1. n. [Drilling Fluids]

Settling of particles in the annulus of a well, which can occur when the mud is static or being circulated. Because of the combination of secondary flow and gravitational forces, weighting materials can settle (sag) in a flowing mud in a high-angle well. If settling is prolonged, the upper part of a wellbore will lose mud density, which lessens the hydrostatic pressure in the hole, so an influx (a kick) of formation fluid can enter the well.

See: Brookfield viscometer,  hydrostatic head,  mud weight,  rheological property,  rheology modifier