1. n. [Drilling Fluids]

A fine-grained, impermeable, sedimentary rock composed of clays and other minerals, usually with a high percentage of quartz. Shale is the most common, and certainly the most troublesome, rock type that must be drilled in order to reach oil and gas deposits. The characteristic that makes shales most troublesome to drillers is its water sensitivity, due in part to its clay content and the ionic composition of the clay. For this reason, oil-base drilling fluids are the mud of choice to drill the most water-sensitive shales.

See: activity of aqueous solutions,  balanced-activity oil mud,  cation exchange capacity,  Chenevert method,  clay,  clay-water interaction,  drilling detergent,  encapsulation,  gumbo,  inhibit,  inhibitive mud,  montmorillonite,  native clay,  oil mud,  pore-pressure transmission,  potassium ion,  relative humidity,  silicate mud,  synthetic-base mud