Acid Bottle Inclinator: A device used in a well to determine the degree of deviation from the vertical of the well bore. The acid is used to etch a horizontal line on the container and from the angle the line makes with the wall of the container, the angle of the well's course can be determined.

Acidizing a well: A technique for increasing the flow of oil from a well. Hydrochloric acid is pumped into the well under high pressure to reopen and enlarge the pores in the oil-bearing limestone formations.

Acre-Foot of Sand: A unit measurement applied to petroleum reserves; an acre of producing formation one foot thick.

ADA Mud: A material which may be added to drilling mud to condition it in order to obtain satisfactory core samples.

Adsorption: The attraction exhibited by the surface of a solid for a liquid or a gas when they are in contact.

Advance Payment Agreement: A transaction in which one operator advances a sum of money or credit to another operator to assist in developing an oil or gas field. The agreement provides an option to the "lender" to buy a portion or all of the production resulting from the development work.

Aerify: To change into a gaseous form; to infuse with or force air into; gasify.

Air Drilling: The use of air as a drilling fluid. In certain types of formations, air drilling is considered a better medium than conventional drilling mud. It is more economical (mud is expensive and the preparation ot the slurry and maintaining the condition is time consuming), drilling rates are higher, penetration is faster, and bit life is longer. Although air does a good job of cooling the bit and bringing out the pulverized rock, it has severe limitations. With air drilling, water in the subsurface formations and downhole gas pressure cannot be controlled. When drilling in an area where these two types of intrusions may occur, a mud system must be on standby to avert possible trouble.

Air Injection Method: A type of secondary recovery to increase production by forcing the oil from the reservoir into the well bore. Because of the dangers inherent in the use of air, this method is not a common practice except in areas where there is sufficient gas for repressuring.

Aliphatics: One of the two classes of organic petrochemicals; the other is the aromatics. The most important aliphatics are in the gasses, ethylene, butylenes, acetylene, and propylene.

Allowable: The amount of oil or gas a well or a leasehold is permitted to produce under proration orders of a state regulatory body.

Annular space: The space between the well's casing and the wall of the borehole.

Annulus of a well: The space between the surface casing and the producing or well-bore casing.

API Gravity: Gravity (weight per unit volume) of crude oil or other liquid hydrocarbon as measured by a system recommended by the API (American Petroleum Institute). API gravity bears a relationship to true specific gravity but is more convenient to work with than the decimal fractions which would result if petroleum were expressed in specific gravity.

Appraisal Drilling: Wells drilled in the vicinity of a discovery or wildcat well in order to evaluate the extent and the importance of the find.

Aquagel: A specifically prepared bentonite (clay) widely used as a conditioning material in drilling mud.

Arbitrage, Product (Petroleum): The buying, selling, or trading of petroleum or products in various markets to make a profit from short-term differences in prices in one market as compared to those in another. A sophisticated method of trading in world petroleum markets.

Areal Geology: The branch of geology that pertains to the distribution, position, and form of the areas of the earth's surface occupied by different types of rocks or geologic formations; also, the making of maps of such areas.

Areometer: An instrument for measuring the specific gravity of liquids; a hydrometer.

Aromatics: a group of hydrocarbon fractions that form the basis of most organic chemicals so far synthesized. The name aromatics derives from their rather pleasant odor. The unique ring structure of their carbon atoms makes it possible to transform aromatics into an almost endless number of chemicals. Benzene, tolulene, and xylene are the principal aromatics and are commonly referred to as the BTX group.

Artificial Drives: Methods of producing oil from a reservoir when natural drives - gas-cap, solution gas, water drive, etc. - are not present or have been depleted. Waterflood, repressuring or recycling, and in-situ combustion are examples of artificial drives.

Asphalt: A solid hydrocarbon found as a natural deposit. Crude oil of high asphaltic content when subjected to distillation to remove the lighter fractions such as naphtha and kerosene, leave asphalt as a residue. Asphalt is dark brown or black in color and at normal temperatures solid.

Asphalt-Base Crude: Crude oil containing very little paraffin wax and a residue primarily asphaltic. Sulfur, oxygen, and nitrogen are often relatively high. This type crude is particularly suitable for making high-quality gasoline, lubrication oil, and asphalt.

Asphaltic Petroleum: Petroleum that contains sufficient amounts of asphalt in solution to make recovery commercially practical by merely distilling off the solvent oils.

Asphaltic Sands: Natural mixtures of asphalts with varying proportions of loose sand. The quantity of bituminous cementing material extracted from the sand may run as high as 12 percent. This bitumen is composed of soft asphalt.

Assembly: A term to describe a number of special pieces of equipment fitted together to perform a particular function; e.g., a drill assembly may include other pieces of downhole equipment besides the drill bit, such as drill collars, dampening subs, stabilizers, etc.

Assignee: A recipient of an interest property or a contract; in oil and gas usage, the recipient of an oil or gas lease; a transferee.

Associated Gas: Gas that occurs with oil, either as free gas or in solution. Gas occurring alone in a reservoir is unassociated gas.

Atmosphere, One: The pressure of the ambient air at sea level; 14.69 pounds per square inch. Air at sea level, 29.92 inches of mercury or 33.90 feet of water.

Back Off: To raise the drill bit off the bottom of the hole; to slack off on a cable or winch line; to unscrew.

Back Pressure: The pressure against the face of the reservoir rock caused by the control valves at the wellhead, hydrostatic head of the fluid in the hole, chokes, and piping. Maintenance of back pressure reduces the pressure differential between the formation and the borehole so that oil moves into the well with smaller pressure loss. This results in the expenditure of smaller volumes of gas from the reservoir, improves the gas-oil ratio, and ensures the recovery of more oil.

Backwashing: Reversing the fluid flow through a filter to clean out sediment that has clogged the filter or reduced its efficiency. Backwashing is done on closed-system filters and on open-bed gravity filters.

Bail: To evacuate the liquid contents of a drill hole with the use of a long cylindrical bucket (bailer).

Balling of the Bit: The fouling of a rotary drilling bit in sticky, gumbo-like shale which causes a serious drag on the bit and loss of circulation.

Barrel: A unit of measure for crude oil and oil products equal to 42 U.S. gallons.

Basement Rock: Igneous or metamorphic rock lying below the sedimentary formations in the earth's crust. Basement rock does not contain petroleum deposits.

Basic Sediment: Impurities and foreign matter contained in oil produced from a well.

Basin: A synclinal structure in the subsurface, once the bed of a prehistoric sea. Basins, composed of sedimentary rock, are regarded as good prospects for oil exploration.

Behind the Pipe: Refers to oil and gas reservoirs penetrate or passed through by wells but never tapped or produced. Behind the pipe refers usually to tight formations of low permeability that, although recognized, were passed through because they were uneconomical to produce at the time.

Bell Nipple: A large swage nipple for attaching casinghead fittings to the well's casing above the ground or at the surface. The bell nipple is threaded on the casing end and has a plain or weld end to take the casinghead valves.

Biochemical Conversion: The use of bacteria to separate kerogen from oil shale. Certain bacteria will biodegrade the minerals in oil shale releasing kerogen from the shale in liquid or semi-liquid form.

Bit: The cutting or pulverizing tool or head attached to the drillpipe in boring a hole in underground formations.

Bitumen, Oil Sands: A heavy, petroleum-like substance found in certain consolidated sand formations at the surface of the earth or at relatively shallow depths where it can be surface mined after the removal of a few feet of overburden. The extraction process is complicated but basically it involves the heating of the oil sands to separate the oil. The oil is floated off and undergoes treatment before it is piped to a refinery.

Bleed: To draw off a liquid or gas slowly. To reduce pressure by allowing fluid or gas escape slowly; to vent the air from a pump.

Bleeder Valve: A small valve on a pipeline, pump or tank from which samples are drawn or to vent air or oil; sample valve.

Block: A contiguous acreage position that also can be designated as a lease or permit.

Blowdown Stack: A vent or stack into which the contents of a processing unit are emptied when an emergency arises. Steam is injected into the tank to prevent ignition of volatile material or a water quench is sometimes used.

Blowing a Well: Opening a well to let it blow for a short period to free the well tubing or casing of accumulations of water, sand, or other deposits.

Blowout: Out of control gas and/or oil pressure erupting from a well being drilled; a dangerous, uncontrolled eruption of gas and oil from a well; a wild well.

Blowout Preventer: A stack or an assembly of heavy-duty valves attached to the top of the casing to control well pressure; a Christmas tree.

Booster Station: A pipeline pumping station usually on a main line or trunk line; an intermediate station; a field station that pumps into a tank farm or main station.

Borehole: The hole in the earth made by the drill; the uncased drill hole from the surface to the bottom of the well.

Bottom Hole Assembly: A drilling string comprised of a drill bit and several drill collars is a simple bottom-hole assembly. Such an assembly may also include a bottom-hole reamer above the bit or above the first drill collar. When in addition to drill collars and reamers there are two or three stabilizers in the string, it is referred to as a packed-hole assembly. The main purpose of a packed hole assembly is to keep the bit drilling as straight down as possible.

Bottom Out: To reach total depth; to drill to a specified depth.

Bottom Hole Pressure: The reservoir or formation pressure at the bottom of the hole. If measured under flowing conditions, readings are usually taken at different rates of flow in order to arrive at a maximum productivity rate. A decline in pressure indicates some depletion of the reservoir.

Bowl: A device that fits in the rotary table and holds the wedges or slips that support a string of tubing or casing.

Bridgeover: The collapse of the walls of the borehole around the drill column.

Bridge Plug: An expandable plug used in a well's casing to isolate producing zones or to plug back to produce from a shallower formation; also to isolate a section of the borehole to be filled with cement when the well is plugged.

Bumper Sub (Fishing): A hydraulically actuated tool installed in the fishing string above the fishing tool to produce a jarring action. When the fishing tool has a firm hold on the lost drill pipe or tubing, which may also be stuck fast in the hole, the bumper sub imparts a jarring action to help free the "fish".

Button Bit: An insert bit; a drill bit with tungsten-carbide or other superhard metal inserts or buttons pressed into the face of the bit's cutting cones.

Casing: Steel pipe used in oil wells to seal off fluids from the borehole and to prevent the walls of the hole from sloughing off or carving. There may be several strings of casing in a well, one inside the other. The first casing put in a well is called surface pipe which is cemented into place and serves to shut out shallow water formations and also as a foundation or anchor for all subsequent drilling activity.

Casing Head: The top of the casing set in a well; the part of the casing that protrudes above the surface and to which the control valves and flow pipes are attached.

Casing Point: A term that designates a time when a decision must be made whether casing is to be run and set or the well abandoned and plugged. In a joint operating agreement, casing point refers to the time when a well has been drilled to objective depth, tests made, and the operator notifies the drilling parties of his recommendation with respect to setting casing and a production string completing the well.

Casing Shoe: A reinforcing collar of steel screwed onto the bottom joint of casing to prevent abrasion and distortion on the wall of the borehole. Casing shoes are about an inch thick and 10 to 16 inches long and are an inch or so larger in diameter in order to clear a path for the casing.

Caving/Cavy Formation: A formation that tends to cave or slough into the well's borehole. In the jargon of cable-tool drillers, "the hole doesn't stand up".

Cellar: An excavation at the drillsite before erecting the derrick to provide working space for the casinghead equipment beneath the derrick floor. Blowout preventer valves are also located beneath the derrick floor in the cellar.

Channeling: A condition that arises in oil production when water bypasses the oil in the formation and enters the well bore through fissures or fractures. There are two general types of channeling: 1) coning off, in which a small amount of oil rides on top of the encroaching water; and 2) bypassing where water breaks through to the well bore through fractures or more permeable streaks or sections of the formation leaving the oil behind.

Christmas Tree: An assembly of valves mounted on the casing head through which a well is produced. The Christmas tree also contains valves for testing the well and for shutting is in if necessary.

Churn Drilling: Another name for cable-tool drilling because of the up and down, churning motion of the drill bit.

Circulate: To pump drilling fluid into the borehole through the drillpipe and back up the annulus between the pipe and the wall of the hole; to cease drilling but to maintain circulation for any reason. When closer inspection of the formation rock just encountered is desired, drilling is halted as circulation is continued to "bring bottoms up".

Circulation: The round trip made by drilling mud; down through the drillpipe and up on the outside of the drillpipe, between the pipe and the walls of the borehole. If circulation is lost, the flow out of the well is less than the flow into the well; the mud may be escaping into some porous formation or a cavity downhole.

Clean Circulation: The circulation of drilling mud free of rock cuttings from the bottom of the borehole. This condition may be caused by a worn bit; circulating to clean the hole or by a broken or parted drillstring.

Closed In: Refers to a well, capable of producing, that is shut in.

Complete a Well: To finish a well so that it is ready to produce oil or gas. After reaching total depth (TD) casing is run and cemented; casing is perforated opposite the producing zone, tubing is run, and control and flow valves are installed at the wellhead. Well completions vary according ti the kind of well, depth, and the formation from which it is to produce.

Concession: An agreement (usually with a foreign government) to permit an oil company to prospect and produce oil in the area covered by the agreement.

Conductor Casing: A well's surface pipe used to seal off near-surface water, prevent the caving or sloughing of walls of the hole, and as a conductor of the drilling mud through loose, unconsolidated shallow layers of sand, clays, and shales.

Connate Water: The water present in a petroleum reservoir in the same zone occupied by oil and gas. Connate water is not to be confused with bottom or edge water. Connate water occurs as a film of water around each grain of sand in granular reservoir rock and is held in place by capillary attraction.

Consortium: An international business association organized to pursue a common objective; (e.g.) to explore, drill, and produce oil.

Crown Block: A stationary pulley system located at the top of the derrick used for raising and lowering the string of drilling tools; the sheaves and supporting members to which the lines of the traveling block and hook are attached.

Darcy: A unit of permeability of rock.

Delineation Wells: Wells drilled outward from a successful wildcat well to determine thee extent of the oil find, the boundaries of the productive formation.

Development: The drilling and bringing into production of wells in addition to the discovery well on a lease. The drilling of development wells may be required by the express or implied covenants of a lease.

Development Clause: The drilling and delay rental clause of a lease/ also express clauses specifying the number of the development wells to be drilled.

Development Well: A well drilled in an area indicated to be productive by the presence of a successful, previously drilled exploratory well. Development wells are relatively low risk.

Directional Drilling: The technique of drilling at an angle from the vertical by deflecting the drill bit. Directional wells are drilled for a number of reasons; to develop an offshore lease from one drilling platform; to reach a pay zone beneath land where drilling cannot be done (e.g. beneath a railroad, cemetery or lake), and to reach the production zone of a burning well to flood the formation (Killer Well).

Discovery Well: An exploratory well that encounters a new and previously untapped petroleum deposit; a successful wildcat well. A discovery well may also open a new horizon in an established field.

Drilling Jars, Hydraulic: A tool used in the drillstring for imparting an upward or downward jar or jolt to the drillpipe should it get stuck in the hole while drilling or making a trip. The jars' action is initiated either by the weight or tension of the drillstring, which the driller can apply.

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

Drowning: A colloquial term fro the encroachment of water at the well bore into a formation that once produced oil but now produces more and more water.

Dry Hole: An unsuccessful well. An exploratory or development well incapable of producing either oil or natural gas in sufficient quantities to justify completion.

Dual Discovery: A well drilled into two commercial pay zones, two separate producing formations, each a different depth.

Economic Depletion: The reduction in the value of a wasting asset by removing or producing the minerals.

Exploitation: The optimization of oil and gas production or establishing additional reserves from producing properties through development drilling or the application of various technological methods.

Exploratory Well: A well drilled to find and produce oil or natural gas in an unproved area. Sometimes called a "wildcat."

Fanning the Bottom (of the borehole): Drilling with very little weight on the drill bit in the hope of preventing the bit from drifting from the vertical and drilling a crooked hole. Fanning the bottom, however, is considered detrimental to the drillstring by some authorities as reduced weight on the bit causes more tension on the drill pipe resulting in pipe and collar fatigue.

Farmin: An arrangement whereby one oil operator buys in or acquires an interest in a lease or concession owned by another operator on which oil or gas has been discovered or is being produced. Often farmins are negotiated to assist the original owner with development costs and to secure for the buyer a source of crude or natural gas.

Farmout: The name applied to a leasehold held under a farmout agreement.

Farmout Agreement: A form of agreement between oil operators whereby the owner of a lease who is not interested in drilling at the time agrees to assign the lease or a portion of it to another operator who wishes to drill the acreage. The assignor may or may not retain an interest (royalty or production payment) in the production.

Field: A geographical area encompassing a group of oil or gas wells.

Field Compression Test: A test to determine the gasoline content of casinghead or wet gas.

Field Potential: The producing capacity of a field during a 24 hour period.

Fish: Anything lost down the drillhole; the object being sought downhole by the fishing tools.

Fluid Loss: A condition downhole in which a water base drilling mud loses water in a highly permeable zone, causing the solids in the drilling fluid to build up on the wall of the borehole. This buildup of mud solids can result in stuck pipe, which often arises when the hydrostatic head or mud pressure is considerably higher than the formation pressure.

Force Majeure Clause: A lease clause providing that cessation or failure of production shall not cause automatic termination of the leasehold, and that the performance of lessee's covenants shall be excused when the failure of production or performance of covenants is owning to causes set forth in the clause. Such clauses usually list acts of God; adverse weather; compliance with federal, state, or municipal laws; wars; strikes; and other contingencies over which the lessee has no control.

Formation: A strata of rocks named after its geographical location and dominant rock type or combination of rock types.

Fracing Fluid: A slurry or foam that carries propant material in suspension downhole under very high pressure to fracture and prop open the small cracks and fissures made in the producing formation by intense pressure. After the propant material (sand grains or microscopic beads) is in place, the pumping of the fracing fluid is discontinued, allowing the fluid to drain out of the formation, leaving to propant behind to hold open the cracks.

Fungible: Products that are or can be commingled for the purpose of being moved by product pipeline. Interchangeable.

Gas-Cut Mud: Drilling mud aerated or charged with gas from formations downhole. The gas forms bubbles in the drilling fluid, seriously affecting drilling operations and sometimes causing loss of circulation.

Gross Acres or Gross Wells: The total number of acres or wells, as the case may be, in which a working or royalty interest is owned.

Jars: A tool for producing a jarring impact in cable tool drilling, especially when the bit becomes stuck in the hole.

Jet Charges: Shaped explosive charges used in perforating casing downhole to permit oil and gas to enter the well bore from the producing formation.

Jetting: Injecting gas into a subsurface formation for the purpose of maintaining reservoir pressure.

Joint: A length of pipe, casing, or tubing usually from 20 to 30 feet long. On drilling rigs, drillpipe and tubing are run the first time (lowered into the hole) a joint at a time; when pulled out of the hole and stacked in the rig, they are usually pulled two, three, or four at a time, depending on the height of the derrick. These multiple-joint sections are called stands.

Junk Basket: A type of fishing tool used to retrieve small objects lost in the borehole or down the casing.

Kelly Joint: The first and the sturdiest joint of the drill column; the thick-walled, hollow steel forging with two flat sides and two rounded sides that fit into a square hole in the rotary table which rotates the Kelly joint and the drill column. Attached to the top of the Kelly or grief stem is the swivel and mudhose.

Kelly Valve, Lower: An automatic valve attached to the lower end of the Kelly joint that opens and closes by mud pressure. The purpose to the valve is to prevent the mud in the Kelly joint from pouring out on the derrick floor each time the Kelly is disconnected from the drillpipe. When the mud pump is stopped, the Kelly valve automatically closes. After a joint of drillpipe is added to the string and the Kelly is made up tight, the pumps are started and the mud pressure opens the Kelly valve and drilling resumes.

Kick: Pressure from downhole in excess of that exerted by the weight of the drilling mud, causing loss of circulation. If gas pressure is not controlled by increasing the mud weight, a kick can violently expel the column of drilling mud resulting in a blowout.

Kill A Well: To overcome downhole pressure in a drilling well by the use of drilling mud or water. One important function of drilling mud is to maintain control over any downhole gas pressures that may be encountered. If gas pressure threatens to cause loss of circulation or a blowout, drilling mud is made heavier (heavied up) by the addition of special clays or other material.

Landing Casing: Lowering a string of casing into a hole and setting it on a shoulder of rock at a point where the diameter of the borehole has been reduced. The beginning of the smaller-diameter hole forms the shoulder on which the casing is landed.

Landowner Royalty: A share of the gross production of the oil and gas on a property by the landowner without bearing any of the cost of producing the oil or gas. The usual landowner's royalty is one-eighth of gross production.

Lease: (1) The Legal instrument by which a leasehold is created in minerals. A contract that, for a stipulated sum, conveys to an operator the right to drill for oil and gas. The oil lease is not to be confused with the usual lease of land or a building. (2) The location of production activity; oil installations and facilities; location of oil field office, tool house, garages.

Lessee: The person or company entitled, under a lease, to drill and operate an oil or gas well.

Liner: In drilling, a length of casing used downhole to shut off a water or gas formation so drilling can proceed. Liners are also used to case a "thief zone" where drilling fluid is being lost in a porous formation.

Location Damages: Compensation paid by an operator to the owner of the land for damages to the surface or to crops during the drilling of a well. Mud pits must be dug, a surface leveled for tanks and rigs, and access roads built, so there are always some location damages to be paid.

Looping a Line: The construction of a pipeline parallel to an existing line, usually in the same right-of -way, to increase the throughout capacity of the system; doubling a pipeline over part of its length, with the new section tied into the original line.

Lose Returns: Refers to a condition in which less drilling mud is being returned from downhole than is being pumped in at the top. This indicates that mud is being lost in porous formations, crevices, or a cavern.

Loss of Circulation: A condition that exists when the drilling mud pumped into the well through the drillpipe does not return to the surface. This serious condition results from the mud being lost in porous formations, a crevice or a cavern penetrated by the drill.

Making Hole: Progress in drilling a well, literally.

Net Acres or Net Wells: The fractional working or royalty interest in property or wells. The gross acres or wells are multiplied by the fractional interest.

Nipple Up: To put together fittings in making a hook up; to assemble a system of pipe, valves, and nipples as in a Christmas tree.

Oil Pool: An underground reservoir or trap containing oil. A pool is a single, separate reservoir with its own pressure system so that wells drilled in any part of the pool affect the reservoir pressure throughout the pool. An oil field may contain one or more pools.

Open Flow: The production of oil or gas under wide-open conditions; the flow of production from a well without any restrictions (valves or chokes) on the rate of flow. Open flow is permitted only for testing or clean-out. Good production practice is to produce a well under maximum efficient rate conditions.

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.

Overshot: A fishing tool; a specially designed barrel with gripping lugs on the inside that can be slipped over the end of tubing or drillpipe lost in the hole.

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 run on the tubing or the casing, and when in position can be expanded mechanically or hydraulically against the pipe wall or the wall of the well bore.

Perforating: To make holes through the casing opposite the producing formation to allow the oil or gas to flow into the well. Shooting steel bullets through the casing walls with a special downhole "gun" is a common method of perforating.

Permeability: A measure of the resistance offered by rock to the movement of fluids through it. A "tight" formation lacks sufficient permeability to allow oil to flow from pores within the rock into the well bore.

Pinch Out: The gradual, vertical "thinning" of a formation, over a horizontal or near-horizontal distance, until it disappears.

Plug: To fill a well's borehole 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; to stop the flow of oil or gas.

Pooling: The bringing together of small, contiguous tracts, resulting in a parcel of land large enough for granting a well permit under applicable spacing regulations.

Production: Oil or gas removed from the ground. Gross production is the production before deducting royalties. Net production equals the gross production, less royalties, multiplied by the fractional working interest.

Productive or Producing Well: A well capable of producing oil or natural gas in economic quantities.

Prospect: A promising area where exploratory wells could be drilled.

Protective String: A string of casing used in very deep wells and run on the inside of the outermost casing to protect against the collapsing of the outer string from high gas pressures encountered.

Proved Reserves: The estimated quantities of crude oil, natural gas and natural gas liquids that have been discovered and determined to be economically recoverable but remain in the ground.

Pulling the Casing: Removing the casing from the hole after abandoning the well. Prior to plugging the well with mud and cement, as much of the casing as can be pulled is retrieved. It is rare that all the casing can be removed from the hole. Often part of the string must be cut off and left in the hole.

Rathole: (1) A slanted hole drilled near the well's borehole to hold the Kelly joint when not in use. The Kelly is unscrewed from the drillstring and lowered into the rathole as a pistol into a scabbard.

Recompletion: The modification of an existing well so that oil or gas can be produced from a different formation.

Reef: A type of reservoir trap composed of rocks, usually limestone, made up of the skeletal remains of marine animals. Reef reservoirs are often characterized by high initial production that falls off rapidly, requiring pressure maintenance techniques to sustain production.

Reservoir: A porous, permeable sedimentary rock formation containing oil and/or natural gas. A structural trap; a stratigraphic trap.

Rheology: The science that treats of the flow of matter. Rheology in drilling refers to the makeup and handling of a drilling mud circulation system; drilling mud control and characteristics.

Royalty or Royalty Interest: An interest in a property that entitles the mineral rights owner to a share of the value of the oil and/or natural gas produced on the property free of production costs.

Seismic: A tool for identifying oil or gas accumulations. Energy waves or sound waves are sent into the earth, and the wave reflections are recorded. Results can indicate the type, size, shape and depth of subsurface rock formations. 2D seismic provides two-dimensional pictures, and 3D seismic provides three-dimensional pictures.

Shut Down/Shut-in-Well: A well is shut down when drilling ceases, which can happen for a variety of reasons: failure of equipment, waiting on pipe, waiting on cement, waiting on orders from the operator, etc. A well is shut-in when its wellhead valves are closed, shutting off production. A shut-in well often will be waiting on tankage or a pipeline connection.

Source Rocks: Sedimentary formations where nearly all the world's petroleum has been found. Nearly 60% of the world's petroleum reserves are in sandstone; the other 40% are in limestone, dolomite, et al.

Swab: To clean out the borehole 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 borehole.

Tool Pusher: A supervisor of drilling operations in the field. A tool pusher may have one drilling well or several under his direct supervision. Drillers are directed in their work by the tool pusher.

Undeveloped Acreage: Lease acreage on which wells have not been drilled or completed to a point that would permit the production of commercial quantities of oil or gas.

Working Interest: The interest in an oil or gas lease that gives the owner the right to drill, produce and conduct operating activities on the property. The working-interest owner is entitled to a share of the production, subject to royalties, costs and risks.

Workover: Remedial work on a well to enhance or restore production.

Zone: An interval of a subsurface formation containing one or more reservoirs; that portion of a formation of sufficient porosity and permeability to form an oil or gas reservoir.

Zone Isolation: A method of sealing off, temporarily, a producing formation while the hole is being deepened.

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