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.
Gas cut mud has always been considered a warning signal, but not necessary a serious problem. There are multiple sources of gas. Gas may enter the mud as a result of the following:
1. Gas in shales that form a base line for a continuous gas unit level.
2. Gas from sands that may cause sudden change in the gas concentration level.
3. Connections gas associated with swabbing on connections.
4. Trip gas associated with swabbing following round trips with the drill string.
5. Gas that enters the mud because of insufficient mud weight to control formation fluids.
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.
A gas cut is inferred only if the mud returning to the surface is significantly less dense than it should be. In the case of the mud logger's measurement, "units" of gas (having virtually no absolute meaning) are reported. For the mud logger's measurement, a direct indication of combustible gases is made, with no direct correlation to mud weight.
Because gas is a compressible material it often gives appearance of being a more serious problem at the surface than actually exists. Many shales contain gas in the pore spaces and furnish a continuous level of gas in the mud. This continuous level of gas forms a base line reference and for given areas it is predictable. Very little attention is paid to this source of gas.
Some gas sands may increase the gas in the mud substantially and may results in severe reductions in surface mud weights. This always concerns the operator and may cause trouble if it comes unexpectedly.