Drilling Glossary

on 21 June 2024. Posted in Drilling Engineering

Tags: Drilling Glossary

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Abandonment - The process of permanently sealing and securing a well that is no longer economically viable or safe to operate.
Abrasive Jetting - A wellbore treatment in which a fluid containing solid particles is used to remove deposits from the surface of the wellbore and / or completion components. The fluid is pumped at high pressure through a downhole tool that uses nozzles to direct jets of the fluid onto the target area. Many tools utilise a controlled rotary motion to ensure the entire internal circumference of the wellbore is treated. Abrasive jetting can also be used to cut completion or wellbore components in which case highly abrasive particles (such as sand) are put into the fluid and then jetted at the target area for a prolonged period of time until the surface is eroded.
Adjustable Kick Off (AKO) sub or Bent Sub - A small segment of drill string which is tilted at an angle rather than being straight. It is run above the bit and below the power section in the drill string and is used for drilling directional for well or for kicking off from a vertical well. Running AKO sub in the string makes it possible to steer the well as needed through sliding. Tilt of the sub depends on tool size, hole size and DLS (Dog Leg Severity) requirement.
Annular Blowout Preventer - A well control device, usually installed above the ram preventers, that forms a seal in the annular space between the pipe and well bore or, if no pipe is present, over the well bore itself.
Annulus - The space between two concentric cylinders within the wellbore. Specifically, it is the space between the drill string (casing) and the wellbore (another casing string).
Annular Capacity - Used to determining the volume of drilling fluids that can be held in the annular space between the drill string (casing) and the wellbore.
Antifoam - A chemical additive used to lower interfacial tension so that trapped gas will readily escape from mud and not form a foam. This additive is used during the preparation of a treatment fluid or when preparing the drilling surface (slurry). Common additives include: Octyl alcohol, aluminum stearate, various glycols, silicones and sulfonated hydrocarbons.
Asphault Mud Additive - Solid or high-viscosity hydrocarbons found in natural deposits or in petroleum refining residue that are used as additives of oil- and water-based muds (drilling fluids).
Associated Gas - Natural gas found in association with oil in a reservoir, either dissolved in the oil or as a cap above the oil.

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Background Gas (Drill Gas) - Expresses the average gas reading that is observed while drilling. It is an indication that formation being drilled contains gas but doesn't necessarily an indication of higher formation pressure. The gas enters the wellbore only when new formation is drilled. It will seep in from already drilled part of the well only if the formation has enough permeability and pressure differential. Background gas doesn't indicate an underbalance condition but may lead to it if total volume of gas in the system lightens the hydrostatic column enough.
Ballooning (Well Breathing) - A term given to the loss/gain situation. Ballooning occurs when the "fracture closure stress" is greater than the mud weight but less than the "equivalent circulation density." In this situation, the well stands full in the static condition. However, fracture extends and takes mud when circulation is established since ECD exceeds the fracture closure stress. When the pumps are stopped and ECD removed, the fracture closes, forcing some or all the mud back into the wellbore.
Barrels of Cuttings per Foot Drilled - The volume of cuttings (rock fragments) generated per foot of well drilled. This is needed for understanding the efficiency of the drilling process, the performance of the drill bit, and the properties of the geological formations being drilled.
Barrel (bbl) - a barrel is a unit of measurement for crude oil and petroleum products, equivalent to 42 US gallons or 159 liters.
Basket - A device placed in the drill or work string that catches debris when a drillable object is being milled or drilled downhole.
Bit - A drilling tool used to cut a cylindrical hole in the earth’s crust, sometimes referred to as a "drill bit" or "ock bit." It can be used during the discovery or extraction process of both crude oil and natural gas.
Bit Breaker - A device used to break or loosen the threaded connections between sections of drill pipe or casing during drilling operations, allowing for the removal or installation of tubulars in the wellbore.
Bitumen - A thick, sticky form of crude oil that is so heavy and viscous that it will not flow unless it is heated or diluted with lighter hydrocarbons. At room temperature, bitumen looks much like cold molasses. It typically contains more sulphur, metals and heavy hydrocarbons than conventional crude oil.
Bitumen Cracking - The process of breaking the large, complex bitumen hydrocarbon molecules into smaller, lighter molecules that are often more easily refined into more useful feedstocks or products.
Block - A machine designed specifically for sucker rod pumping. An engine or motor (prime mover) is mounted on the unit to power a rotating crank. The crank moves a horizontal member (walking beam) up and down to produce reciprocating motion. This reciprocating motion operates the pump.
Blowout - Occurs when down-hole pressure gas is not properly balanced with the weight of the drilling mud; the uncontrolled flow of gas, oil, or other fluids.
Blowout Preventer - A stack or an assembly of heavy-duty valves attached to the top of the casing to control well pressure.
Bore - The inside diameter of a pipe or drilled hole.
Borehole - The openhole or uncased portion that is created in well drilling.
Bottomhole - The deepest part (the bottom) of the well.
Bottom Hole Assembly (BHA) - A set of tools between the drill bit and drill pipe. One of the main objectives of BHA is to have enough weight to transfer to the bit for enhancing the bit's ability to cut hard rocks. A conventional BHA for drilling vertical wells is designed as a tapered string with a larger diameter and heavier drill collars above the bit. A well-designed BHA produces a smooth hole, minimizes vibration, maintains the intended profile of the well and improves bit performance.
Bottom Hole Pressure - The pressure at the bottom of a borehole. It is caused by the hydrostatic pressure of the wellbore fluid and, sometimes, by any backpressure held at the surface, as when the well is shut in with blowout preventers. When mud is being circulated, bottomhole pressure is the hydrostatic pressure plus the remaining circulating pressure required to move the mud up the annulus.
Bottom Hole Temperature - The temperature of the circulating fluid (air, mud, cement, or water) at the bottom of the wellbore after several hours of circulation. This temperature is lower than the bottomhole static temperature. Therefore, in extremely harsh environments, a component or fluid that would not ordinarily be suitable under bottomhole static conditions can be used with great care in circulating conditions. Similarly, a high-temperature well can be cooled down in an attempt to allow logging tools to function. The BHCT is also important in the design of operations to cement casing because the setting time for cement is temperature-dependent. The BHCT and bottomhole static temperature (BHST) are important parameters when placing large volumes of temperature-sensitive treatment fluids.
Bottom Plug - A cement wiper plug that precedes cement slurry down the casing. The plug wipes drilling mud off the walls of the casing and prevents it from contaminating the cement.
Branch Drilling - Drilling from an existing well path towards a new well target.
Break Out - To unscrew one section of pipe from another section, especially drill pipe while it is being withdrawn from the wellbore. During this operation, the tongs are used to start the unscrewing operation.
Bridge Plug - A downhole tool, composed primarily of slips, a plug mandrel, and a rubber sealing element, that is run and set in casing to isolate a lower zone while an upper section is being tested or cemented.
Brine - Water that has a quantity of salt, especially sodium chloride, dissolved in it; salt water.
Buoyancy Factor - Used to account for the buoyant effect of the drilling fluid on the drill string, casing, and other downhole equipment. The buoyant force reduces the effective weight of the drill string in the wellbore, affecting both the load on the drilling rig and the downhole equipment's behavior.

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Casing - A steel pipe that’s placed in well after drilling is completed to prevent the well hole from caving in. Casing also prevents fluids from moving from one formation (like groundwater) to another and helps in well control.
Casinghead - The adapter between the first casing string and either the BOP stack (during drilling) or the wellhead (after completion). This adapter may be threaded or welded onto the casing and may have a flanged or clamped connection to match the BOP stack or wellhead.
Casing Bowl - A wellhead component or a profile formed in wellhead equipment in which the casing hanger is located when a casing string has been installed. The casing bowl incorporates features to secure and seal the upper end of the casing string and frequently provides a port to enable communication with the annulus.
Casing Head Gas - Gas produced with oil wells or from wells located in oil reservoirs, not to be confused with gas from a gas well.
Casing Hanger - A circular device with a frictional gripping arrangement of slips and packing rings used to suspend casing from a casing head in a well.
Casing Point - Point in time after a well has been drilled to its objective depth where the operator must decide whether to commit additional dollars to “setting pipe” and attempt a completion or to abandon the well as non-commercial.
Casing Pressure - The pressure in a well that exists between the casing and the tubing or the casing and the drill pipe.
Casing Spool - A wellhead component used in flanged wellhead assemblies to secure the upper end of a casing string. Casing spools or bowls are available in a wide range of sizes and pressure ratings and are selected to suit the specific conditions.
Casing String - An assembled length of steel pipe configured to suit a specific wellbore. The sections of pipe are connected and lowered into a wellbore, then cemented in place. The pipe joints are typically approximately 40 ft [12 m] in length, male threaded on each end and connected with short lengths of double-female threaded pipe called couplings. Long casing strings may require higher strength materials on the upper portion of the string to withstand the string load. Lower portions of the string may be assembled with casing of a greater wall thickness to withstand the extreme pressures likely at depth. Casing is run to protect or isolate formations adjacent to the wellbore.
Catwalk - The elevated work area adjacent to the vdoor and ramp on a drilling rig where pipe is laid to be lifted to the derrick floor by the catline or by an air hoist.
Cellar - A pit in the ground to provide additional height between the rig floor and the well head to accommodate the installation of blowout preventers, ratholes, mouseholes, and so forth. It also may collect drainage water and other fluids for subsequent disposal.
Cement - A powder consisting of alumina, silica, lime, and other substances that hardens when mixed with water. Extensively used in the oil industry to bond casing to the walls of the wellbore.
Cementing - To prepare and pump cement into place in a wellbore. Cementing operations may be undertaken to seal the annulus after a casing string has been run, to seal a lost circulation zone, to set a plug in an existing well from which to push off with directional tools or to plug a well so that it may be abandoned. Before cementing operations commence, engineers determine the volume of cement (commonly with the help of a caliper log) to be placed in the wellbore and the physical properties of both the slurry and the set cement needed, including density and viscosity. A cementing crew uses special mixers and pumps to displace drilling fluids and place cement in the wellbore.
Cement Bond - The adherence of casing to cement and cement to formation. When casing is run in a well, it is set, or bonded, to the formation by means of cement.
Cement Casing - To fill the annulus between the casing and wall of the hole with cement to support the casing and prevent fluid migration between permeable zones.
Cement or Set Pipe - To fix the casing firmly in the hole with cement, which is pumped through the drill pipe to the bottom of the casing and up into the annular space between the casing and the walls of the well bore. After the cement sets (hardens) it is drilled out of the casing. The casing is then perforated at the level of the zone of expected petroleum accumulation to allow oil and gas to enter the well bore.
Cement Slurry - A cement slurry is a mixture of water, cement, and additives that is pumped into the annulus between the casing and the formation. The water is the base fluid that hydrates the cement and provides the mobility of the slurry. The cement is the main solid material that hardens and bonds the casing to the formation. The additives are chemicals that modify the performance and characteristics of the slurry, such as density, rheology, fluid loss, setting time, strength, and durability. The selection and proportion of these components depend on the well design, formation characteristics, and operational requirements.
Cement Squeeze - A method whereby perforations, large cracks, and fissures in the wall of the bore hole are forced full of cement and sealed off.
Chemical Flooding - A method of improved oil recovery in which chemicals dissolved in water are pumped into a reservoir through injection wells to mobilize oil left behind after primary or secondary recovery and to move it toward production wells.
Choke - A wellhead choke controls the surface pressure and production rate from a well. Chokes are usually selected so that fluctuations in the line pressure downstream of the choke have no effect on the production rate. This requires that flow through the choke be at critical flow conditions. Under critical flow conditions, the flow rate is a function of the upstream or tubing pressure only. For this condition to occur, the downstream pressure must be approximately 0.55 or less of the tubing pressure.
Christmas Tree - A complex arrangement of valves, spools, and fittings used to control the flow of oil and gas from a well. This equipment is installed on top of a wellhead in the cellar, which is a pit dug around the wellbore at the surface level.
Circulate - To pass from one point throughout a system and back to the starting point. For example, drilling fluid is circulated out of the suction pit, down the drill pipe and drill collars, out the bit, up the annulus, and back to the pits while drilling proceeds.
Circulating System - The complete, circuitous path that the drilling fluid travels. Starting at the main rig pumps, major components include surface piping, the standpipe, the kelly hose (rotary), the kelly, the drillpipe, drill collars, bit nozzles, the various annular geometries of the openhole and casing strings, the bell nipple, the flowline, the mud-cleaning equipment, the mud tanks, the centrifugal precharge pumps, and, finally, the positive displacement main rig pumps.
Circulating Pressure - The pressure generated by the mud pumps and exerted on the drill stem.
Coalesce - To grow, as in the process of droplet growth, through small drops merging when they come in contact. If this occurs repeatedly, a continuous liquid phase forms. Through this phenomenon, emulsions break and form two distinct liquid phases that tend to separate. In oil-base mud, the water phase is dispersed as small droplets, with oil as the continuous (external) phase. A stable oil mud will remain dispersed under normal drilling conditions because when droplets contact each other, they do not coalesce due to the strong emulsifier film around each droplet. However, when the emulsion film around each droplet becomes weakened, droplets will begin to coalesce. If not corrected, this can lead to total emulsion breakdown with solids becoming water-wetted.
Cold Flaring - Controlled emission of cold gas.
Completion - The process of making a well ready for production after drilling operations. It usually involves the installation of permanent wellhead and downhole equipment.
Completion Fluid - Low-solids fluid or drilling mud used when a well is being completed. It is selected not only for its ability to control formation pressure, but also for the properties that minimize formation damage.
Connection Gas - A term used for gas observed on bottoms up after making a pipe connection. It indicates the amount of gas that enters the well bore during connection. Pipe connection causes loss annular pressure losses effect as the pumps are turned off. The bottom hole pressure in this static condition is equivalent to only the hydrostatic pressure of mud column. Gas feeds into the well due to loss of hydrostatic pressure caused by loss of annular friction pressure. Connection gas is an indication that formation pressure is more than hydrostatic pressure of column of mud in hole in static condition. Increasing trend of connection gas would indicate the need of increasing mud weight to balance the formation pressure.
Containment Dike - A structure constructed around an oil tank to contain the oil in case the tank collapses or leaks.
Cooler - Heat exchangers used to reduce gas and liquid wellstream temperatures to allow further processing and meet pipeline specifications.
Corrosion Cap (Temporary Abandonment Caps or TA Cap) - They are used for suspending an offshore well at the seabed. Once an offshore well is drilled and needs to be temporarily suspended for moving the rig off location, leaving the casing or wellhead open at the seabed could allow botanical growth or corrosion of exposed threads. This could make it difficult to reconnect to the well for any future operations. Hence a corrosion cap is installed to protect the threads and seal areas. Corrosion caps are installed and retrieved with a running tool. Corrosion caps can also offer pressure containment if a check valve is installed in the stem of the cap. Normally after disconnecting casing strings at MLS, the corrosion cap is installed for the topmost threads. However, caps can be installed for each casing string for isolating the string and its annular space if required.
Creaming - The separation of phases of an emulsion with the lighter phase on top and denser phase on bottom. When oil muds are stagnant, the less-dense oil phase rises and the denser aqueous phase settles. This behavior is not necessarily related to emulsion weakness, nor does it portend breaking, as does coalescence.
Cuttings - As the drilling bit is rotated and the weight is applied, it grinds the rock into small pieces called cuttings. Mud pumped through the drill string brings the cuttings to surface where they are removed from drilling mud using solid control equipment.
Cyclic Steam Stimulation - Injecting steam into a well in a heavy-oil reservoir which introduces heat and thins the oil, allowing it to flow through the same well.

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Derrick - A large load-bearing structure, usually of bolted construction. In drilling, the standard derrick has four legs standing at the corners of the substructure and reaching to the crown block. The substructure is an assembly of heavy beams used to elevate the derrick and provide space to install blowout preventers, casingheads, and so forth.
Desander - A centrifugal device for removing sand from drilling fluid to prevent abrasion of the pumps. It may be operated mechanically or by a fast-moving stream of fluid inside a special cone-shaped vessel, in which case it is sometimes called a hydrocyclone.
Developmental Well - A well drilled in a proven area of an oil or gas reservoir with the goal of maximising economic production and recovery of reservoirs known reserves. They are typically drilled to the depth of a stratigraphic horizon that is known to be productive. The probability of success increases as more development wells are drilled. Development wells differ to exploratory wells as they are wider in diameter and drill deeper.
Directional Drilling - The process of drilling a deviated well to reach Bottom Hole Location (BHL) that is different from surface location. Directional drilling is carried out for several reasons. It could be either for accessing specific BHL where the surface location is obstructed or inaccessible or for reaching several BHLs from one surface location or for enhancing reservoir production from a well. Directional drilling is a specialized service. It needs many special tools and skills for designing & achieving the intended direction of a well.
Directional Survey - This is carried out in a well to measure angle and direction of a well at a certain depth. Surveys are essential part of directional drilling to establish actual well path and compare it with the planned trajectory.
Discovery Well - An exploratory well that results in production from a previously unknown deposit.
Displacement Fluid - In well cementing, the fluid, usually drilling mud or salt water, that is pumped into the well after the cement is pumped into it to force the cement out of the casing and into the annulus.
Drill - To bore a hole in the earth, usually to find and remove subsurface formation fluids such as oil and gas.
Drill Collar - Heavy wall rigid pipes that are used as part of bottom hole assembly above drill bit. Drill collars are stiff tubulars and are added in the drilling assembly so that more weight on bit can be applied on bit for drilling without the risk of buckling the drill string. Drill collars can be either plain or spiral. Spiral drill collars have spiral groove machined on its outer surface. This reduces the contact area of the drill collar with hole, reducing chances of differential sticking.
Drillpipe - Tubular steel conduit fitted with special threaded ends called tool joints. The drillpipe connects the rig surface equipment with the bottomhole assembly and the bit, both to pump drilling fluid to the bit and to be able to raise, lower and rotate the bottomhole assembly and bit.
Drillstem - The drill pipe. In rotary drilling, the bit is attached to the drill stem or drill column which rotates to “dig” the hole.
Drillstring - The combination of the drillpipe, the bottomhole assembly, and any other tools used to make the drill bit turn at the bottom of the wellbore.
Drillstring Capacity - Used to determining the volume of drill string drilling fluid.
Drilling - The process of creating a hole or wellbore in the earth’s surface to access underground hydrocarbons.
Dry Gas - Almost pure methane gas, lacking water and with few heavy components.
Dull Bit Grading - The method of evaluating the condition of a drill bit used for drilling after it is pulled out of the hole.
Drilling Bit Types - Designed to cut through various types of rock and earth formations. There are several types of drilling bits used in the industry, each tailored to specific conditions and requirements. The main types of drilling bits are listed below.
Drilling Cost per Foot -
Drilling Fluid - Any of a number of liquid and gaseous fluids and mixtures of fluids and solids (as solid suspensions, mixtures and emulsions of liquids, gases and solids) used in operations to drill boreholes into the earth. Synonymous with "drilling mud" in general usage, although some prefer to reserve the term "drilling fluid" for more sophisticated and well-defined "muds." Classifications of drilling fluids has been attempted in many ways, often producing more confusion than insight.
Drilling Mud - A special mixture of clay, water, and chemical additives pumped down hole through the drill pipe and drill bit. The mud cools the rapidly rotating drill bit; lubricates the drill pipe as it turns in the well bore; carries rock cuttings to the surface; and serves as plaster to prevent the wall of the bore hole from crumbling or collapsing. Drilling mud also provides the weight or hydrostatic head to prevent extraneous fluids to entering the well bore and to control down-hole pressures that might be encountered.
Drilling Rig - A drilling component that isn’t permanently fixed to the seabed. For example, a drillship or a jack-up unit. The term drilling rig also can refer to the derrick and its associated machinery. Although this term is usually used in offshore activities, you may also hear it during discussions of onshore for land-based drilling activities.
Drilling String Displacement - Is when either pulling out of a hole or trip in a hole for any kind of pipes such as drill pipe, casing or tubing, you should know how much fluid to displace the volume of the drill string.
Dry Hole - A wellbore which is not capable of producing oil and/or gas in commercial quantities. A well may qualify as a dry hole either before or after completion.
Dull Bit Grading - A method of evaluating the condition of a drill bit used for drilling after it is pulled out of the hole. The method uses a system developed by the International Association of Drilling Contractors (IADC). The pulled-out bit is inspected to identify wear and damage to the cutting structure and other parts of the bit. A precise dull bit grading can help to identify the effects of operating parameters and drilling dysfunctions in drilling a particular formation type. It provides vital clues to improve bit design and optimize bit selection for different drilling scenarios.

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Electric Log - An electrical survey made on uncased holes. A special tool is lowered into the hole that ejects an electrical current into the rock and records its resistance to the current. Geologists use the survey data to determine the nature of the rock and its contents.
Embrittlement - Applies to drilling equipment whereby steel components become less resistant to breakage and significantly weaker in tensile strength as a result of prolonged exposure to gaseous or liquid hydrogen sulphide. The weakness is caused when hydrogen ions get between the grain boundaries of the steel, where they then form molecular hydrogen - this takes up more space and reduces the strength of the bonds between the grains. Molecular hydrogen can cause metal to crack when subjected to tensile stress. Hydrogen embrittlement can also be referred to as “acid brittleness.”
Emulsion - A dispersion of one immiscible liquid into another through the use of a chemical that reduces the interfacial tension between the two liquids to achieve stability.
Enhanced Oil Recovery - A process which uses sophisticated techniques to increase the amount of oil that can be recovered from an oil reservoir. It restores formation pressure, and improves oil displacement / fluid flow in the reservoir.
Exploration - The process of searching for new oil and gas reserves through geophysical surveys, remote sensing, seismic studies, exploratory drilling, and various additional methods.

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Feedstock - Raw material supplied to a refinery or oil sands upgrader. For example, crude oil is a feedstock in a refining process which produces gasoline.
Filtrate Loss - Drilling fluid consists of solids in a liquid phase. Filtrate loss is the loss of the liquid phase into the rock. Operationally, the industry does not differentiate between seepage and filtrate losses, and both are collectively referred to as seepage loss. Filtration control materials are added to the mud system, but filtrate loss cannot be stopped unless effective blockage of the pore throat is achieved.
Fireflooding - A method of thermal recovery in which a flame front is generated in the reservoir by igniting a fire at the sandface of an injection well. Continuous injection of air or other gas mixture with high oxygen content will maintain the flame front. As the fire burns, it moves through the reservoir toward production wells. Heat from the fire reduces oil viscosity and helps vaporize reservoir water to steam. The steam, hot water, combustion gas and a bank of distilled solvent all act to drive oil in front of the fire toward production wells.
Fishing - Refers to the act of retrieving objects from the borehole. For retrieval to be successful, the operation requires an understanding of the dimensions and nature of the “fish” to be recovered, the wellbore conditions, the tools and techniques to be employed and the process by which the recovered fish will be handled at surface.
Flaring - Controlled burning of gas for safety purposes.
Float Joint - A full-sized length of casing placed at the bottom of the casing string that is usually left full of cement on the inside to ensure that good cement remains on the outside of the bottom of the casing. If cement were not left inside the casing in this manner, the risk of overdisplacing the cement (due to improper casing volume calculations, displacement mud volume measurements, or both) would be significantly higher. Hence, the well designer plans on a safety margin of cement left inside the casing to guarantee that the fluid left outside the casing is good-quality cement. A float collar is placed at the top of the float joint and a float shoe placed at the bottom to prevent reverse flow of cement back into the casing after placement. There can be one, two or three joints of casing used for this purpose.
Flowback - A water-based fluid from the fracking process that flows back to the earth’s surface after completing the hydraulic fracturing of a shale gas reserve.
Flow Line - A surface pipeline carrying oil, gas or water that connects the wellhead to a manifold or to production facilities, such as heater-treaters and separators.
Flow Test - Also called a well test is a period of time during which the production of the well is measured, either at the well head with portable well test equipment or in a production facility.
Fluid Contact - The interface that separates fluids of different densities in a reservoir. Horizontal contacts are usually assumed, although tilted contacts occur in some reservoirs. The contact between fluids is usually gradual rather than sharp, forming a transition zone of mixed fluid. A mixed-fluid reservoir will stratify according to fluid density, with gas at the top, oil in the middle, and water below. Production of fluids often perturbs the fluid contacts in a reservoir.
Formation Pressure - The force exerted by fluids or gas in a formation, recorded in the hole at the level of the formation with the well shut in. Also called reservoir pressure or shut-in bottomhole pressure.
Formation Temperature - Is the temperature of geological formations, typically within the Earth's subsurface. This temperature is used various fields such as geology, petroleum engineering, and geothermal energy. Formation temperature helps in understanding the geothermal gradient, which is the rate at which the Earth's temperature increases with depth.
Fracture or Fracturing - Procedure to stimulate production by forcing a mixture of fluid and proppant into the formation under high pressure. Fracturing creates artificial fractures in the reservoir rock to increase permeability and porosity, thereby allowing the release of trapped hydrocarbons.
Fracturing Fluid - A fluid used in the fracturing process. Under extremely high hydraulic pressure, frac fluids are pumped downward through production tubing or drill pipe and forced out below a packer or between two packers. The pressure causes cracks to open in the formation, and the fluid penetrates the formation through the cracks. Sand grains, aluminum pellets, walnut shells, or similar materials are carried in suspension by the fluid into the cracks. When the pressure is released at the surface, the fracturing fluid returns to the well but leaves behind the propping agents to hold open the formation cracks.
Free Point - The location in the wellbore where the drill string is not stuck and can move freely. Determining the free point is crucial when a portion of the drill string becomes stuck, as it helps in planning the recovery and freeing operations. In other words, it is the highest point above which the pipe is not stuck and can move freely.
Free Point Indicator - A specialized tool used in the oil and gas industry, particularly in wellbore operations. It helps determine the free point of stuck pipe or tubing within a well. When drilling or performing wellbore operations, the drill string, casing, or tubing can become stuck due to various reasons, such as differential sticking, key seating, or mechanical obstruction.
Free Point vs Stuck Depth - The terms "free point" and "stuck depth" refer to specific aspects of the drill string's condition within the wellbore when part of the string becomes immobilized.
Fullbore - A description of the internal area and surfaces of a tool or tubular assembly through which there is an unimpeded internal diameter. In some cases, fullbore is used to describe the form of a nominal internal diameter that extends over the length of the tool or interval without any variation. In other applications, the term simply implies an ability to pass a ball or similar item of a stated drift diameter through the assembly.

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Gas Anchor - A tubular, perforated device attached to the bottom of a suckerrod pump that helps to prevent gas lock. The device works on the principle that gas, being lighter than oil, rises. As well fluids enter the anchor, gas breaks out of the fluid and exits from the anchor through perforations near the top. Remaining fluids enter the pump through a mosquito bill (a tube within the anchor), which has an opening near the bottom. In this way, all or most of the gas escapes before the fluids enter the pump.
Gas-cut Mud - Drilling mud aerated or charged with gas from formations down hole. The gas forms bubbles in the drilling fluid. Gas-cut mud may indicate commercial quantities of gas present in the formation.
Gas Injection - The injection of gas into a reservoir to maintain formation pressure by gas drive and to reduce the rate of decline of the original reservoir drive. One type of gas injection uses gas that does not mix (is not miscible) with the oil. Examples of these gases include natural gas, nitrogen, and flue gas. Another type uses gas that does mix (is miscible) with the oil. The gas may be naturally miscible or become miscible under high pressure. Examples of miscible gases include propane, methane enriched with other light hydrocarbons, methane under high pressure, and carbon dioxide under pressure. Frequently, water is also injected in alternating steps with the gas.
Gas Kick - Pressure from down hole in excess of that exerted by the weight of the drilling mud, causing loss of circulation. If the gas pressure is not controlled by increasing the mud weight, a kick can violently expel the column of drilling mud resulting in a blow-out.
Gas Lift - A production enhancement technique in which gas is injected into the production tubing to reduce hydrostatic pressure and increase the flow of oil or gas from the well.
Gas Lock - A condition sometimes encountered in a pumping well when dissolved gas, released from solution during the upstroke of the plunger, appears as free gas between the valves. If the gas pressure is sufficient, the standing valve is locked shut, and no fluid enters the tubing.
Gas-oil Contact - The interface that separates fluids of different densities in a reservoir. Horizontal contacts are usually assumed, although tilted contacts occur in some reservoirs. The contact between fluids is usually gradual rather than sharp, forming a transition zone of mixed fluid. A mixed-fluid reservoir will stratify according to fluid density, with gas at the top, oil in the middle, and water below. Production of fluids often perturbs the fluid contacts in a reservoir.
Gas-water Contact - A bounding surface in a reservoir above which predominantly gas occurs and below which predominantly water occurs. Gas and water are somewhat miscible, so the contact between gas and water is not necessarily sharp and there is typically a transition zone between 100% gas and 100% water in reservoirs.
Geological Structure - Layers of sedimentary rocks which have been displaced from their normal horizontal position by the forces of nature into folds, fractures and faults. Geological structures are the logical places to find accumulations of oil and gas.
Gravel Packing - A method of well completion in which a slotted or perforated liner, often wire-wrapped, is placed in the well and surrounded by gravel. If open hole, the well is sometimes enlarged by underreaming at the point where the gravel is packed. The mass of gravel excludes sand from the wellbore but allows continued production.
Groundwater - Groundwater is the water found underground in the cracks and spaces in soil, sand and rock. It is stored in and moves slowly through geologic formations of soil, sand and rocks called aquifers.
Gusher - An oil well with so much pressure that oil flows out of the well head into the air. Now less common thanks to improved drilling methods.

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Heavy Crude Oil - Oil with a gravity below 28 degrees API.
Heavy Weight Drill Pipe - As the name suggests, HWDP is a tubular with drill pipe dimensions but has a heavier wall. It is used as the intermediate member of the drill string between drill collars and drill pipes. HWDP usually has longer tool joint than drill pipe. Its weight is less than the drill collar but more than drill pipe. Having HWDP in the drill string has multiple advantages. It reduces the required number of drill collars in the string while maintaining drill string's ability to apply required weight on bit for drilling. It has less wall contact than drill collar which reduces torque and the chances of differential sticking. HWDP provides smooth weight transition from drill collars to drill pipes and can be run through the hole angle much easily due to less rigidity.
Hoisting System - The system on the rig that performs all the lifting on the rig, primarily the lifting and lowering of drill pipe out of and into the hole. It is composed of drilling line, traveling block, crown block, and drawworks.
Horizontal Drilling - A drilling technique in which the wellbore is drilled directionally through the reservoir rock to maximize contact with hydrocarbon-bearing formations.
Hydraulic Disconnect - A downhole tool designed to allow the lower and upper tool string sections to be parted, which enables the retrieval of the running string. Hydraulic disconnects rely on the application of a predefined pressure through the running string to activate a release mechanism. Sometimes, a ball will be used to block circulation through the tool string, which enables the application of the release pressure.
Hydraulic Fracturing - An operation in which a specially blended liquid is pumped down a well and into a formation under pressure high enough to cause the formation to crack open, forming passages through which oil can flow into the wellbore.
Hydrocarbon - A naturally occurring organic compound comprising hydrogen and carbon which can occur as gases, liquids or solids. Some examples include; Methane, Ethane, Propane, Butane, Pentane, Hexane, Coal, Bitumen and Asphalt.
Hydrochloric Acid - A type of acid commonly used in oil and gas well stimulation, especially in carbonate formations (rock where the main mineral content is calcite and aragonite). The effects of hydrochloric acid enable it to be used in many treatments, often with chemical additives that enhance its performance or allow greater control of the treatment. Treatments are commonly conducted with 15% or 28% solutions of hydrochloric acid.
Hydrogen Sulfide Cracking - A type of corrosion that occurs when metals are exposed to hydrogen sulfide gas; it is characterized by minute cracks that form just under the metal’s surface.
Hydrostatic Pressure - The pressure a fluid exerts on an immersed object floating at a certain depth in a liquid. It is a concept in fluid mechanics and is related to the density of the fluid and the height of the fluid column.

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Impression Block - A block with lead or another relatively soft material on its bottom. It is made up on drill pipe or tubing at the surface, run into a well, and set down on the object that has been lost in the well. The block is retrieved and the impression is examined. The impression is a mirror image of the top of the fish; it also indicates the fish’s position in the hole, for example, whether it is centered or off to one side. From this information, the correct fishing tool may be selected.
Infill Drilling - The process of drilling additional wells between existing wells in a producing oil or gas field to increase overall production and recoverable reserves.
Inflow Test (Negative Pressure Test) - This is aimed at testing the barriers placed in the well for ensuring well integrity. The barriers in a well are installed to avoid fluid flowing past the barrier from the formation into the well or vice versa. To ensure well integrity, it is important to test whether the barrier fulfills this requirement.
Injection Pressure - The pressure at which fluids are injected into a wellbore or reservoir during secondary or enhanced oil recovery operations to stimulate production.
Intermediate Casing - A casing string set in place after the surface casing and before the production casing. The intermediate casing string provides protection against caving of weak or abnormally pressured formations and enables the use of drilling fluids of different density necessary for the control of lower formations.

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Jacking Gear Unit - The individual reduction gear assembly, comprising drive motor, coupling, enclosed reduction gearing and main pinion normally attached to the jack-house.
Jackup Rig - An offshore drilling system commonly used in water depths of 50-330 feet. They are made up of a drilling rig, floating barge, and are fitted with long support legs that can be raised or lowered independently of each other. If the ocean bottom is firm, an independent leg will be used whereas if the ocean bottom is soft, the rig will be mat supported.
Jar - A tool used in drilling or workover and is either hydraulic or mechanical. The tools purpose is to deliver a sharp upward or downward blow to the tools below it. An example of its use is to free a tool or bit that is stuck. A jar accelerator, booster or intensifier can be used to speed up or increase the effect of the jar.
Joint - A length of pipe, casing, or tubing usually from 20 to 30 feet long. On a drilling rig, drill pipe and tubing are lowered into the hole the first time one joint at a time. When pulled from the hole and stacked in the rig, they are usually pulled two, three, or four at a time. These multiple-joint sections are called stands.
Junk Basket - A large, rectangular steel box, usually with sides made of expanded metal to allow you to see what’s inside. The junk basket is used by the rig crew to store an assortment of relatively small parts of the drilling rig, ranging from drill bits to crossover subs to lifting subs.

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Kelly - The first and sturdiest joint of the drill column; a thick-walled, hollow steel forging with two flat sides and two rounded sides. When fitted into the square hole in the rotary table will rotate the kelly joint and thence the drill column and drill bit. Attached to the top of the kelly is the swivel and mud hose.
Kelly Bypass - A system of valves and piping that allows drilling fluid to be circulated without the use of the kelly.
Kelly Hose and Mud Hose - A flexible, steel-reinforced, rubber hose connecting the mud pump with the swivel and kelly joint on the drilling rig. Mud is pumped through the mud hose to the swivel and down through the kelly joint and drill pipe to the drill bit at the bottom of the hole.
Key-seat - A small-diameter channel worn into the side of a larger diameter wellbore. Often this results from a sharp change in direction of the wellbore, or if a hard formation ledge is left between softer formations that then enlarge over time. The diameter of the channel is generally similar to the diameter of the drill pipe, which causes problems for the larger diameters of drilling tools such as tool joints, drill collars and bits, as they unable to pass through the channel and may become stuck in the key-seat. Preventive measures include ensuring that any turns in the wellbore are gradual and smooth. The solution to key-seating is to enlarge the worn channel so that the larger tools will fit through it.
Kick - An unexpected influx of formation fluids (such as water, oil, or gas) into the wellbore during drilling operations. This influx occurs when the pressure exerted by the drilling fluid (mud) in the wellbore is less than the pressure of the fluids in the formation being drilled.
Kill Line - A high-pressure pipe connecting the mud pump and the well, through which drilling fluid can be pumped into the well to control a threatened blowout. During well control operations, kill fluid is pumped through the drillstring and annular fluid is taken out of the well through the choke line, which controls the fluid pressure. If the drillpipe is inaccessible, it may be necessary to pump heavy drilling fluid into the top of the well, wait for gravity to force the fluid to fall, and then remove fluid from the annulus, via both the kill line and choke line. The kill line also provides a measure of redundancy for the operation. In floating offshore operations, the choke and kill lines exit the subsea BOP stack and run along the outside of the riser to the surface.
Killing - If for any reason, the overbalance in a well is lost and the hydrostatic pressure in the well drops below the formation pressure, the formation fluid called influx can enter the well. Well killing is the process of placing the heavier fluid back in the well by displacing out the influx for regaining primary control of the well.

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Land Rig - Any drilling rig that is located on dry land.
Lay Down Pipe - To pull drill pipe or tubing from the hole and place it in a horizontal position on a pipe rack.
Lead Tongs - The pipe tongs suspended in the derrick or mast and operated by a chain or a wire rope connected to the makeup cathead or the breakout cathead.
Leak-off Test (LOT) - A LOT is a pressure integrity test that is carried out to determine the strength of the formation or fracture pressure. The leak-off test provides important information required for safely drilling the next hole section by establishing the maximum mud weight, kick tolerance, maximum allowable surface pressure for shutting in a well, etc.
Liquid Petroleum Gas - Light hydrocarbon material, gaseous at atmospheric temperature and pressure, held in the liquid state by pressure to facilitate storage, transport, and handling. Commercial liquefied gas consists essentially of either propane or butane or mixtures thereof.
Log - A systematic recording of data, such as a driller’s log, mud log, electrical well log, or radioactivity log. Many different logs are run in wells to obtain various characteristics of downhole formations.
Loss of Circulation - A condition that exists when drilling mud pumped into the well through the drill pipe does not return to the surface. This serious condition results from the mud being lost in porous formations, a crevice or cavern penetrated by the drill bit.
Loss of Circulation Material - Material that is added to the drilling mud when circulation is lost to assist in plugging the breached area of the well bore.

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Mast - A portable derrick that is capable of being raised as a unit, as distinguished from a standard derrick, which cannot be raised to a working position as a unit. For transporting by land, the mast can be divided into two or more sections to avoid excessive length extending from truck beds on the highway.
Master Valve - A large valve located on the Christmas tree and used to control the flow of oil and gas from a well. Also called master gate.
Methane - The lightest and most abundant of the hydrocarbon gases and the principal component of natural gas. Methane is a colorless, odorless gas that is stable under a wide range of pressure and temperature conditions in the absence of other compounds.
Monkeyboard - The derrickhand's working platform. As pipe or tubing is run into or out of the hole, the derrickhand must handle the top end of the pipe, which may be as high as 90 feet (27 meters) or higher in the derrick or mast.
Mudlog - A progressive analysis of the well-bore cuttings washed up from the bore hole by the drilling mud. Rock chips are retrieved and examined by the geologist. Modern drilling operations include an electronic evaluation of the mud itself that indicates the presence of hydrocarbons in the mud along with the analysis of the well-bore cuttings.
Mud - The name given to drilling fluid which is mainly a mixture of water, or oil distillate, and ‘heavy’ minerals such as Bentonite or Barites. Mud is pumped into a well at densities calculated to provide a hydrostatic pressure sufficient to overcome downhole formation pressures.
Mud Acid - A mixture of hydrochloric and/or hydrofluoric acids and surfactants used to remove wall cake from the wellbore.
Mud Cake - The sheath of mud solids that forms on the wall of the hole when liquid from mud filters into the formation. Also called filter cake or wall cake.
Mud Centrifuge - A device that uses centrifugal force to separate small solid components from liquid drilling fluid.
Mud Line Suspension (MLS) - MLS systems are installed at or near mudline (seabed) in offshore wells . MLS supports the weight of casing strings that are run in the well as per well configuration and makes it possible to use a surface BOP and wellhead stack while drilling an offshore well. It also facilitates safely suspending the well at the seabed, disconnecting and reconnecting on a later date for future operations.
Mud Logging - The process of monitoring and analyzing drilling mud and cuttings to evaluate subsurface formations, detect hydrocarbons, and provide real-time data to drilling engineers and geologists.
Mud Pit - Excavations near the rig into which drilling mud is circulated. Mud pumps withdraw the mud from one end of the pit as the circulated mud, bearing rock chips from the bore hole, flows in at the other end. As the mud moves toward the suction line, the cuttings drop out leaving “clean” mud ready for another drip down the well bore.
Mud Pump - A large, high-pressure reciprocating pump used to circulate the mud on a drilling rig. A typical mud pump is a two or three-cylinder piston pump whose replaceable pistons travel in replaceable liners and are driven by a crankshaft actuated by an engine or a motor.
Mud Return Line - A trough or pipe that is placed between the surface connections at the wellbore and the shale shaker.
Mud Tank - One of a series of open tanks, usually made of steel plate, through which the drilling mud is cycled to remove sand and fine sediments.
Mud Weight - The weight per unit volume of the drilling fluid used to cool and lubricate the drill bit, carry cuttings to the surface, and maintain pressure to prevent well blowouts.
Multibranch Well - A well drilled to produce and/or inject from several well paths simultaneously.

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Natural Gas - A highly compressible, highly expansible mixture of hydrocarbons with a low specific gravity and occurring naturally in a gaseous form.
Negative Pressure Test (Inflow Test) - It is aimed at testing the barriers placed in the well for ensuring well integrity. The barriers in a well are installed to avoid fluid flowing past the barrier from the formation into the well or vice versa. To ensure well integrity, it is important to test whether the barrier fulfills this requirement.
Nitrogen Injection - a secondary or tertiary recovery technique in which nitrogen gas is injected into a reservoir to maintain pressure, displace oil, and improve sweep efficiency.

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Oblique Drilling - Drilling of an exploration well whose path is not planned to be drilled vertically.
Offset Well - A well location adjoining another well site.
Oil and Gas Lease - A contract between an oil operator and a mineral owner that gives the operator the right to drill for oil and gas on the property.
Oil Gravity - The most widely used indicator of a crude oil’s worth to the producer. Normally, the price that a producer receives for his oil depends on its gravity, the less dense oils (higher gravity) being the most valuable.
Oil Mud - A drilling mud, such as, oil-base mud and invert-emulsion mud, in which oil is the continuous phase. It is useful in drilling certain formations that may be difficult or costly to drill with waterbase mud.
Oil Sands - unconventional petroleum deposits of bitumen mixed with sand, clay, and water that require specialized extraction methods such as surface mining or in-situ techniques.
Oil-water Contact - 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.
Open Hole - An uncased well bore; the section of the well bore below the casing; a well in which there is no protective string of pipe.
Overboard Water - Another name for produced water or brine produced from oil and gas wells, water produced from a wellbore that is not a treatment fluid. The characteristics vary and use of the term often implies an unknown composition. It is generally accepted that water within the pores of shale reservoirs is not produced due to its low relative permeability and that fact that its mobility is lower than that of gas.
Overbalanced Drilling - Is a conventional way of drilling, where the pressure (force per unit area) in the wellbore while drilling is maintained above the formation pressure. Difference between the hydrostatic pressure in the well and the fluid pressure in the formation being drilled is called overbalance. Overbalance is considered the primary barrier for well control and is maintained to disallow flow of formation fluid into the wellbore. Different companies have different policies for maintaining minimum level of overbalance depending on type of well and expected reservoir fluid.
Overshot - A fishing tool that is attached to tubing or drill pipe and lowered over the outside wall of pipe or sucker rods lost or stuck in the wellbore. A friction device in the overshot, usually either a basket or a spiral grapple, firmly grips the pipe, allowing the fish to be pulled from the hole.

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Packer - An expanding plug used in a well to seal off certain sections of the tubing or casing when cementing, acidizing, or when a production formation is to be isolated. Packers are hung on the tubing or the casing and when in position can be expanded hydraulically or mechanically against the pipe wall or the wall of the well bore.
Packer Fluid - A liquid, usually salt water or oil, but sometimes mud, used in a well when a packer is between the tubing and the casing. Packer fluid must be heavy enough to shut off the pressure of the formation being produced, and should not stiffen or settle out of suspension over long periods of time, and must be non-corrosive.
Peak Gas - Peak Gas is a term used to express the maximum gas observed during a drilling break.
Perforating - To create holes in the 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.
Permeability - The property of a material that determines its ability to allow the flow or passage of substances, such as liquids, gases, or electromagnetic fields, through it. It is a measure of how easily a substance can permeate or pass through a given material.
Pinch-out - The disappearance or wedging out of a porous, permeable formation between two layers of impervious rock.
Pipe Ramp - An angled ramp for dragging drill pipe, casing and other materials up to the drilling floor or bringing such equipment down.
Plug Back - To place cement in or near the bottom of a well to exclude bottom water, to sidetrack, or to produce from a formation higher in the well. Plugging back can also be accomplished with a mechanical plug set by wireline, tubing, or drill pipe.
Plugged and Abandoned - To fill a well’s bore hole with cement or other impervious material to prevent the flow of water, gas or oil from one strata to another when a well is abandoned.
Plugging a Well - To fill a well’s bore hole with cement or other impervious material to prevent the flow of water, gas or oil from one strata to another when a well is abandoned.
Pole Mast - A portable mast constructed of tubular members. A pole mast may be a single pole, usually of two different sizes of pipe telescoped together to be moved or extended and locked to obtain maximum height above a well. Double-pole masts give added strength and stability.
Polished Rod - The topmost portion of a string of sucker rods. It is used for lifting fluid by the rod-pumping method. It has a uniform diameter and is smoothly polished to seal pressure effectively in the stuffing box attached to the top of the well.
Pony Collar - A simply as shorter version of a drill collar. It can be either plain or spiral. Pony collar is used in the string to adjust the spacing between the stabilization points or other tools as needed.
Porosity - The measure of empty spaces or voids within a material. It is a property that describes the amount of empty space, or pores, present in a substance relative to its total volume. These pores can be interconnected or isolated, and they can vary in size, shape, and distribution.
Pressure Gradient for Drilling - The rate of change of pressure with respect to distance. It is typically used to describe pressure variations within a fluid that is not in hydrostatic equilibrium.
Production Rig - A portable servicing or workover unit, usually mounted on wheels and self-propelled. A wellservicing unit consists of a hoist and engine mounted on a wheeled chassis with a self-erecting mast. A workover rig is basically the same, with the addition of a substructure with rotary, pump, pits, and auxiliaries to permit handling and working a drill string.
Production Well - Collective term for wells used to recover petroleum, including injection wells, observation wells and possible combinations of these.
Productivity Test - A combination of a potential test and a bottomhole pressure test the purpose of which is to determine the effects of different flow rates on the pressure within the producing zone of the well to establish physical characteristics of the reservoir and to determine the maximum potential rate of flow.
Proven Reserves - Oil and gas reserves that have not been produced but have been located and have been determined recoverable.
Possum Belly - A receiving tank situated at the end of the mud return line. The flow of mud comes into the bottom of the device and travels to control mud flow over the shale shaker.
Pumping Unit - Also called a pumpjack, oil horse, donkey pumper, horsehead pump, rocking horse, beam pump, dinosaur, grasshopper pump, Big Texan, thirsty bird or jack pump is the overground drive for a reciprocating piston pump in an oil well. It is used to mechanically lift liquid out of the well if not enough bottom hole pressure exists for the liquid to flow all the way to the surface.

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Rabbit - An instrument or device that is dropped, pulled, or pushed through a section of pipe to ensure that it is free of obstruction.
Ram - The closing and sealing component on a blowout preventer. One of three types, blind, pipe, or shear, may be installed in several preventers mounted in a stack on top of the wellbore. Blind rams, when closed, form a seal on a hole that has no drill pipe in it; pipe rams, when closed, seal around the pipe; shear rams cut through drill pipe and then form a seal.
Rat Hole - A slanted hole drilled near the well’s bore hole to hold the kelly joint when not in use. The kelly is unscrewed from the drill string and lowered into the rat hole as a pistol into a scabbard.
Raw Crude Oil - Crude oil direct from the wellbore, before it is treated in a gas separation plant. It usually contains nonhydrocarbon contaminants.
Recovery Well - A well used for production or injection.
Recycled Gas - Residual gas that remains entrained in the drilling fluid despite being circulated to surface. At the surface, it remains in the mudstream, which is suctioned from the mud pit and recirculated into the wellbore.
Re-completion - The process of completing a zone or zones that were bypassed during the original completion. Many reasons exist for originally bypassing a zone; one example would be that the original zone opened up was a high-pressure, high volume zone that would overpower other zones in the well. Once the high-pressure zone is depleted, the operator can then re-complete the well in the lower pressure zones that are typically up the hole, shallower in the well.
Re-entry - The process of re-entering a plugged and abandoned well to attempt to complete zones that were bypassed by the original operator. Many reasons exist for the original operator bypassing a zone; one example would be that when the original well was drilled gas pipelines were not present in the vicinity of the well where now they are, so the original operator did not open up known gas producing zones.
Reservoir - The underground formation where oil and gas has accumulated. It consists of a porous rock to hold the oil or gas, and a cap rock that prevents its escape.
Reservoir Pressure (Downhole Pressure) - The pressure at the face of the producing formation when the well is shut in. It is equal to the shut-in pressure (at the wellhead) plus the weight in pounds of the column of oil in the hole. The hydrostatic pressure exerted by a column of oil 5,000 feet high, for example, would be several thousand pounds. In a flowing well, the reservoir pressure would be sufficient to overcome the pressure of the hydrostatic head.
Rich Gas - A mixture of wet and dry gas (methane, ethane, propane, butanes, etc.).
Rig - The machine used to drill a wellbore. In onshore operations, the rig includes virtually everything except living quarters. Major components of the rig include the mud tanks, the mud pumps, the derrick or mast, the drawworks, the rotary table or topdrive, the drillstring, the power generation equipment and auxiliary equipment. Offshore, the rig includes the same components as onshore, but not those of the vessel or drilling platform itself.
Rig Down - A term meaning to disassemble or take apart after operations are complete. This term applies to any complex operation where multiple parts were assembled for an activity.
Rig Up - A term meaning to assemble or put together in preparation to go into operation. This term applies to any complex operation where multiple parts are assembled for an activity.
Roughneck - Drill crew members who work on the derrick floor, screwing together the sections of drillpipe when running or pulling a drillstring.
Roustabout - Drill crew members who handle the loading and unloading of equipment and assist in general operations around the rig.
Round Trip - Pulling the drill pipe from the hole to change the bit and running the drill pipe and new bit back in the hole. On deep wells, round trips or “a trip” may take 24 hours, three 8-hour shifts.
Run Ticket - A record of the oil run from a lease tank into a connecting pipeline.

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Safety Slide - A device normally mounted near the monkey board to afford the derrickhand a means of quick exit to the surface in case of emergency. It is usually affixed to a wireline, one end of which is attached to the derrick or mast and the other end to the surface. To exit by the safety slide, the derrickhand grasps a handle on it and rides it down to the ground.
Saline Groundwater - Deep groundwater that is high in dissolved salt and unsuitable for domestic or agricultural uses.
Salt Water Disposal Well - A well, sometimes a formerly producing well, that is used to inject produced salt water into a formation or formations for disposal.
Secondary Recovery - A broad term encompassing any method of extracting oil from a reservoir after a well or field has exhausted its primary production.
Sedimentary Basin - An extensive area where substantial amounts of sediments occur. Most sedimentary basins are geologically depressed areas. The sediments are usually thickest in the middle, thinning toward the edges.
Seepage Loss - It is a slow and steady loss of volume of drilling fluid. In general, it is termed seepage loss if the loss rate is less than 30 barrels per hour BPH. Seepage losses are caused in highly permeable rocks. Seepage losses can be stopped by blocking the pore throats of the rock with solids or adding "lost circulation material (LCM)" to the mud system. The flow of mud into the pore throat of the rock is stopped when they are sufficiently blocked by the solid particles in the mud.
Separation Factor (Error Ratio) - Used for assessing the collision risk of two wellbores. It is calculated as a ratio of center to center distance and sum of semi-major axis of error ellipse of the two wellbores. Separation factor of 1 indicates that the error ellipses of the two well bores touch each other. Separation factor of less than 1 means that error ellipses overlap and separation factor of more than 1 means that the ellipses don't overlap.
Service Load - In tubular design represent worst-case axial loading due to all selected burst and collapse service loads as a function of depth along with thermally induced axial strains.
Shale Gas - Natural gas produced from gas shale formations.
Short Drill Collar (Pony Collar) - Is a simply a shorter version of a drill collar. It can be either plain or spiral. Short drill collar is used in the string to adjust the spacing between the stabilization points or other tools as needed.
Shut-in Pressure - The surface force per unit area exerted at the top of a wellbore when it is closed at either the Christmas tree or the BOP stack. The pressure may be from the formation or an external and intentional source. The SIP may be zero, indicating that any open formations are effectively balanced by the hydrostatic column of fluid in the well. If the pressure is zero, the well is considered to be dead, and can normally be opened safely to the atmosphere.
Sidewall - Relating to being held against, or taken from, the side of the borehole. The term also describes a measurement that must be made by pressing the sonde against the side of the borehole in order to minimize borehole effects, as, for example, a sidewall epithermal neutron log.
Sliding - The process of drilling without rotating drill string. The bit still rotates as it is connected to motor. Sliding is carried out when attempts are being made to adjust the well trajectory. While drilling a directional well with motor, the bit is run on a bent sub and hence is at an inclination rather than being vertically down. The bit is oriented in the required direction and is rotated through motor without rotating the string. This makes it possible to drill ahead with bit pointing in specified direction.
Slip Crushing - When drill pipe, casing or tubing are run in or pulled out of hole, they are set on slips at rotary bushing for connecting or disconnecting pipes. When pipe is sitting on slips, in addition to tensile load due to buoyed weight of string, the pipe also experiences radial load or hoop stress exerted by slip segments. The damage to the pipe that can be caused due to this additional stress by slip segments is called "slip crushing effect." Calculations and analysing tools help in determining the maximum load that can be placed on the slips without damaging tubulars subjected to a tensile load. Slip crushing is affected by the pipe diameter, slip angle, slip length and the coefficient of friction between slip and the master bushing.
Slurry - A mixture of suspended solids and liquids. Muds in general are slurries, but are seldom called that. Cement is a slurry and is often referred to as such.
Slurry Density - The weight per unit volume of a cement slurry, usually given in units of \(kg/m^3\) or \(lbm/gal\). Typical oil or gas well slurries have densities of 1380 \(kg/m^3\) to 2280 \(kg/m^3\) [11.5 \(lbm/gal\) to 19.0 \(lbm/gal\) ], although special techniques, such as foamed cementing and particle-size distribution cementing, extend this range to 840 \(kg/m^3\) to 2760 \(kg/m^3\) [7 \(lbm/gal\) to 23 \(lbm/gal\) ].
Solid Expandable Tubulars - These are casing or liners, which are expanded downhole after running the pipe in the well. Metallurgically, the tubular expansion process is equivalent to cold-working steel tubular to increase the diameters to the required size downhole. The process is complicated and requires overcoming operational hurdles for the expansion of pipe in a downhole environment. However, the technology provides the distinct advantage of extending the well depth where well conditions limit the number of regular-size casings that can be used to reach planned well depth.
Solution Gas - Natural gas that is found with crude oil in underground reservoirs. When the oil comes to the surface, the gas expands and comes out of the solution.
Sour Gas - Natural gas containing significant amounts of hydrogen sulfide (H2S) or other sulfur compounds, which can be corrosive and toxic.
Sour Crude Oil - A crude oil containing hydrogen sulfide, carbon dioxide or mercaptans.
Specific Gravity for Drilling - The density or ratio of any substance to another substance. When calculating the specific gravity of a liquid or solid, water is normally the comparison (water has a specific gravity of 1).
Spinning Chain - A relatively short length of chain attached to the tong pull chain on the manual tongs used to make up drill pipe. The spinning chain is attached to the pull chain so that a crew member can wrap the spinning chain several times around the tool joint box of a joint of drill pipe suspended in the rotary table. After crew members stab the pin of another tool joint into the box end, one of them then grasps the end of the spinning chain and with a rapid upward motion of the wrist "throws the spinning chain" that is, causes it to unwrap from the box and coil upward onto the body of the joint stabbed into the box. The driller then actuates the makeup cathead to pull the chain off of the pipe body, which causes the pipe to spin and thus the pin threads to spin into the box.
Spud - To start the actual drilling of a well. The first section of the hole is drilled with a large-diameter spudding bit down several hundred feet to accommodate the surface pipe which may be 8 to 20 inches in diameter, depending upon the depth to which the well will ultimately be drilled. The surface pipe is cemented into this hole to protect the surface formations which might contain potable water.
Squeeze a Wall - A technique to seal off with cement a section of the well bore where a leak or incursion of water or gas occurs; forcing cement to the bottom of the casing and up the annular space between the casing and the wall of the bore hole to seal off a formation or plug a leak in the casing: a squeeze job.
Spinning Chain - A relatively short length of chain attached to the tong pull chain on the manual tongs used to make up drill pipe. The spinning chain is attached to the pull chain so that a crew member can wrap the spinning chain several times around the tool joint box of a joint of drill pipe suspended in the rotary table. After crew members stab the pin of another tool joint into the box end, one of them then grasps the end of the spinning chain and with a rapid upward motion of the wrist "throws the spinning chain" that is, causes it to unwrap from the box and coil upward onto the body of the joint stabbed into the box. The driller then actuates the makeup cathead to pull the chain off of the pipe body, which causes the pipe to spin and thus the pin threads to spin into the box.
Spinning Chain - A relatively short length of chain attached to the tong pull chain on the manual tongs used to make up drill pipe. The spinning chain is attached to the pull chain so that a crew member can wrap the spinning chain several times around the tool joint box of a joint of drill pipe suspended in the rotary table. After crew members stab the pin of another tool joint into the box end, one of them then grasps the end of the spinning chain and with a rapid upward motion of the wrist "throws the spinning chain" that is, causes it to unwrap from the box and coil upward onto the body of the joint stabbed into the box. The driller then actuates the makeup cathead to pull the chain off of the pipe body, which causes the pipe to spin and thus the pin threads to spin into the box.
Squeeze Cementing - The forcing of cement slurry by pressure to specified points in a well to cause seals at the points of squeeze. It is a secondary cementing method that is used to isolate a producing formation, seal off water, repair casing leaks, and so forth.
Squeeze Packer - Used for carrying out squeeze jobs below packer. It is commonly used for remedial cement squeeze jobs. Packer can be set either mechanically or hydraulically. It supports the squeeze pressure and avoids back flow.
Stabbing Guide - A handling tools used while running in drill pipe, casing or tubing. It help align male-to-female connection of two separate pipes. Stabbing guides have two sections, a thru-bore (top section) and a counterbore (bottom section). Both thru-bore and counterbore are made of a strong, durable, yet somewhat elastic material. The top section guides the pin end to align with the box end of the other pipe. The bottom end of the stabbing guide helps the pin end slide over and sit in the box end of the pipe.
Stabilizers - These are thick-walled hollow cylindrical components with stabilizing blades on their circumference. They are the components of the bottom hole assembly and are used to provide stability to the drill string. Stabilizers help avoid unintentional sidetracks, minimize vibrations, and enable weight and torque transfer to the bit efficiently by ensuring that the drill string rotates at or near the center of the borehole. Stabilizers have box and pin ends with API connections to be able to easily install at the desired position in the drilling assembly.
Stand - When tubing is pulled from a well it will often be pulled in multiple joint lengths, often in three-joint lengths of up to ninety-feet. These multiple joints of tubing are called “stands” and are stacked in the rig during the “trip”.
Steam Flooding - A thermal recovery method in which steam is injected into a reservoir through injection wells and driven toward production wells. The steam reduces the viscosity of crude oil, causing it to flow more freely. The heat vaporizes lighter hydrocarbons; as they move ahead of the steam, they cool and condense into liquids that dissolve and displace crude oil. The steam provides additional gas drive. This method is also used to recover viscous oils. Also called continuous steam injection or steam drive.
Steam Injection - An improved recovery technique in which steam is injected into a reservoir to reduce the viscosity (“thickness”) of the crude oil.
Stimulation - The action of attempting to improve and enhance a well's performance by the application of horsepower using pumping equipment, placing sand in artificially created fractures in rock, or using chemicals such as acid to dissolve the soluble portion of the rock.
Stratified Flow - A multiphase-flow regime in horizontal or near-horizontal wells in which the fluids are separated into different layers, with lighter fluids flowing above heavier fluids. Stratified flow is more likely to occur at low flow rates and in flat or downhill sections of horizontal wells. In uphill sections, and as the flow rate increases, the interface between the fluids becomes mixed and irregular, hence the term wavy stratified flow is often used.
Stratigraphic Trap - A type of reservoir capable of holding oil or gas, formed by a change in the characteristics of the formation loss of porosity and permeability, or a break in its continuity, the seal, which forms the, trap or reservoir.
Stress Caging - The technique of strengthening the wellbore so that the hole can be drilled without inducing downhole losses. Stress caging not only helps avoid downhole complications by avoiding loss of circulation but could also reduce the number of casing strings required for drilling the well to the planned target depth.
Stress Corrosion Cracking - A form of corrosion in which susceptible types of metals will break by a combination of stress within the metal and the specific type of corrosion. Sulfide corrosion of ferrous alloys and chloride corrosion of stainless steels are two common type of SCC. When high-strength steel remains in contact with hydrogen sulfide (or sulfide ion) in a water-mud environment, sulfide SCC may occur. Tool joints, hardened parts of blowout preventers and valve trim are particularly susceptible to brittle failure caused by sulfide SCC. For this reason, along with toxicity risks of hydrogen sulfide gas, it is essential that water muds be kept entirely free of soluble sulfides and especially hydrogen sulfide at low pH.
Stripper Well - An oil well in the final stages of production.
Stuck Depth - The specific point or section along the drill string where the pipe becomes immobilized and cannot move. This is the depth at which the drill string is held in place by an obstruction or formation that causes it to be stuck, preventing further drilling operations or movement of the drill string.
Stuffing Box - A device that prevents leakage along a piston, rod, propeller shaft, or other moving part that passes through a hole in a cylinder or vessel. It consists of a box or chamber made by enlarging the hole and a gland containing compressed packing. On a well being artificially lifted by means of a sucker rod pump, the polished rod operates through a stuffing box, preventing escape of oil and diverting it into a side outlet to which is connected the flow line leading to the oil and gas separator or to the field storage tank. For a bottomhole pressure test, the wireline goes through a stuffing box and lubricator, allowing the gauge to be raised and lowered against well pressure. The lubricator provides a pressure-tight grease seal in the stuffing box.
Sucker Rod - Steel rods that are screwed together to form a “string” that connects the pump inside a well’s tubing down hole to the pumping jack on the surface.
Sulphur - A yellow mineral extracted from petroleum for making fertilizers, pharmaceuticals and other products.
Surface Pipe - Another term for surface casing, a large-diameter, relatively low-pressure pipe string set in shallow yet competent formations for several reasons. First, the surface casing protects fresh-water aquifers onshore. Second, the surface casing provides minimal pressure integrity, and thus enables a diverter or perhaps even a blowout preventer (BOP) to be attached to the top of the surface casing string after it is successfully cemented in place. Third, the surface casing provides structural strength so that the remaining casing strings may be suspended at the top and inside of the surface casing.
Sulfate Light Crude - Light crude oil containing sulfur compounds such as hydrogen sulfide.
Sulfate Resistance - The ability of set cement to resist deterioration in the presence of sulfate ions.
Sulfate-resistance Cement - A cement in which the amount of tricalcium aluminate is controlled as specified by API Specification 10A.
Sulfide Stress Cracking - A type of spontaneous brittle failure in steels and other high-strength alloys when they are in contact with moist hydrogen sulfide and other sulfidic environments. Tool joints, hardened parts of blowout preventers and valve trim are particularly susceptible. For this reason, along with toxicity risks of hydrogen sulfide gas, it is essential that water muds be kept entirely free of soluble sulfides and especially hydrogen sulfide at low pH. Sulfide stress cracking is also called hydrogen sulfide cracking, sulfide cracking, sulfide corrosion cracking and sulfide stress-corrosion cracking. The variation of the name is due to the lack of agreement in the mechanism of failure. Some researchers consider sulfide-stress cracking a type of stress-corrosion cracking, while others consider it a type of hydrogen embrittlement.
Surge - While running in the drill string or casing in the well, the downward motion of the string forces the drilling fluid out of the flowline. This upward flow, causes frictional force with the wellbore wall and pipe that acts downwards. At the same time, the fluids immediately adjacent to the string are dragged downwards. This creates a piston effect and adds additional pressure to the hydrostatic pressure. This additional pressure is called "surge pressure." Excessive surge pressures can increase the bottom hole pressure to high levels and induce losses.
Surrounding Bed - A formation layer above or below the layer being measured by a logging tool. The term is used in particular to describe the adjacent layers above or below a horizontal well. In a vertical well, the term shoulder bed is more common. The term adjacent bed is used in both cases.
Suspended Well - A well that is not currently producing oil or gas but has been safely secured and may produce in the future.
Swabbing - To clean out the bore hole of a well with a special tool attached to a wire line. Swabbing a well is often done to start it flowing. By evacuating the fluid contents of the hole the hydrostatic head is reduced sufficiently to permit the oil in the formation to flow into the bore hole.
Sweet Gas - Natural gas that does not contain hydrogen sulfide [H₂S] or significant quantities of carbon dioxide [CO₂].
Sweet Oil and Gas - Petroleum containing little or no hydrogen sulphide.
Synthetic Crude Oil - A mixture of hydrocarbons, similar to crude oil, derived by upgrading bitumen from oil sands.
Synthetic Natural Gas - A gas obtained by heating coal or refining heavy hydrocarbons. Synthetic natural gas is abbreviated SNG.

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Tar Sands - An inaccurate term for oil sands, which are a type of petroleum deposit containing a naturally occurring mixture of sand, clay, and water, soaked with a form of petroleum called bitumen.
Telescoping Mast - A portable mast that can be erected as a unit, usually by a tackle that hoists the wireline or by a hydraulic ram. The upper section of a telescoping mast is generally nested (telescoped) inside the lower section of the structure and raised to full height either by the wireline or by a hydraulic system.
Tertiary Recovery - The third major phase of crude oil recovery that involves using more sophisticated techniques, such as steam flooding or injection of chemicals, to increase recovery.
Tight Gas - Gas with very low flow rates. Found in sedimentary layers of rock that are cemented together so tightly that it greatly hinders the extraction. Getting tight gas out usually requires enhanced technology like hydraulic fracturing where fluid is pumped into the ground to make it more permeable.
Tight Hole - A drilling well about which all information, depth, formations encountered, drilling rate, logs, is kept secret by the operator.
Tipping the Bit - Removing the bit from the hole and running it in again. This requires that all of the drill pipe in the hole be pulled up. This pipe is stacked vertically in the derrick. Usually the bit is replaced with a new one once it is out of the hole.
Tongs - The large wrenches used for turning when making up or breaking out drill pipe, casing, tubing, or other pipe; variously called casing tongs, rotary tongs, and so forth according to the specific use. Power tongs or power wrenches are pneumatically or hydraulically operated tools that serve to spin the pipe up and, in some instances, to apply the final makeup torque.
Traps - A mass of porous, permeable rock, sealed on top and both sides by non-porous, impermeable rock, that halts the migration of oil and gas, causing them to accumulate.
Transition Zone - An area in which water is too shallow for acquisition of marine seismic data with towed streamers, such as near the shoreline, marshes and lagoons. In some cases, source explosives can be rammed into the unconsolidated sediments of transition zone environments rather than drilling more costly shot holes. Likewise, hydrophones can be placed by ramming to couple the receiver to the Earth better and to save time and money during survey acquisition.
Treatment Fluid - A fluid designed and prepared to resolve a specific wellbore or reservoir condition. Treatment fluids are typically prepared at the wellsite for a wide range of purposes, such as stimulation, isolation or control of reservoir gas or water. Every treatment fluid is intended for specific conditions and should be prepared and used as directed to ensure reliable and predictable performance.
Trip Gas - The term used to define the gas observed on bottoms up after making a trip. It represents the amount of gas that feeds into the well during the trip while the well is in static condition. Gas feeds into the well due to loss of hydrostatic pressure caused by loss of annular friction pressure or swabbing effect during the trip.
Tracer - A substance added to reservoir fluids to permit the movements of the fluid to be followed or traced. Dyes and radioactive substances are used as tracers in underground water flows and sometimes helium is used in gas. When samples of the water or gas taken some distance from the point of injection reveal signs of the tracer, the route of the fluids can be mapped.
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). This is one of two primary depth measurements used by the drillers, the other being measured depth.
Tubing - Small diameter pipe that is installed in the casing. Tubing increases the viscosity of fluid thus a well’s flow capabilities.

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Underbalanced drilling (UBD) - The process of drilling a well by maintaining a hydrostatic head in the wellbore intentionally lower than the fluid pressure in the formation. The difference between the fluid pressure in the formation and the hydrostatic pressure in the well is called "Underbalance." The hydrostatic head can either be naturally under balance or by injecting natural gas, nitrogen, or air. If there is enough porosity and permeability in the rock, the formation fluid flows into the wellbore and up to the surface while drilling.
Unloading a Well - Removing fluid from the tubing in a well, often by means of a swab, to lower the bottomhole pressure in the wellbore at the perforations and induce the well to flow.
Upgrading - The process of converting heavy oil or bitumen into synthetic crude oil.

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Vapor Extraction (vapex) - A non-thermal heavy oil manufacturing method that consists of a solvent vapor being used to reduce the viscosity of heavy oil. The solvent is injected into the heavy oil and subsequently expands and dilutes. The diluted heavy oil will then drain to the lover horizontal well.
Vibroseis - An exploration technique of thumping the ground with vibrator vehicles to produce seismic shock waves under the earth’s surface. The shaking gets translated into seismic data to provide insights into the subsurface for oil, gas and resource exploration.
Viscosity - The measure of the internal friction/resistance to the flow of a liquid. Lower viscosity fluids flow easily in pipes where high viscosity fluids have a higher pressure drop. Viscosity of fluids is typically temperature dependent and is not affected as dramatically by pressure as gas viscosity. In fact, typically as a liquid temperature increases, the velocity decreases. When a gas temperature increases, the viscosity increases!
Vugular Loss - If the pore throats of rock are very big, severe levels of losses are encountered where even complete circulation can be lost. If the pore throats are larger than 1/16” in diameter, the rock is termed a vugular rock. These vugular pore throats cannot be easily plugged and losses are harder to control. Since vugular losses are in significant volume, they are measured in ‘Barrels Per Minute (BPM)’ rather than ‘Barrels Per Hour (BPH)’. Vugular-sized pore throats are commonly found in carbonate, gravel, or any uncompacted formation.

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Washout - An openhole section or an enlarged region of a wellbore. A washout is usually larger than the size of the drillbit and can be caused by excessive bit jet velocity, soft formations, in-situ rock stresses, mechanical damage amongst other reasons. They can become more severe with time but using an appropriate mud time can minimize them.
Water-wet - Pertaining to the adhesion of a liquid to the surface of a solid. In water-wet conditions, a thin film of water coats the surface of the formation matrix, a condition that is desirable for efficient oil transport. Treatments that change the wettability of the formation from water-wet to oil-wet can significantly impair productivity.
Water Cut - The percentage of water produced along with oil or gas from a well, used to assess reservoir performance, monitor fluid production, and optimize production operations.
Water Drive Reservoir - An oil reservoir or field in which the primary natural energy for the production of oil is from edge, or bottom-water in the reservoir.
Water Flood - One method of secondary recovery in which water is injected into an oil reservoir to force additional oil out of the reservoir rock and into the well bores of producing wells.
Wellbore - The hole drilled in the ground either on land or offshore to explore or produce oil, gas, or water. A wellbore is drilled using a drill bit a through several formation layers and rock types. Drilling fluid is used and is designed to provide the required hydrostatic pressure for ensuring well bore stability, avoiding the influx of formation fluid, removing drilled cuttings, and cooling downhole. Based on types of formations, pore pressure, and fracture pressure estimations, wellbores are cased with steel casing and cement at predetermined intervals. Part of a wellbore can be left uncased also depending on the design, rock type, and objectives of the well.
Wellbore Tortuosity - A ratio of actual distance between two point in a wellbore and the straight line distance between the same two points. Inclination and azimuth measurements define the profile of the well whereas tortuosity is a measure of crookedness and indicates the well quality. High wellbore tortuosity not only affect the drilling process, drilling efficiency and casing running but also impact effectiveness of completions and production equipment in achieving reservoir return rates.
Well Abandonment - The process of permanently suspending operations in a well. It is the process of permanently abandoning a well by setting a set of abandonment plugs. Usually, when a well is drilled to the intended target depth and after evaluation if it is determined that the well will not be completed for production in near future, the well is abandoned. A set of cement and/or mechanical plugs are placed in the well to ensure complete isolation as per regulatory requirements. Every company has its own guidelines and policies for well abandonment. The type and number of plugs depend on the type of well and expected reservoir pressure, fluid, etc.
Well Cellar - A structural component used in oil and gas drilling operations. It is a pit or depression dug into the ground around the wellhead.
Wet Gas - Natural gas that contains less methane (typically less than 85% methane) and more ethane and other more complex hydrocarbons.
Wildcat - A well drilled in an area where no oil or gas production exists.
Workover - Operations on a producing well to restore or increase production. Tubing is pulled and the casing at the bottom of the well is pumped or washed free of sand that may have accumulated. In addition to washing out sand and silt that has clogged the face of the formation, a workover may also include an acid treatment, hydrofracing, or plugging off a depleted zone and opening a new zone for production.
Workover Rig - A truck-mounted mast with winches, cables and sheaves capable of pulling tubing, as well as performing the other functions of a workover.

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Yield Point - A term used in materials science and engineering to describe the point at which a material begins to deform plastically under stress, without any additional increase in the applied stress.
Yield Strength - The minimum stress that leads to permanent deformation of the material. It is important to note that the yield strength of a material is not a fixed value, but rather depends on various factors such as the composition, processing, and testing conditions of the material.

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Z Factor - The factor used to compensate for change in density of gas with temperature and pressure not accounted for in the ideal gas law.
Zero Gas - Gas recommended by the manufacturer, which is free of flammable gases, and interfering and contaminating substances, the purpose of which is calibration/adjustment of the apparatus zero.
Zero Point - Zero point is the gas concentration at which the gas analysis unit registers zero.

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