Shale shaker screening is dependent on a constant flow of drilling fluid with cuttings. The fluid must pass through the screen, and the cuttings must either pass through or be rejected by the screen. Gas cutting in the drilling mud can have up to three different effects that upset the screening process.
1. Gas heading can cause volume surges in the mud flow that exceed the ability of the screen to handle fluid flow. This is usually from gas, intermixed in the mud, rapidly expanding at the surface and pushing large surges of drilling fluid out the flowline. Gas busters and gas separators are the solutions to this problem.
2. Gas cutting from tiny gas bubbles entrained in the drilling fluid can cause screen blinding when the bubbles expand to fill the area between the screen wires. This problem is usually handled by a degasser that removes the entrained gas from the drilling fluid.
3. Foaming associated with gas cutting leaves a film of very light, wet foam on the shaker screen. The foam is too light to be gravity-pulled through the screen and carries extra liquid off the end of the shaker. Often the fluid loss is not significant and can be ignored. In other cases shaker sprays and defoamers are needed to break the foam. However, shaker sprays tend to wash extra cuttings through the screen.
A fourth problem, not related to gas, is an extreme rise in the viscosity of the drilling fluid from saltwater or salt. This may cause the drilling fluid to flow over and not pass through the shaker screen. This problem often requires the bypassing of the contaminated mud to a reserve tank.