Circulating drilling fluid that is pumped through the drill string to the bit and back up the annular space of the borehole removing cuttings, cooling the bit, preventing caving and controlling entry of oil and gas. Drilling muds divide into two main categories; conductive water-based and resistive oil-based. This division limits the use of micro resistivity imaging and dipmeter devices to those boreholes drilled with a water-based mud. Within oil-based muds, dipmeters are either induction devices or are run with scrapers that penetrate the formation allowing current return. Acoustic based borehole televiewers can function in either oil or water based fluids. A new generation of resistivity imagers have recently been developed that can function in oil based muds. Mud types and properties (e.g. density, viscosity, filtrate and composition) can all influence dipmeter and borehole image quality. Acoustic borehole televiewers, in particular, suffer strong acoustic attenuation in very dense muds.
RABIA, H. 1985. Oilwell Drilling Engineering, Principles & Practice. Graham & Trotman, London, p322.
See also mud cake.