Glossary

D-exponent

drilling exponent. Unitless number derived from rate of penetration normalised for the effect of weight on a bit, hole diameter, rotary speed and mudweight. Can be indicative of changes in formation pore pressure gradient or lithology.

Daisy chain

the process by which a cargo of oil or oil products is sold many times before being delivered to the customer

Delineation well

an exploration well drilled as part of a carefully planned programme with the objective of appraising the value of an oil or gas discovery. Delineation, or step-out wells, are drilled so that the probable outline of the oil or gas field may be delineated.

Derrick

steel structure mounted over the borehole to support the drill pipe and other equipment which is lowered and raised during drilling operations

Derrick floor

a raised dais at the base of the derrick carrying the drill table; often called the drilling or rig floor.

Development drilling

extension of a drilling programme once a strike has been established and proved to be capable of economic production. Development wells, which may be vertical or deviated, are usually drilled from permanent platforms. Some development wells may be production wells, others injection wells.

Development phase

the phase in which a proven oil or gas field is brought into production by drilling production wells.

Deviated well or directional well

borehole that is intentionally deviated from vertical in order to reach some remote or inaccessible subsurface target.

DF

drill floor; see BDF.

Dip

a measure of the inclination of rock strata with respect to the horizontal.

Distillates

the products of condensation during the fractional distillation process (gaseous fuels, naphtha, gasoline, kerosene and gas oils.

Distillation

(fractional distillation) a process based on the difference in boiling points of the liquids in the mixture to be separated. Successive vaporisation and condensation of crude oil in a fractionating column will separate out the lighter products, leaving a residue of fuel oil or bitumen. Distillation is carried out in such a way as to avoid any cracking. It is the basic process that takes place in an oil refinery.

Diurnal storage

literally, daily storage. Refers to short-term or peak storage in pipelines or gas holders, as opposed to seasonal storage.

Doghouse

this name was originally given to a shelter alongside a land-based rig, but offshore installations also have their doghouses -usually a small room on the edge of the rig floor used as office, tool store and for coffee breaks. Doghousing' refers to idle conversation, time-wasting at work.

Dogleg

a sharp deviation in direction of the wellbore.

Dope

thread sealing compound used in connecting pipe fittings. On the rig it refers to the metalised grease used on drillpipe, collar and casing connections.

Down dip

a term used where one well is below another well due to inclination of the strata in which the reservoir lies.

Down time

time during which no drilling is possible due to adverse weather conditions, while downhole equipment is being changed, during well logging, etc.

Downhole

in a well bore.

Downhole safety valve

a valve fitted to the production tube of a well some distance below the sea bed in order to permit flow to be stopped in an emergency.

Downstream

those activities which take place between the loading of crude oil at the export terminal and the use of the oil by the end-user. This encompasses the ocean transportation of crude oil, supply and trading, refining and the distribution and marketing of the oil products. See also upstream.

Draw-down

the differences between the static and the flowing bottom-hole pressures in a well

Drawworks

the control centre from which the driller operates the drilling machinery.

Drawworks

the part of the drilling tool that actually cuts through the rock.

Drill collars

lengths of extra-heavy steel tubing located immediately above the drill bit in order to maintain pressure on the bit and keep the drill string in tension.

Drill ship

a ship fitted with a drilling derrick that is used to drill in waters that are too deep for a jack-up or semi-submersible rig.

Drill stem test (DST)

a test whereby the nature and quantity of the formation fluids in a possible oil- or gas-bearing stratum are determined by allowing them to flow to the surface through the drill string under carefully controlled conditions.

Drill string

lengths of steel tubing roughly 10 metres long screwed together to form a pipe connecting the drill bit to the drilling rig. The string is rotated to drill the hole and also serves as a conduit for drilling mud. Also called the drill pipe.

Drilling contractor

a company that undertakes to provide the equipment and manpower needed to drill a well on behalf of an operator.

Drilling crew

the team who actually work on or above the derrick floor during drilling operations. Generally a driller plus four roughnecks.

Drilling engineer

the engineer who designs the casing and is responsible for the drilling programme for a well.

Drilling line

the wire rope connecting the drawworks, crown block and travelling block that is used to raise or lower drill pipe or casing from or into a well.

Drilling mud

a mixture of clays, water and chemicals pumped down the drill string and up the annulus during drilling in order to lubricate the system, carry away rock cuttings, maintain the required pressure at the bit end, provide an aid to formation evaluation, etc.

Drilling platform

an offshore platform used to drill exploration and development wells but lacking the processing facilities of a production platform.

Drilling table

the table on the derrick floor whereby the drill pipe is rotated; also called the rotary table.

Dry gasfield

the production from such a reservoir will yield dry/lean gas and very small quantities of condensate; typically less than 10 barrels per million cubic feet

Dry hole

a well drilled without finding oil or gas.

Dry natural gas

natural gas consisting principally of methane and devoid of readily condensable constituents (low in associated liquids). Dry gas contains less than 0.1 gal natural gas liquid vapours per 1000 cu ft

Dynamic positioning

the method whereby a vessel is kept on station by computer-controlled thruster propellers rather than by anchors.