Plain-language definitions of common terms used in drilling, geoscience, reservoir engineering and HSE. Choose a letter to jump to it.
A
- API gravity
- A measure of how heavy or light a petroleum liquid is compared with water. Higher API gravity means a lighter oil; above 10° it floats on water.
- Annulus
- The space between two concentric tubulars, or between the drill string or casing and the wall of the hole. Drilling fluid returns to surface through it.
- Anticline
- An arch-shaped fold in rock layers. Anticlines are among the most common structural traps for oil and gas.
B
- Barrel (bbl)
- The standard volume unit for oil: 42 US gallons, or about 159 litres.
- Bottomhole assembly (BHA)
- The lower part of the drill string: the bit, drill collars, stabilisers and tools such as MWD and mud motors.
- Blowout
- An uncontrolled release of formation fluids from a well after pressure-control systems have failed.
- Blowout preventer (BOP)
- A stack of large valves on the wellhead that can seal the well during drilling to stop an uncontrolled flow.
C
- Casing
- Steel pipe run into the hole and usually cemented in place. It supports the hole wall, isolates zones and provides pressure integrity.
- Cementing
- Pumping cement slurry into the annulus between casing and the hole to hold the casing and seal off formations.
- Christmas tree
- An assembly of valves and fittings on top of a completed well that controls the flow of produced fluids.
- Completion
- The equipment and operations that prepare a drilled well for safe, efficient production or injection.
- Core
- A cylindrical sample of rock cut from the formation, used to measure rock and reservoir properties directly.
D
- Directional drilling
- Deliberately steering a well away from vertical to reach a target, including deviated, horizontal and multilateral wells.
- Drill string
- Drill pipe, heavyweight pipe and the BHA joined together. It turns the bit and carries drilling fluid down the well.
- Drilling fluid (mud)
- The fluid circulated through the well. It cools the bit, carries cuttings to surface, controls formation pressure and keeps the hole stable.
E
- Equivalent circulating density (ECD)
- The effective density the formation feels while fluid is circulating. It equals the mud weight plus the annular friction pressure, expressed as a density.
F
- Formation
- A body of rock with consistent characteristics that can be mapped and given a name.
- Fracture gradient
- The pressure per unit depth at which a formation breaks down and takes fluid. It sets the upper limit of the drilling window.
H
- HAZOP
- Hazard and Operability study: a structured team review of a process or design that identifies hazards and operability problems.
- Hydrocarbon
- A compound made of hydrogen and carbon. Crude oil and natural gas are both mixtures of hydrocarbons.
- Hydrogen sulphide (H₂S)
- A toxic, flammable gas found in some reservoirs. It needs special detection, protective equipment and materials.
K
- Kick
- An unplanned flow of formation fluid into the wellbore, which happens when well pressure falls below formation pressure. Left uncontrolled, a kick can become a blowout.
L
- Lithology
- The physical character of a rock, such as its mineral content, grain size and texture (for example sandstone, shale or limestone).
- Logging while drilling (LWD)
- Measuring formation properties with tools in the BHA while the well is being drilled.
M
- Measured depth (MD)
- Depth measured along the actual path of the wellbore.
- Mud weight
- The density of the drilling fluid, usually in ppg or SG. It is the main barrier against formation pressure.
- Measurement while drilling (MWD)
- Downhole tools that measure well direction and drilling data and send them to surface in real time.
N
- Non-productive time (NPT)
- Rig time lost to unplanned events such as equipment failure, stuck pipe or waiting.
P
- Permeability
- A rock’s ability to let fluids flow through it, measured in darcies or millidarcies.
- Permit to work (PTW)
- A formal written system that controls high-risk work by defining the hazards, the precautions and who has authorised the job.
- Pore pressure
- The pressure of the fluids in the pore spaces of a formation. It sets the lower limit of the drilling window.
- Porosity
- The fraction of a rock’s volume that is pore space, which can hold oil, gas or water.
- Personal protective equipment (PPE)
- Equipment worn to reduce exposure to hazards, such as hard hats, safety glasses, gloves and coveralls.
R
- Rate of penetration (ROP)
- How fast the bit drills, usually in m/hr or ft/hr.
- Reservoir
- A porous, permeable rock body that holds hydrocarbons trapped beneath a seal.
S
- Seismic survey
- Sending sound energy into the ground and recording the reflections to image the rock layers below the surface.
- Source rock
- An organic-rich rock, often shale, that has produced hydrocarbons under heat and pressure.
- Stuck pipe
- When the drill string can no longer be moved up, down or rotated. It is a leading cause of NPT.
T
- True vertical depth (TVD)
- The vertical distance from the surface reference to a point in the well. It is used for all pressure calculations.
- Trap
- A geological arrangement (structural or stratigraphic) that stops hydrocarbons from migrating further and lets them accumulate.
- Tripping
- Pulling the drill string out of the hole or running it back in, for example to change a bit.
U
- Underbalanced drilling (UBD)
- Drilling with wellbore pressure deliberately kept below formation pressure, so that reservoir fluids flow to surface under control while drilling.
W
- Well control
- The methods and equipment used to keep formation pressure under control, to prevent kicks and to handle them safely when they happen.
- Wellbore
- The drilled hole itself, including any open-hole and cased sections.
- Workover
- Major maintenance or remedial work on an existing well to restore or improve production.
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