Gas cut mud is any drilling mud which is found to contain gases after drilling through porous underground formations containing gas. The presence of “foreign” gas within the mud reduces the bulk density of the mud leading to a slight decrease in the mud’s hydrostatic pressure, especially in sections of the well close to the surface.
A gas cut mud can be physically observed with gas bubbling out of the drilling mud as the mud is circulated out of the borehole. However, it is important to note that even though “gas shows” are seen as the mud returns to the surface, a gas cut scenario rarely leads to a kick.
This is because during gas cutting, hydrostatic pressure reduction is only felt close to the surface because that is where the largest gas expansion takes place. A kick requires the hydrostatic pressure to drop throughout the entire mud column and not just close to the surface so as to permit more gases and fluids to flow into the hole.
Gas Cutting and a Kick
Gas cutting of the mud can take place when circulation is stopped for a long period of time (such as when conducting a round trip) or through the release of gases previously trapped within the pore spaces of rock cuttings. It may not be necessary to increase the mud weight to control a gas cut, a simple bottoms-up circulation is usually enough to remove the gas from the borehole.
However, if after bottoms up circulation, gas cutting still continues then it may not be a gas cut after all, but a kick. A gas will expand when the pressure exerted on it is lowered. This is why gas cut is usually felt around the topmost 200 feet of the hole.
The entrained gas will circulate upwards with the drilling mud and when it gets close to the surface where the pressure in the hole nears that of atmospheric pressure, the gas will expand occupying more space and forcing drilling mud out of the hole (pit gain).
This gas expansion is what effectively lowers mud hydrostatic pressure close to the surface. But then, the overall hydrostatic pressure from the top to the bottom of the hole may only be reduced by a small margin because the effect of gas cut mud is majorly felt close to the surface and not throughout the entire borehole.
Also, the reduction of hydrostatic pressure due to gas cut mud may be further hindered by the presence of drilled cuttings within the circulating drilling mud. As gas cutting slightly reduces hydrostatic pressure, the drilled cuttings may slightly increase density and mud hydrostatic pressure balancing out the effect of gas cutting. This is why gas cut mud rarely leads to a kick, although that is still a possibility.
Controlling Gas Cut Mud
When gas shows are noticed with mud returns at the surface, stop circulating and make a flow check. If the well does not flow after pumps are down, then circulate bottoms-up to remove the gas from the hole and de-gas the mud as soon as it reaches the surface to avoid re-circulating gas cut mud back into the hole.
If gas cutting stops after circulating bottoms-up, resume drilling and continue to de-gas the mud. Finally, pump drilling mud with the correct mud weight down the hole. However if gas cutting does not cease after circulating bottoms up, then it may be necessary to treat the case as a kick scenario.