Oil and gas glossary

The stack of valves on the wellhead that can close the well if the mud column no longer holds the formation.

The tools on the bottom of the drill string, below the drill pipe.

Steel pipe run into the well and cemented in place to hold the hole open and to isolate formations.

The bottom end of a casing string.

The drill string held against the borehole wall by the difference between mud pressure and a lower pore pressure.

How sharply the wellbore turns, over a standard course length.

The fluid circulated down the drill string and up the annulus while drilling.

The density of a static mud column that would exert the same pressure as the mud does while it is circulating.

The wellbore pressure, per unit of true vertical depth, at which the formation fractures and starts to take drilling fluid.

Formation fluid flowing into the well because pressure in the wellbore has fallen below the pore pressure.

Drilling fluid leaving the wellbore into the formation instead of returning to the pits.

Distance along the wellbore from the surface depth reference to a point in the well.

The density of the drilling fluid.

The part of a mud's flow resistance that rises in proportion to how fast the mud is sheared.

The pressure of the fluid in the pore space of the rock.

How quickly the bit is deepening the hole.

The pressure on the mud as it is pumped into the drill string, read at the standpipe.

Pulling the drill string out of the hole, or running it back in.

The vertical distance from the depth reference on the rig down to a point in the well.

The compressive force applied to the bit.

Keeping formation fluids from flowing to surface in an uncontrolled way, and removing a kick once one has entered.

Whether the hole stays close to bit size while it is drilled, instead of caving in or fracturing.

The extra resistance a mud keeps, in the Bingham model, as the shear rate falls.