Drilling Glossary

Abnormal Pressure

1. n. [Geology]

A subsurface condition in which the pore pressure of a geologic formation exceeds or is less than the expected, or normal, formation pressure. When impermeable rocks such as shales are compacted rapidly, their pore fluids cannot always escape and must then support the total overlying rock column, leading to abnormally high formation pressures. Excess pressure, called overpressure or geopressure, can cause a well to blow out or become uncontrollable during drilling. Severe underpressure can cause the drillpipe to stick to the underpressured formation.

See: compaction,  geopressure gradient,  geostatic pressure,  hydrostatic pressure,  normal pressure,  pressure gradient

2. n. [Drilling]

Reservoir pore fluid pressure that is not similar to normal saltwater gradient pressure. The term is usually associated with higher than normal pressure, increased complexity for the well designer, and an increased risk of well control problems. Pressure gradients in excess of around 10-lbm/galUS equivalent fluid density [0.52 psi/foot of depth] are considered abnormal. Gradients below normal are commonly called subnormal.

See: barite,  blowout,  well control

Adjustable Choke

1. n. [Drilling]

A valve usually used in well control operations to reduce the pressure of a fluid from high pressure in the closed wellbore to atmospheric pressure. It may be adjusted (opened or closed) to closely control the pressure drop. Adjustable choke valves are constructed to resist wear while high-velocity, solids-laden fluids are flowing by the restricting or sealing elements.

See: choke,  choke line,  choke manifold,  well control

2. n. [Well Completions]

A valve, located on or near the Christmas tree that is used to control the production of fluid from a well. Opening or closing the variable valve influences the rate and pressure at which production fluids progress through the pipeline or process facilities. The adjustable choke is commonly linked to an automated control system to enable the production parameters of individual wells to be closely controlled. 

See: choke