A drilling fluid (or mud) that has gas (air or natural gas) bubbles in it, resulting in a lower bulk, unpressurized density compared with a mud not cut by gas. The density of gas-cut mud can be measured accurately using a pressurized mud balance. Defoamer chemicals added to the mud or a mechanical vacuum pump degasser can liberate the trapped gas. [..]

Drilling mud permeated with bubbles of gas from downhole. The circulation of such mud can be severely impaired, seriously affecting drilling operations.