glossary

Vertical angle between the direction of the magnetic field and the horizontal plane. Commonly called magnetic dip angle (see inclination)

A geophysical instrument used to measure the intensity, in both the horizontal and vertical directions, of the earth magnetic field.

The length of the wellbore, as if determined by a measuring stick. This measurement differs from the true vertical depth of the well in all but vertical wells. Since the wellbore cannot be physically measured from end to end, the lengths of individual joints of drill pipe, drill collars and other drillstring elements are measured with a steel tape measure and added together. Measured depth should not be confused with total depth (also called true vertical depth).

Measurement-while-drilling collars are used to measure depth (see measured depth), inclination and azimuth of the borehole.

The lightest and most abundant of the hydrocarbon gases and the principal component of natural gas. Methane is a colorless, odorless gas that is stable under a wide range of pressure and temperature conditions in the absence of other compounds.

The movement of newly generated hydrocarbons out of their source rock is primary migration, also called expulsion. The further movement of the hydrocarbons into reservoir rock in a hydrocarbon trap or other area of accumulation is secondary migration.

Abbreviation for a thousand standard cubic feet per day, a common measure for volume of natural gas. Standard conditions are normally set at a temperature of 60°F and a pressure of 14.7 psia.

A positive displacement drilling motor that uses hydraulic horsepower of the drilling fluid to drive the drill bit. Mud motors are used extensively in directional drilling operations.

The primary component of the mud is commonly water (fresh or saline) or oil. Based on the mud composition, drilling mud can basically be subdivided into water-based mud (see water-base drilling fluid), non-water based mud and gaseous drilling mud. The mud type may affect MWD measurements and telemetry. See also Oil-base drilling fluids

The mass per unit volume of a drilling fluid, synonymous with mud density. Mud weight controls hydrostatic pressure in a wellbore and prevents unwanted flow into the well. The weight of the mud also prevents collapse of casing and the openhole. Excessive mud weight can cause lost circulation by propagating, and then filling, fractures in the rock.

A well that has more than one branch radiating from the main borehole.