GLOSSARY

 

ABANDONMENT

Final plugging of wells, and/or permanent dismantling, etc. of a production platform or other installation.

AMERICAN PETROLEUM INSTITUTE (API)

API is the world's foremost authority on oil industry standards and practices. "API Gravity" is a reference system for the density of crude oils and constituent hydrocarbons.

ANTICLINE

A geological structure sometimes described as a dome or inverted saucer. Such a structure would be an anticline with "four way dip closure" which if covered by an impermeable layer of rock would make a potential oil or gas reservoir. Not all anticlines are this "perfect" shape.

APPRAISAL WELL

A well drilled to confirm the size or quality (commercial potential) of a hydrocarbon discovery. Before development, a discovery is likely to need at least two or three such wells.

ASSOCIATED GAS

Natural gas found as part of or in conjunction with other constituents of crude oil, as opposed to gas found on its own. The expression has come to include natural gas necessarily produced along with crude oil.

BACK OFF

• In drilling, to pull the drill-string out of, or partly out of, the borehole.
• To unscrew a joint of drillpipe.
• To slacken off a line or block.

BARREL/BARRELAGE

42 US Gallons (approximately 159 litres). The traditional unit of measure of oil volume.

BIT

A drilling bit. Those chiefly in use are the steel roller-cutter, and the diamond-insert bit and PDC bit for hard formations, which penetrates by scratching or abrading the rock rather than by crushing or pulverising like the roller bit. There is also the annular diamond-insert core bit, for cutting and retrieving rock samples (in conjunction with a core barrel).

BLOWOUT

Uncontrolled or uncontrollable release of downhole pressure upward through the well-bore or casing. Although the main danger is fire, the gases are also toxic, and in floating operations a gas blowout may include a threat to the stability of the rig itself. (See Mud)

BLOWOUT PREVENTER (BOP)

An emergency shut-off valve installed on the wellhead during drilling or testing of a well, which incorporates hydraulic pipe rams capable of closing the space around the drillpipe against very high pressures.

BOTTOMS UP

Circulation of drilling fluid in a well, until the bottom hole mud and cuttings reach the surface.

BRIDGE PLUG

A down hole packer assembly used in a well to seal off or isolate a particular formation for testing, acidizing, cementing, etc. also a type of plug used to seal off a well temporarily while the wellhead is removed.

CALIPER

A tool for checking casing in a well for deformation before e.g. running drilling tools, which might become stuck, or packers which might leak.

CASING ETC.

The steel pipes with which a well is lined, for protection against collapse of the borehole, and unwanted leakage into or from rock formation, or at the surface. "Joints" of casing are around 33ft/10metres long and are normally screwed together as they are run into the well. It is normal to set large diameter casing, (such as 20") called the Conductor (or surface) Pipe after the well has penetrated the layers nearest the surface, and cement it into place, after which the drilling continues with a smaller diameter bit, etc.

The next string of casing, the "Surface String" is cemented inside the previous string and down to the new Casing Point which may be at, say 1,000/1,500m and forms the base for the wellhead. This is followed by one or more intermediate strings depending on the target depth and expected conditions in the well. Finally the Inner/Production String is set and cemented through the reservoir zone, and perforated to allow hydrocarbons to enter the well.

CASING POINT

The depth of the lower end of a string of casing.

CASING SHOE

A reinforced section of casing run into a well at the lower end of a string, to protect against bucking or deformation.

CEMENT ETC.

Cement is used to "set" casing in the well bore and seal off unproductive formations and apertures.

CHOKE

An aperture restricting flow in a well or flowline.

CHRISTMAS TREE

The manifold, or arrangement of pipework connections and valves which is installed on the wellhead prior to production. As well as outlets for production, the tree will provide for the injection of mud to "kill" the well, and for the insertion of downhole maintenance tools and wirelines.

CIRCULATION (DRILLING)

The passage of fluids, primarily drilling mud, down the interior of the drill-stem and back to surface viz the annulus.

CLOSURE

Four-way (all round) closure or seal is necessary, over the top and down the gradients on the sides of a potential reservoir, before it can trap or retain hydrocarbons. Closure may be structural as in an anticline, or may be partly due to an impermeable fault, or stratigraphic trapping.

COMMERCIAL WELL

A well capable of producing profitably.

COMPLETION

Installation in a well of production tubing and equipment, wellhead and Christmas Tree.

CONDENSATE (GAS CONDENSATE)

Light hydrocarbon fractions produced with natural gas which condense into liquid at normal temperatures and pressures associated with surface production equipment.

CONING

If an oil well is produced at excessive rates the reduction in reservoir pressure may tend to draw up underlying water towards the well in a cone like shape, Like-wise gas can be drawn downwards from an overlying gas cap.

CORE/CORE BARREL

A vertical section of reservoir or other rock taken in drilling a well, for detailed study and analysis. In order to retrieve the core as intact as possible, it is cut from the rock by an annular core bit. The central column of rock passes through the centre of the bit and, as the bit cuts deeper, is received by a hollow cylindrical Core Barrel above the bit, where it is retained and protected by a series of rubber baffles. When the bit has cut deep enough to fill the core barrel, it is withdrawn from the hole and the core extracted. In this way the actual sequence of rock strata is preserved.

CUTTINGS

The small chips or flakes of rock retrieved from a well by the circulation of the mud. They are studied and logged by the well-site geologist.

DARCY

The unit of measurement of rock permeability, i.e. the extent to which it will allow a fluid to flow through it.

The permeability of most oil and gas reservoir rocks is measured in millidarcies, (thousandths of a Darcy)

DELINEATION WELL

A name for an appraisal well, usually one drilled specifically to determine the boundary of a discovered reservoir.

DEPTH MAP

A relief map of a sub-surface geological structure where the contours relate to depths from the surface datum level. This is a further interpretation of a seismic time map.

DEVELOPMENT WELL

Any well drilled in the course of extraction of reservoir hydrocarbons.

DEVIATED WELL/HOLE

A well whose path has been deliberately diverted from the vertical. Although relatively costly to drill, they are used particularly offshore to reach distant parts of a reservoir from a single platform. Deviated, or directional drilling up to 60º to 70º from the vertical is now fairly common. Greater deviation is possible with special equipment.

DIFFERENTIAL PRESSURE

The difference between the pressure in a well due to the mud column and the pressure in the surrounding rock at any point. See also Sticking.

DIP (DIPMETER)

The inclination from the horizontal of the to surface of a geological structure. A Dip Meter indicates dip relative to a well bore.

DOWN DIP

An area of a structure where the top of the formation is lower than the point under consideration.

DOWN HOLE

Down a well. The expression covers any equipment, measurement, etc., in a well or designed for use in one.

DOWNTIME

A period when any equipment is unserviceable or out of operation for maintenance etc.

DRILL PIPE

Pipe, usually of 3.5 in. to 5 in. outside diameter, which is supplied in "joints" normally of around 33 ft./10m. in length, each being fitted with thicker, or "up-set" reinforced threaded couplings at each end, "male and female" or "pin and box" respectively. To save time, drill pipe in use but not in the well is stacked in stands.

DRY HOLE

An unsuccessful well. Sometimes called a "Duster".

EXPLORATION/EXPLORATION WELL

Exploration is the process of identifying a prospective hydrocarbon region and structure, mainly by reference to regional, and specific, geochemical, geological and geophysical (seismic) surveys. An Exploration Well is a well drilled to test a potential but unproven hydrocarbon trap or structure where good reservoir rock and a seal or closure combine with a potential source of hydrocarbons.

FACIES

In geology, the composition and characteristics of a rock formation.

FAULT/FAULT BLOCK

A discontinuity in a rock formation caused by fracturing of the earth's crust. In oilfield terms a Fault Block is a compartment of a rock formation surrounded or partly surrounded by faults, which may have sealed in hydrocarbons separately from the rest of the formation.

FILTER CAKE/FILTRATE

Build up of mud solids or filtrate on the wall of a well. This helps seal and stabilize the rock face, but too much can cause sticking of the drill string. See also Differential Pressure.

FISH/FISHING

Any unwanted object down a well, commonly the lower end of a drill string which has broken off. "Fishing" is trying to recover the Fish, using various attachments to the drill stem or wireline, known as fishing tools.

FLOWING BOTTOM HOLE PRESSURE

Bottom hole pressure measured at a given flow rate.

FOOTAGE/FOOTAGE RATE

Penetration rate in drilling. Footage Rate may also be a form of remuneration under a drilling contract. Often referred to as ROP (Rate of Penetration).

FORMATION

A rock deposit or structure of homogeneous origin and appearance.

FORMATION DAMAGE

Damage to the reservoir rock around a well due to e.g. plugging with mud, or infiltration by water from the well.

FRACTURING

The process of cracking open the rock formation around a well bore to increase productivity. This is normally done by applying hydraulic pressure down the well bore.

GAS KICK

Increase of down hole pressure above that exerted by the column of drilling fluid in a well, allowing gas to escape to the surface. If not controlled this could develop into a blowout.

GAS/OIL RATIO (GOR)

The proportional amount of gas to oil liquid occuring in production from a reservoir, usually expressed as cubic feet per barrel.

IMPERMEABLE ROCK

A rock with restricted or poorly-communicating pore spaces, such that hydrocarbons will not flow through it.

IN PLACE

Description of the total hydrocarbon content of a reservoir, as distinct from 'Reserves' which can be 'recovered' or produced. Oil or gas in place (OIP, GIP) before the start of production is known as Oil or Gas Originally in Place or Initially in Place. (e.g. STOOIP = Stock Tank Oil Originally in Place; GIIP = Gas Initially in Place).

JARS

Down hole tools inserted in the drill-string when fishing to jerk or jar the fish free by repeated sudden blows. They may also be used while drilling to avoid the drill-string becoming stuck.

JOINT

A single length of pipe. Coupled or welded to other lengths, joints become a string. The term joint may also refer to the couplings or joints themselves.

JUNK

Any unwanted object 'lost' down a well.

LITHOLOGY

The study of rocks and hence the description of different formations encountered by a well.

LOG/LOGGING ETC

There are various applications, but chiefly:

• Written chronological records such as shift logs, maintenance logs, tour sheets, and mud logs which also record cuttings recovered from a well
• Various devices for taking measurements of formations, physical conditions, and fluids encountered by a well, together with the records produced by them. The main types of well logs are Electrical, Magnetic, Mechanical, Sonic, Nuclear.

MAKE UP/BREAK OUT

To assemble/screw together the sections of joints of a string of pipe. 'Breaking out' is the opposite.

MARGINAL

A well, development, etc. whose commercial profitability is in doubt.

MIGRATION

• Hydrocarbons are often found in formations other than those in which their organic source was deposited. This movement often over considerable distances is known as migration.
  
• A process applied to data recorded, e.g. in a "3-D" seismic survey, to adjust for the effects of the "oblique" angle at which it was gathered.

MILLIDARCY

See Darcy

MUD

Mud is 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. (See e.g. Gas Kick). In addition, the mud is continuously circulated down to the bit, and returns in the annular space outside the drill-string, bringing with it rock cuttings for inspection and keeping the well clean. It is also engineered to maintain a thin protective layer of filter-cake on the bore hole wall, without excessive weight which would decrease the weight on the bit and hence penetration ( see Drill String), and also possibly lead to differential sticking and formation damage. Mud is pumped from the mud pit (or tank) via the standpipe, rotary hose and gooseneck to the swivel, and into the drill stem. On return from down hole it is recovered and rock cuttings removed by the shale shakers before re-circulation. A Mud Log is the record of mud make-up and analysis of cuttings recovered.

NODDING DONKEY

The colloquial name for conventional onshore wellhead production beam pumps.

OIL IN PLACE

See In Place

OIL COLUMN/GAS COLUMN

The vertical distance between points of highest and lowest known oil or gas in a reservoir.

OIL/WATER CONTACT

The lower end of the oil column in a reservoir with underlying water. This may be graduated or occur in formations where it is hard to detect.

ON STREAM

When production is flowing, or plant is in operation.

OPEN FLOW

Producing a well without chokes or beans. Unrestricted production normally for testing or maintenance purposes.

OPEN HOLE

An uncased section of well borehole.

PACKER

A seal used to isolate a section of a well, eg for testing or production from one of several formations. Packers are also used in operations such a cementing and acidizing.

PAY ZONE/HORIZON

A formation containing producible hydrocarbons.

PERFORATION/PERFORATING GUN

Holes punched in the casing of a well at the pay zone to be produced, to allow oil or gas to enter the well.

A Perforating Gun is a cylindrical tool loaded with explosive charges which are triggered opposite the pay zone, perforating the casing in many places.

PERMEABILITY

The degree to which a body of rock will permit a fluid to flow through it. It is a function of the shape of the capillary pore spaces and the degree to which pores are connected.

PETROLOGY

The study of rocks, their origin, chemical and physical properties and distribution.

PINCH OUT

The thinning out and disappearance over a distance of a formation, eg an oil bearing sandstone between layers of impermeable rock.

PIPE RACK

Where stands of drill pipe are stacked vertically in a derrick ready for use. Racks or frames are also sometimes used to store tubulars horizontally in yards and or offshore decks, and when transporting them offshore.

PIPE RAMS

Hydraulic rams in a blowout preventer which are shaped to fit around the drill-stem and seal the annulus. Blind Rams are designed in extreme emergency to shear through the drill pipe and seal the well completely.

PLUG/PLUG AND ABANDON

To seal a well, or part of a well with cement, e.g. before producing from a higher formation, sidetracking, or leaving the well permanently sealed and abandoned.

POOL

An Oil Pool is a reservoir or group of reservoirs sharing a common pressure system.

POROSITY

The volume of free space between the grains of a rock capable of holding fluid, (gas or liquid). It is expressed as a percentage of total gross rock volume.

PRESSURE BOMB

A down hole pressure recording capsule used in well monitoring.

PRODUCTION CASING STRING

The innermost steel lining of a well cemented in place and perforated for production in the pay zone. Note that production tubing is inserted inside this casing.

PRODUCTION TESTING

A production test concerns the capability to produce (productivity) of a well and its effects on the reservoir produced. A production test may continue for several months where extensive data is necessary prior to final commitment to development expenditures etc.

PRODUCTION TUBING STRING

The string of pipe installed inside the casing of a production well, to a point just above the reservoir through which the fluids are produced. It may be 2" to 5" diameter or more, depending on the production flow and pressures anticipated.

PRODUCTION WELL/PRODUCER

A development well specifically for the extraction of reservoir fluids.

PRODUCTION WELLHEAD AND TREE

The assembly of casing head, tubing head, connections and well-control valves fitted to a producing well. The "Christmas Tree" is the name given the complete assembly of valves, connecting flanges etc.

PRODUCTIVITY/PRODUCTIVITY INDEX

The continuous productive capacity of a well. The Index is measured as volume produced (e.g. barrels per day) divided by the drop in pressure (p.s.i.) to achieve that flow rate starting with a "shut in" pressure.

PULLING OUT

Retrieving and stacking the drill-string on reaching target depth.

PUP JOINT

A joint of pipe of non-standard length, to make up a string of tubulars to an exact required total length.

RATHOLE

A hole in the drilling floor in which the kelly joint is kept when not in use.

RECOVERY FACTOR

The ratio between the volumes of oil and /or gas produced and produceable from a reservoir and the oil and/or gas originally in place.

RE ENTRY

Inserting the drilling, testing or logging string etc. into the wellhead.

RESERVES/RECOVERABLE RESERVES

Oil or gas that it is anticipated can be produced. Technical Reserves are theoretically producible at a gross operating margin by e.g. normal primary or secondary recovery methods, while Commercial Reserves are restricted to volumes recoverable at an acceptable profitability. The detailed definitions are important in oil and gas financial and other information.

RESERVOIR

A porous, fractured or cavitied rock formation with a geological seal forming a trap for producible hydrocarbons. A common exploration maxim is that a prospective target must possess a related Source rock, Structure and Seal.

RESERVOIR PRESSURE

The pressure at reservoir depth in a shut-in well.

REWORKING A WELL

Maintenance work on a well to stimulate production. This may involve cleaning out silt deposits etc., or stimulation techniques such as fracturing or acidizing.

RIG

A collective term to describe the permanent equipment needed for drill a well. It has come to include the onshore and offshore vehicles, mobile platforms, or vessels on which the equipment is installed.

ROLLER BIT

A rotary drilling bit which penetrates by pulverising the rock with its toothed wheels.

ROTARY TABLE

The heavy turntable at the centre of a drilling-rig floor, which is rotated by the main rig power supply, and in turn rotated the kelly.

ROUND TRIP

Recovering the drill string from the bottom of the well to the surface and returning it to continue drilling. This may be e.g. to replace the bit. "Tripping" is arduous and interrupts "making hole".

SEAL

See Reservoir. An impermeable fault or stratum of rock beneath or behind which hydrocarbons can accumulate.

SECONDARY POROSITY

Porosity developed after the original deposition of a formation, for instance by the action of water on soluble components of the rock or well stimulation techniques.

SEPARATOR

A process vessel used to separate gases and various liquids in a hydrocarbon stream. A Wellhead Separator is the first process vessel in a production operation, operating at or near wellhead pressures.

SHALE SHAKERS

See Mud. Screens for extracting rock cuttings from circulating drilling mud.

SHOE

The strengthened fitting on the lower end of a string of casing to protect the tubulars and help direct the cement to the annulus.

SHOW

Evidence of hydrocarbons in a formation such as fluorescence in cuttings or gas in the mud returning from down hole.

SHUT-IT PRESSURE

The pressure in a shut-in well; static pressure.

SIDE-TRACK/SIDE-TRACKED WELL

A well re-drilled from an intermediate depth. Wells are re-directed or sidetracked for various reasons, but usually because of technical problems deeper in the original well.

SIDEWALL CORING

Obtaining rock samples from the sides of a well bore using a special tool.

SOLUTION GAS

See Associated Gas

SPUD

To start drilling a new well (or re-start)

SQUEEZE

Inserting cement under pressure into the poorly sealed annulus of a well, past the existing material.

STAND OF PIPE

A short length of drill pipe pre-assembled to save time in operations and stored vertically in the derrick ready for use, usually in lengths of three joints.

STICKING

Jamming of the drill string in the well borehole caused by usually a high differential pressure and a build-up of mud solids on the rock face.

STRADDLE PACKER

A rubber packer or seal isolating a section of a well for production testing of the formation in question.

STRING

See Joint, Drill-string, Casing, etc. Any number of connected joints of tubulars run in the well.

STRUCTURE

• A geological formation which, if sealed against leakage, could be a potential structural or stratigraphic trap or hydrocarbons.
• A man-made load-bearing construction, such as an offshore platform, usually designed by structural engineers.

SUSPENDED WELL

A well, usually a successful discovery well which is left temporarily sealed or plugged to be re-entered for further testing or for production purposes.

SWABBING

Reducing pressure in a well to clean or stimulate it.

TIME MAP

A contoured map of a subsurface geological formation based on the time taken to reflect seismic impulses rather than the subsequently computed/interpreted depths on a depth map.

TIE-IN

The action of connecting one pipeline to another or to equipment. Hence "pipeline tie in" commonly describes the connection itself.

TOOLPUSHER

The chief engineer of a drilling crew, with overall responsibility for the rig in the field.

TOTAL DEPTH (T.D)

• The target depth for a well.
• The achieved (drilled) depth in a well at any one time.

TOUR

• Drilling or other shift - usually 12 hours
• Any longer period of duty, such as say, 2 weeks offshore, or a period of assignment to a remote or foreign location.

TRIP GAS

High pressure gas in a well which must be carefully controlled when withdrawing the drill string (making a trip).

TWIST OFF

Breaking the drill pipe due to fatigue or excessive torque.

UPDIP

An area of a structure where the top of the formation is higher than the point under consideration.

VISCOSITY/VISCOUS

The resistance of a fluid to flow, due to the mutual adherence of its molecules.

WATER SATURATION

The proportion of water in the pore spaces of a reservoir. See Porosity.

WELLHEAD

"Wellhead" is descriptive of a location or function rather than a specific item of equipment. See Production Wellhead.

WELL TESTING

Testing in an exploration or appraisal well is directed at estimation of reserves in communication with that well, in addition to well productivity.

WILDCAT WELL

An exploration well in an unproved area.

WIRE LINE

A system in which a flexible cable and reel is used to lower log or maintenance equipment down a well, rather than a rigid drill string, with considerable savings of equipment, manpower and time.

WORKOVER

A maintenance job on a well usually to replace equipment or to stimulate production.

ZONE

The interval between two depths in a well containing a reservoir or other distinctive characteristics.