Short, plain-language definitions of the terms used across The Well Log. Where a term is explained in depth in an article, the definition links to it.
A to C · D to F · G to K · L to O · P to R · S to Z
A to C
Annulus. The space between two tubular or wellbore surfaces, such as between the drill string and the hole wall or between two casing or tubing strings. During conventional drilling, returning fluid flows up the annulus.
APWD (annular pressure while drilling). A downhole measurement of annular pressure while drilling, often used to calculate or verify ECD. Also called PWD.
BHA (bottomhole assembly). The lower part of the drill string: the bit plus the tools above it, such as drill collars, stabilisers, a mud motor or rotary steerable system, and MWD/LWD tools.
BOP (blowout preventer). A pressure-control device used to close and seal a well during a well-control event. A BOP stack typically combines annular and ram-type preventers. On conventional deepwater wells, the subsea BOP stack sits on the seabed and connects to the floating rig through the marine riser.
Casing. Steel pipe run into the well and usually cemented in place to protect the hole drilled so far, isolate formations and provide pressure integrity for the next section.
Casing shoe. The bottom end of a casing string. The formation just below it is often tested (FIT or LOT) after the shoe is drilled out.
Cementing. Pumping cement into the annulus between casing and hole to support the casing and isolate formations from each other.
Completion. The equipment and configuration that prepare a well to produce or inject safely, such as sand-control equipment, packers, production tubing, safety valves and downhole gauges.
Conductor. The first and largest-diameter string in a well, often 30 or 36 in in deepwater designs. It provides the structural foundation for the wellhead and may be jetted, driven or drilled and cemented into place.
Cuttings. Rock fragments produced by the bit and carried to surface by the drilling fluid.
D to F
Differential pressure. The difference between two pressures, for example between wellbore pressure and formation pressure.
Drilling break. A sudden increase in rate of penetration, often caused by a change in formation drillability. It can coincide with porous, hydrocarbon-bearing or pressure-changing intervals, so it is interpreted together with gas, flow and other drilling data.
Drilling fluid (mud). The fluid circulated through the drill string and back up the annulus. It cools and lubricates the bit, transports cuttings, supports wellbore stability and, in conventional overbalanced drilling, provides the hydrostatic pressure used to control formation fluids.
Drilling window. The allowable range of wellbore pressure between a lower limit, set by pore pressure or wellbore stability, and an upper limit, usually set by fracture pressure. Also called the pressure window or mud-weight window.
ECD (equivalent circulating density). The effective density exerted by a circulating fluid against the formation, including the annular friction pressure above the point considered. See Hydrostatic Pressure and ECD.
EMW (equivalent mud weight). A pressure at a given depth expressed as the mud weight that would produce it. Used to compare pore pressure, fracture pressure, mud weight and ECD on one scale.
FIT (formation integrity test). A pressure test after drilling out a casing shoe that confirms the formation can withstand a predetermined design pressure, without necessarily reaching leakoff.
Flow check. Stopping the pumps and observing the well to confirm whether it is static or flowing.
Formation pressure (reservoir pressure). The pressure of the fluids in a formation; in a producing reservoir it is usually called reservoir pressure. See also pore pressure.
Fracture gradient. The pressure gradient at which a formation is expected to begin taking fluid through induced fracturing, commonly expressed in psi/ft, SG or equivalent mud weight.
FPSO (floating production, storage and offloading unit). A floating facility that receives produced fluids, processes them, stores the oil and offloads it to export tankers. FPSOs are widely used in deepwater Angola.
G to K
Gravel pack. A sand-control completion in which gravel is placed around a screen to hold back formation sand while allowing fluids to flow.
Hole cleaning. The removal of cuttings from the well by the drilling fluid. Poor hole cleaning can raise ECD and cause stuck pipe.
Hole enlargement (underreaming). Opening the hole to a larger diameter than the bit, usually with a reamer in the BHA, to gain clearance for the next casing string.
Hole section. A length of hole drilled with one bit size between two casing points, for example the 17 1/2 in section.
Hydrostatic pressure. The pressure exerted by a column of fluid at rest. In oilfield units, 0.052 × mud weight (ppg) × TVD (ft) gives pressure in psi.
Influx. Formation fluid that has entered the wellbore. An influx that requires well-control action is a kick.
Jetting. A method of installing a conductor by pumping fluid through a bit or jetting assembly inside it to loosen and remove soil beneath the conductor while controlled weight advances the pipe into the seabed.
Kick. An unplanned flow of formation fluid into the wellbore, which must be detected and controlled. See Kicks, SIDPP and Kill Mud Weight.
Kick tolerance. The maximum design influx that can be circulated out of the well without exceeding the allowable pressure at the weakest exposed formation, commonly evaluated at the previous casing shoe.
Kill mud weight. The mud density calculated after a kick to provide enough hydrostatic pressure to balance formation pressure and restore primary well control. See Kicks, SIDPP and Kill Mud Weight.
L to O
LOT (leak-off test). A pressure test after drilling out a casing shoe that is continued until the formation starts to take fluid, to estimate its pressure limit.
Lost circulation. Partial or complete loss of drilling fluid from the wellbore into the formation, for example into induced or natural fractures, vugs or highly permeable zones.
LWD (logging while drilling). Measurement of formation properties, such as resistivity and gamma ray, with tools in the BHA while drilling.
MD (measured depth). The length of the wellbore measured along its path. In deviated wells it is greater than TVD.
MPD (managed pressure drilling). An adaptive drilling method used to precisely manage the annular pressure profile within defined formation limits, using techniques such as surface backpressure, fluid density, rheology or circulating friction.
Mud logging. Surface-based monitoring and geological evaluation during drilling, including cuttings description, gas measurements, drilling parameters, pit and flow trends, and correlation of observations with depth.
Mud weight. The density of the drilling fluid, usually in pounds per gallon (ppg) or specific gravity.
MWD (measurement while drilling). Downhole measurement of drilling and directional data, typically inclination, azimuth and toolface and, depending on the system, pressure, temperature and other drilling parameters, transmitted or stored while drilling.
Open hole. A section of the well that has been drilled but not yet cased.
Overbalance. The amount by which wellbore pressure exceeds formation pore pressure.
Overburden stress. The vertical stress at a given depth caused by the weight of the overlying rock and fluids, including the water column offshore.
P to R
PDC bit (polycrystalline diamond compact). A fixed-cutter bit with synthetic diamond cutters that shear the rock. Widely used in modern drilling.
Pore pressure. The pressure of the fluids in the pore space of a rock. If wellbore pressure falls below it in a permeable formation, fluids can flow into the well.
PPFG. Pore-pressure and fracture-gradient profile plotted against depth. These curves form the primary pressure boundaries used to define the drilling window and casing design.
Pressure window. See drilling window.
Primary well control. Preventing influx by maintaining adequate wellbore pressure, usually through the hydrostatic pressure of the drilling fluid and, while circulating, the ECD.
PWD (pressure while drilling). A downhole measurement of annular pressure near the BHA, used to check ECD and hydraulics in real time.
Reservoir drill-in fluid (RDF). A drilling fluid designed for the reservoir section, formulated to limit formation damage and to be cleaned up before production.
Riser (marine riser). The large-diameter pipe that connects a floating rig to the subsea BOP, guiding the drill string and returning mud to the rig.
Riser margin. Additional mud density intended to compensate for the loss of hydrostatic head if the drilling-fluid column in the marine riser were replaced by seawater. In very deep water, the required margin may be limited by formation strength and the available pressure window.
Riserless drilling. Drilling the top sections of a deepwater well before the riser and BOP are installed, with returns released at the seabed.
ROP (rate of penetration). The speed at which the bit deepens the hole, usually in metres or feet per hour.
S to Z
Sand control. Techniques that stop formation sand from entering a producing well, such as gravel packs and standalone screens.
Secondary well control. Detecting a kick, shutting in the well with the BOP system and circulating the influx out using established procedures.
Shale inhibition. The ability of a drilling fluid to limit the reaction of water-sensitive shales, such as swelling and dispersion, to help keep the hole stable.
SIDPP and SICP. Shut-in drillpipe pressure and shut-in casing pressure: surface pressures observed after shutting in a kick. SIDPP is commonly used with the current mud weight and TVD to calculate kill mud weight, while SICP reflects the annular side of the shut-in system.
Spud. The start of drilling a new well.
Subsea tree (christmas tree). The assembly of valves on a subsea wellhead that controls production from or injection into a completed subsea well.
Surge and swab. Transient wellbore-pressure changes caused by pipe movement: running pipe into the hole can increase pressure (surge), while pulling pipe out can reduce pressure (swab). Tripping speeds are managed to limit both effects.
TD (total depth). The final depth of a well or of a hole section.
Tieback. A development concept in which a new well, field or subsea system is connected to existing production infrastructure, such as an FPSO, rather than requiring a new standalone host.
Trip. Pulling the drill string out of the hole, or running it back in, for example to change the bit or BHA.
Tubing hanger. The component that suspends the production tubing in the wellhead or subsea tree system and seals the annulus around it.
TVD (true vertical depth). The vertical distance from a reference point to a point in the well. Hydrostatic pressure depends on TVD, not MD.
Underbalance. A condition in which wellbore pressure is below formation pressure. Underbalanced drilling maintains this condition deliberately, with formation fluids brought to surface under control.
Well control. The practices and equipment used to prevent uncontrolled flow of formation fluids. In conventional drilling, primary well control maintains adequate wellbore pressure to prevent influx; if a kick occurs, secondary well control uses the BOP system and established shut-in and circulation procedures to regain control.
Wellbore stability. The ability of the hole to keep its size and shape without collapsing, breaking down or swelling. It depends on rock strength, in-situ stresses, mud weight and fluid chemistry.
Wellhead. The equipment at the top of the well, on the seabed in subsea wells, that supports the casing strings and provides the interface for the BOP and later the tree.
Further reading
For broader definitions, the SLB Energy Glossary is a widely used industry reference. SPE PetroWiki and API standards are also useful references for drilling, well-control and well-construction terminology.
Suggestions for terms to add are welcome through the Contact page.