Box 9.3
Basic Solids Control Equipment for handling Gas-Cut Mud
Gas busters are a simple cylinder or baffle box at the flowline where
mixed drilling fluid and gas are roughly separated while flowing. The
drilling fluid goes to the shale shaker, and the gas is allowed to flow away or is sent to a flare line.
Drilling Mud Separators are holding tanks where mixed water, oil, and gas are allowed to separate by gravity. They have evolved in the last 50 years from simple open tanks to complex closed and pressurized tanks.
Separators can be informally divided into two groups: (1) atmospheric,
or unpressurized, and (2) pressurized, or closed
Degassers are somewhat different devices from the preceding two. The
degasser is a tank in which a vacuum and/or spray removes entrained
gas from the mud system. Degassers handle much smaller gas volumes
than do gas busters or separators but do a more complete job of
removing the gas.
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GENERAL COMMENTS ON GAS CUTTING
Solids-control equipment can be severely affected by gas in the drilling
mud. This condition is misinterpreted and misunderstood in most
field applications. The primary problems caused by gas cutting in solids
control are blinding of the shaker screens and degradation of pump
output to hydrocyclones and centrifuges. Gas cutting always occurs
during drilling of a gas-bearing formation.
If there is enough gas to displace drilling fluid to the surface (and
increase pit volume), bottom-hole pressure is reduced. This occurs when
the pressure exerted by the drilling fluid is less than the formation pressure and there is some significant permeability. This condition requires surface control, gas busters or separators, and a degasser.
If there is no pit volume increase but the drilling fluid is gas cut and
the flowline mud density reduced, bottom-hole pressure is not significantly reduced and this condition in general calls for only a degasser. (see inclined letter )
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