ABDANDONMENT COST
The costs associated with abandoning a well. Such costs are specified in the authority for expenditure (AFE), and typically cover the plugging of wells; removal of well equipment, production tanks and associated installations; and surface remediation.

ABSORPTION OIL
A light liquid hydrocarbon used to absorb or remove the heavier liquid hydrocarbons from a wet gas stream. Absorption oil is also called wash oil.

ACETIC ACID
An organic acid used in oil- and gas-well stimulation treatments. Less corrosive than the commonly used hydrochloric acid, acetic acid treatments can be more easily inhibited or retarded for treatments of long duration. This is necessary particularly in applications requiring the protection of exotic alloys or in high-temperature wells. In most cases, acetic acid is used in conjunction with hydrochloric acid and other acid additives.

ACID FRAC
A hydraulic fracturing treatment performed in carbonate formations to etch the open faces of induced fractures using a hydrochloric acid treatment. When the treatment is complete and the fracture closes, the etched surface provides a high-conductivity path from the reservoir to the wellbore.

ACQUISTION LOG
The log that is actually recorded while taking the measurements. It is distinct from a playback, which is produced later on from digital data.

ALLOWABLE
The maximum permitted amount of oil or gas a well or field is able to produce per day. It is typically set by state regulating agencies, after considering the economic market for the product, the maximum efficient rate (MIER), and other factors.

ANNULAR FLOW
A multiphase-flow regime in which the lighter fluid flows in the center of the pipe, and the heavier fluid is contained in a thin film on the pipe wall. The lighter fluid may be a mist or an emulsion. Annular flow occurs at high velocities of the lighter fluid, and is observed in both vertical and horizontal wells. As the velocity increases, the film may disappear, leading to mist or emulsion flow. When the interface between the fluids is irregular, the term wavy annular flow may be used.

ANNULAR GAS FLOW
A flow of formation gas in the annulus between a casing string and the borehole wall. Annular gas flows occur when there is insufficient hydrostatic pressure to restrain the gas. They can occur in uncemented intervals and even in cemented sections if the cement bond is poor. After cementing, as the cement begins to harden, a gel-like structure forms that effectively supports the solid material in the cement slurry. However, during this initial gelling period, the cement has no appreciable strength. Hence, with the solid (weighting) material now supported by the gel structure, the effective density of the slurry that the reservoir experiences falls rather suddenly to the density of the mix water of the cement, which is usually freshwater, whose density is 8.34 lbm/gal, or a gradient of 0.434 psi/ft of vertical column height. Various chemical additives have been developed to reduce annular gas flow.

ASSIGNMENT (ASSIGN)
Legal document transferring an interest in a property from one party to another. The receiving party is the ‘assignee’; the transferring party is the ‘assignor’.

BACKFLOW
Fluid flow in the borehole from one zone into another in response to pressure differences between the zones. Any time the wellbore pressure rises above the average pressure in any zone, backflow will occur. Analysis of buildup tests involving backflow is either impossible or extremely difficult and usually requires expert input to determine useful information from such tests.

BARREL (BBL)
The standard unit of oil measurement in the U.S. oil industry. A barrel equals 42 U.S. gallons, or 159 liters. [In some countries oil is measured using the metric system: volumes in liters, or weights in metric tons.]

BATCH TREATMENT
The pumping of a specific amount of treatment fluid, such as cement slurry, stimulation fluid, well completion fluid or chemical corrosion inhibitor.

BED
A layer of sediment or sedimentary rock, or stratum. A bed is the smallest stratigraphic unit, generally a centimeter or more in thickness. To be labeled a bed, the stratum must be distinguishable from adjacent beds.

BED THICKNESS
The thickness of a layer or stratum of sedimentary rock measured perpendicular to its lateral extent, presuming deposition on a horizontal surface. Because sediment deposition can occur on inclined surfaces, apparent or measured bed thickness might differ from true bed thickness. The thickness of a given bed often varies along its extent.

BEHIND PIPE
If a well drills through several pay zones and are completed in the deepest productive reservoir, casing is set all the way down to the producing zone. Viewed from (a perspective) inside the borehole, reserves in the shallower pay zones up the hole are behind the casing (pipe).

BLOWOUT PREVENTERS
Heavy-duty equipment installed at the wellhead during drilling and completion operations, to prevent the possibility of a blowout, by sealing the annular space between the drill pipe and the casing. Pipe rams, shear rams, or blind rams may be activated to choke off flow from the borehole.

BOREHOLE
The hole created by drilling (boring) of a well.

BOTTOM-HOLE PRESSURE
The pressure in a well, measured by an instrument that is lowered into the borehole on a wire line. It may be measured under flowing or shut-in conditions.

CASH DISTRIBUTIONS
Moneys paid by an oil and gas partnership to its partners according to the terms of the partnership agreement. These distributions are net of state severance tax.

CALCIUM TEST
A quantitative analytical procedure for water-mud filtrate and for calcium in a oil-mud.

CASING
Large diameter steel pipe placed in a borehole to support the side or walls of the hole and to prevent them from caving in. It not only prevents the loss (flow) of drilling fluids from the borehole into rocks penetrated by the borehole, but also prevents rock fluids from flowing into the borehole.

CEMENT
Fluid cement is mixed at the surface, pumped to the bottom of a cased well, forced to flow around the lower end of the casing and up into the space between the casing and the borehole. When the cement solidifies (sets), it holds the casing in place, and provides support.

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

CHECK VALVE
A mechanical device that permits fluid to flow or pressure to act in one direction only. Check valves are used in a variety of oil and gas industry applications as control or safety devices. Check valve designs are tailored to specific fluid types and operating conditions. Some designs are less tolerant of debris, while others may obstruct the bore of the conduit or tubing in which the check valve is fitted.

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

COMPLETION (COMPLETE A WELL)
After the drilling of a successful well, the ‘completion’ includes all the work required to make the well ready for commercial production.

“CREEK-OLOGY”
Refers to locating the next well, not strictly on the basis of science and geology, but also in part, on the basis of gross natural features on the surface of the land. (The bend in a creek, for example, may represent the surface expression of a fault or a structural trap deep beneath the surface). A large corporation might never admit it to its shareholders, but even after all the scientific data are collected and analyzed (at considerable expense) the results are often not conclusive; the final decision on where to drill may include a good measure of "creekology".

CRUDE OIL
A naturally occurring mixture of liquid hydrocarbons as it comes out of the ground (before or after any dissolved gas has been separated from it, but prior to any process of distilling or refining). Greenish crude is usually high in paraffin (wax) content; blackish oil is more likely to be asphaltic. Different types of source rock generate different types of crude oils.

DEPLETION
The value of a naturally occurring mineral deposit is a function of (I) the market value of the mineral, and (II) the concentration of the mineral in the deposit. Physical depletion is the exhaustion of a mineral deposit through production of the mineral. Economic depletion is the reduction in the value of the mineral deposit as it becomes exhausted (less concentrated) through production.

DOWNHOLE
Refers to equipment or mechanical operations that take place, down inside a borehole (as contrasted with those at the surface).

DOWN TIME
Time lost during drilling, usually as a result of equipment breakdown.

DRILL BIT
A tool with very tough steel or diamond teeth that grind rock into small chips during drilling. The diameter of the bits used to drill a well may range from more than 22 inches at the upper part of the hole, to less than four inches at total depth.

DRILL PIPE
A special grade of extra strong steel pipe threaded on both ends that comes in lengths of about 30 feet and diameters from about 6½ to 2½ inches.

DRILL-STEM TEST (DST)
When a well is drilled into a potential pay zone, it may be desirable to try to make the zone flow, before continuing to drill ahead. To ‘test’ the zone, drilling is halted, and the string of drill pipe is pulled out of the borehole. The drill bit is removed and a special testing device is attached to the end of the drill string, which is lowered back into the borehole.

DRILLING RIG
The surface equipment used to drill for oil or gas. It consists of a TOWER framework (derrick) which supports the WINCH (pulley system) used to lift and lower drill pipe, a ROTARY TABLE that turns the drill string (and drill bit connected to the drill string), ENGINES to drive the winch and rotary table, and PUMPS to control the flow of the drilling mud.

FEET OF PAY
The thickness of the pay zone penetrated in a well. In the case of an oil column floating on water, it is the thickness of the layer of oil (‘the oil column’) above the oil-water contact.

FORMATION
A layer of rock having characteristics that is distinct and recognizable. The rock layer is thus mappable, even among other layers of similar rocks. The thickness can range from a few feet to hundreds of feet. Distinctive features might include mineral composition; texture, diagnostic plant or animal remains (fossils) contained in the formation, etc.

FRACTURING
A procedure undertaken to attempt to increase the flow of oil or gas from a well. A fluid (usually crude oil,, diesel oil. Or water) is pumped into the reservoir, with such great force that the reservoir rock is physically broken and split open. Usually the ‘frac fluid’ carries small pellets or beads mixed in with it; the idea is for them to get caught in the fractures and prop them open (the beads or pellets are called the propping agent or proppant). As the pumping pressures are gradually released at the surface, the natural reservoir pressures will force the ‘frac fluid’ out of the reservoir, and back into the well. This procedure is also called hydraulic fracturing. ‘To frac a well’ means to hydraulically fracture a reservoir in a well.

HYDROCARBONS
A large class of organic compounds composed of hydrogen and carbon. Crude oil, natural gas, and natural gas condensate are all mixtures of various hydrocarbons, among which methane (CH4) is the simplest.

(Oil and Gas) LEASE
A contract by which the owner of the mineral rights to a property conveys to another party, the exclusive right to explore for and develop minerals on the property, during a specified period of time. The conveying party is the lessor; the recipient is the ‘lessee’.

MUD (DRILLING MUD)
A fluid mixture of clays, chemicals, and weighting materials suspended in fresh-water, salt-water, or diesel oil.

OVERRIDING ROYALTY (‘OVERRIDE”)
A revenue interest in oil and gas, created out of a working interest. Like the lessor’s royalty, it entitles the owner to a share of the property. Responsibilities of the operator and other working-interest owners are enumerated in the joint operating agreement.

PERMEABILITY
The measurable capacity of a rock to allow a fluid to flow through it. Along with porosity, it is one of the two most critical properties of the reservoir rock. To be commercially producible, oil or gas must be able to flow from the reservoir rock into the well.

PETROLEUM
Crude oil. Naturally occurring liquid hydrocarbons from which gasoline, kerosene, and countless other ‘petrochemicals’ are produced. Non-hydrocarbon compounds (often containing sulfur and nitrogen) may also be present. In the broadest sense, petroleum refers to all naturally occurring hydrocarbons including oil, natural gas, condensate and their derived products.

POROSITY
The percentage (by volume) of holes or voids in a rock. Commercially productive reservoir rocks typically have porosity’s ranging from about 5% - 35%. The higher the porosity, the more oil or gas that can be contained in the pore spaces, the better the quality of the reservoir rock.

PROSPECT
The hypothesis that a naturally occurring, commercially exploitable accumulation of oil or gas exists, at a clearly defined underground location. One or more geologic maps describe it. A single well should be sufficient to test the hypothesis. Surface area of a prospect could range from a ten-acre tract enough for a single well, to a tract covering many square miles and requiring dozens of wells to fully develop its reserves.

RESERVOIR
Any rock having enough porosity and permeability to contain appreciable hydrocarbons. Most commercial reservoirs are in sandstone and limestone (limestone/dolomite), rarely in shale or igneous rock.