perforation The communication tunnel created from the casing into the reservoir formation, through which oil and/or natural gas is produced. The most common method uses jet perforating guns equipped with shaped explosive charges. However, other perforating methods include bullet perforating, abrasive jetting or high-pressure fluid jetting.

Permeability A measure of the resistance of rock to the movement of fluids. Rocks may have holes or void spaces in them (see 'Porosity'), but if these holes do not connect, the permeability can be drastically reduced.

Piezometer A device for measuring pore water pressure (i.e. measuring the location of the water table). Some types of piezometers can also be used for collecting water samples. As a result, wells designed specifically for collecting water samples are often referred to, incorrectly, as piezometers.

Pill A small volume of drilling mud used for a specific purpose in a drilling operation. Various types of pills are needed from time to time on the rig, such as to stop circulation loss or free stuck drillpipe.

Pitless Adaptor A device installed in a vertical well casing to allow water to be piped horizontally below the frost line to its use point (usually a home).

Plug A seal of cement (or other impervious material) deliberately placed in a wellbore to prevent the flow of water, natural gas, or oil from one strata to another when a well is abandoned in one zone.

Porosity The percentage of pore volume or void space, or that volume within rock that can contain fluids. Effective porosity is the interconnected pore volume in a rock that contributes to fluid flow in a reservoir. It excludes isolated pores. Total porosity is the total void space in the rock whether it contributes to fluid flow. Thus, effective porosity is typically less than total porosity.

Potentiometric Surface Theoretical (imaginary) surface of the static head of ground water in an aquifer.

Production Casing The last string of casing set in a well; production casing is tubular steel pipe connected by threads and couplings that lines the total length of the wellbore to ensure safe control of production, prevent water from entering the wellbore and keep rock formations from 'sloughing' into the wellbore.

Production Tubing Steel pipe inside the casing used to flow the petroleum from the producing zone to the surface.

Pumping Level The level of water in a well when pumping is in progress.

Radius of Influence The horizontal radial distance from a well to the points in an aquifer where there is no observable influence from pumping.

Ream To enlarge a hole. Reaming may be necessary for several reasons. Perhaps the most common reason for reaming a section of a hole is that the hole was not drilled as large as it should have been at the outset. This can occur when a bit has been worn down from its original size, but might not be discovered until the bit is tripped out of the hole, and some under gauge hole has been drilled. Some plastic formations may slowly flow into the hole over time, requiring the reaming operation to maintain the original hole size.

Recompletion The operation of sealing off old perforations in order to complete the well in another formation higher in the wellbore.

Recovery The fraction of hydrocarbons that can or has been produced from a well, reservoir or field; also, the fluid that has been produced.

Reserve Pit An earthen-berm storage area for discarded drilling mud. These small reservoirs are used for several reasons. First, when properly arranged, most of the solids in the mud settle out and a suction hose may be placed in the reserve pit to have additional fluid available to pump into the hole in an emergency. At the end of drilling operations, and perhaps at intermediate times during drilling, the fluids and solids in the reserve pit must be carefully discarded, usually by transfer to a properly certified landfill. If the mud is benign, the solids (mostly clay), and liquids (water), may be plowed and tilled back into the local soil. This technique of disposal and reclamation is known as 'land farming.'

Reservoir A subsurface body of rock having sufficient porosity and permeability to store and transmit fluids. Sedimentary rocks are the most common reservoir rocks because they have more porosity than most igneous and metamorphic rocks and form under temperature conditions at which hydrocarbons can be preserved. A reservoir is a critical component of a complete petroleum system.

Retrievable Bridge Plug A type of downhole isolation tool that may be unset and retrieved from the wellbore after use, such as may be required following treatment of an isolated zone. A retrievable bridge plug is frequently used in combination with a packer to enable accurate placement and injection of stimulation or treatment fluids.

Retrievable Packer A type of packer that is run and retrieved on a running string or production string, unlike a permanent production packer that is set in the casing or liner before the production string is run.

Returns Mud that comes back to the surface and exits through the flowline after being pumped down the drillpipe. 'Lost Returns' is the situation in which some or all the mud does not come back to the surface, which indicates that mud is being lost into weak or fractured formations downhole.

Reverse Circulation To circulate fluid down the wellbore annulus, with returns being made up the tubing string. Reverse circulation often is used to remove debris from the wellbore since the high fluid flow rate inside the tubing string enables the recovery of large

Rig The machine used to drill a well. The rig includes virtually everything except living quarters. Major components of the rig include the mud tanks and pumps, the derrick, the draw works, the rotary table, the drillstring, the power generation equipment and auxiliary equipment.

Rotary Drilling A well drilling method achieved by the rotary action of a drill bit. The ground-up rock is removed by circulating drilling mud which may be forced down the drill pipe and out via the annular space between the drill pipe and the hole. If casing is installed as the drilling proceeds then reverse-rotary drilling may be used with the drilling fluid being pumped down the outside of the drill pipe and returned to the surface upwards through the drill pipe.

Rotary Drilling with Casing Hammer A casing hammer is a well-established alternative for advancing casing in situations where the hole cannot otherwise be kept open. A rotary top drive rig may be equipped with a casing hammer powered by compressed air, hydraulic or mechanical power that drives the casing as drilling progresses. Typically, a DTH hammer or tri-cone bit is used to open a hole ahead of the casing and is then retracted inside the casing while the casing is driven with the casing driver. The striking action of the casing hammer can be reversed to aid casing removal.

Rotary Table The revolving or spinning section of the drill floor that provides power to turn the drillstring in a clockwise direction. The rotary motion and power are transmitted through the kelly bushing and the kelly to the drillstring. When the drillstring is rotating, the drilling crew commonly describes the operation as simply, 'rotating to the right' or 'turning to the right.'

Saltwater Disposal Well A well drilled for the sole purpose of disposing saltwater waste. The saltwater is pumped down the wellbore into saltwater formations deep enough not to pollute shallow fresh water sands.

Saturated Zone The zone within sediment and rock formations where all voids are filled with water. The level below the water table in an unconfined aquifer. The saturated zone may be considered to include water held above the water table by capillary rise. Soils and the vadose zone in rock formations are not fully saturated. Saturated thickness describes the vertical extent of an aquifer below the water table.

Screen A cylinder of steel or plastic material used to allow water to enter a well while preventing sediment or rock particles from entering the well. A screen operates something like a sieve. Well screens may be wire wrapped, louver or perforated, and can be made from different materials and at different opening sizes. The selection of well screen design and opening size may depend on characteristics of the geologic formation, required yield and the thickness of the aquifer.

Sidewall Core A core taken from the side of the borehole, usually by a wireline tool. Sidewall cores may be taken using percussion or mechanical drilling. Percussion cores (the most common) are taken by firing hollow bullets into the formation. The bullets are attached to the tool by fasteners, and are retrieved, along with the core inside, by pulling up the tool and the fasteners. Percussion coring tools typically hold 20 to 30 bullets, but two or three tools can be combined on one run in the hole.

Slug A volume of mud that is more dense than the mud in the drillpipe and hole annulus. A slug is used to displace mud out of the upper part of the drillpipe before pulling pipe out of the hole and is mixed in the pill pit by adding additional weighting material (barite) to a few barrels of mud from the surface pits. The 'pill' is pumped into the top of the drillstring to push mud downward, out of the pipe, thus keeping the upper stands of pipe empty.

Specific Capacity The rate of discharge of a well per unit depth of drawdown. Expressed as gallons per minute per foot, (liters per minute per meter). It is used as a measure of well efficiency. The ideal for a well is high discharge and low drawdown.

Spud To start the actual drilling of a well.

Standpipe A rigid metal conduit that provides the high-pressure pathway for drilling mud to travel approximately one-third of the way up the derrick, where it connects to a flexible highpressure hose (kelly hose). Many large rigs are fitted with dual standpipes so that downtime is kept to a minimum if one standpipe requires repair.

Surface Casing The first string of casing put into a well; it is cemented into place and serves to shut out shallow water formations and as a foundation for well control.

Test Hole Test holes are typically used in applications of engineering geology whereas test wells are used in gorund water investigations to obtain information about geologic and/or hydrologic conditions. Test holes are usually drilled at a small diameter. Based on the information obtained, productions wells of a larger diameter may be installed.

Test Well A well used to assess and/or test the geologic and hydraulic properties of an aquifer. A series of test wells may be drilled in order to determine the most effective location for a (much more expensive) production well. Test wells are usually of a smaller diameter than production wells.

Tight Hole A well that the operator requires be kept as secret as possible, especially the geologic information.

Total Depth The bottom of a particular hole section, hole or wellbore.

Trip The complete operation of removing the drillstring from the hole and running it back in the hole. This operation is typically undertaken when the bit becomes dull or broken. The rig crew removes the drillstring 90-120 feet at a time (a 'stand'), by unscrewing every third or fourth drillpipe or drill collar connection. When the stand is unscrewed from the rest of the drillstring, it is carefully stored upright in the derrick by the fingerboards at the top and careful placement on wooden planks on the rig floor. After the drillstring has been removed from the hole, the bit is unscrewed and a new bit is screwed onto the bottom of the drill collars. The drill collars and drillpipe are 'run in the hole' (RIH). Once on bottom, drilling starts again. The duration of this operation depends on the total depth of the hole. A general estimate for a round trip requires one hour per thousand feet of hole, plus an hour or two for handling collars and bits. At that rate, a round trip in a 10,000 foot well might take 12 hours.

Tremie Pipe A pipe used to carry materials (usually grout) to a specific depth in a drilled hole. Tremie pipes are slowly withdrawn as the grout is placed in the well.

True Vertical Depth The vertical distance from a point in the well (usually the current or final depth) to a point at the surface, usually the elevation of the rotary kelly bushing (RKB).

Tubing wellbore tubular used to produce reservoir fluids. Tubing is assembled with other completion components to make up the production string. The production tubing selected for any completion should be compatible with the wellbore geometry, reservoir production characteristics and the reservoir fluids.

Underground Blowout The uncontrolled flow of reservoir fluids from one reservoir into the wellbore, along the wellbore, and into another reservoir. This cross flow from one zone to another can occur when a high-pressure zone is encountered, the well flows, and the drilling crew reacts properly and closes the blowout preventer. Pressure in the annulus then builds up to the point at which a weak zone fractures. Depending on the pressure at which the fracturing occurs, the flowing formation can continue to flow and losses continue to occur in the fractured zone. Underground blowouts are historically the most expensive problem in the drilling arena, eclipsing the costs of even surface blowouts. It may prove necessary to drill a second kill well to remedy an underground blowout.

Virgin Pressure The original, undisturbed pressure of a reservoir before fluid production.

Washout An enlarged region of a borehole. A washout in an openhole section is larger than the original hole size or size of the drill bit. Washout enlargement can be caused by excessive bit jet velocity, soft or unconsolidated formations, in-situ rock stresses, mechanical damage by BHA components, chemical attack and swelling or weakening of shale as it contacts fresh water.

Water Well An engineered device created to access subsurface water. Wells may be bored, or drilled (horizintally or vertically) or constructed as a vertical or horizontal shaft.

Well Screen A steel or plastic device that admits water to a well from the surrounding geologic formations but which prevents or reduces the likelihood of sediment entering the well. Design and selection of well screens is based of geologic and hydraulic criteria.

Well Rehabilitation The process of using mechanical or chemical techniques to restore declining well yield caused by biological and or chemical encrustation of well casing and/or the gravel pack or rock formations immediately adjacent to the well bore.

Well Sealing Unused wells may need to be sealed in order to protect aquifers from surface contaminants, or to prevent comingling of waters from different aquifers in the same well, or from aquifers interconnected by different wells.

Well A hole in the ground made to gain access to an aquifer to obtain water for economic use. Wells may be dug (mostly old wells less than 50 feet deep) or drilled.

Well Point A screened cylinder (usually steel and less than 4 inches in diameter) that is driven into the ground and which can serve to access ground water.

Well Development The application of techniques after and during the drilling process that bring the well to its maximum yield capacity and achieve maximum well efficiency.

Wellbore The cased portion of a borehole.

Wellhead The equipment used to maintain surface control of a well.

Whipstock An inclined wedge placed in a hole to force the drill bit to start drilling in a direction away from the hole axis. The whipstock must have hard steel surfaces so that the bit will preferentially drill through either casing or rock rather than the whipstock itself. Most whipstocks are set on the bottom of the hole or on top of a high-strength cement plug.

Wildcat An exploration well. The significance of this type of well to the well planners is that by definition, little if anything about the subsurface geology is known with certainty. If a wildcat is especially far from another wellbore, it may be described as a 'rank wildcat.'

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

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