Solids Control Equipment and Gas Cutting

1. Shale Shakers And Gas Cutting

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.

2. Desanders, Desilters, And Gas Cutting

The operation of any desilter or desander is dependent on the head
pressure, which is in part a function of the volume pumped by the
centrifugal pump. Centrifugal pumps are very sensitive to gas cutting.
The gas collects in the reduced-pressure area at the center of the impeller
and quickly reduces the pump output. In cases in which there is very low
fluid head on the suction of the centrifugal pump, a small amount of gas
in the drilling fluid may gas-lock the pump and stop or limit the flow of
drilling fluid.

Reduced flow from the centrifugal pumps tends to dump whole mud
from the bottom of the desilter or desander hydrocyclone to waste.
Gas locking of the centrifugal pump stops desilter or desander action.
Any time there is gas in the flowline, a degasser should be used ahead of
the solids-control feed pumps.

3. Decanter Centrifuges And Gas Cutting

The centrifuge feed is typically a Moyno pump, also called a progressing
cavity pump. The Moyno pump is a positive displacement pump and
does not gas-lock, as does a centrifugal pump, but the input feed is
reduced. The reduction is in direct proportion to the gas in the mud and
the feed pressure to the centrifuge. The gas at atmospheric pressure in
the mud is compressed by the Moyno pump to the feed pressure. The
compression of the gas reduces the output of the pump (Box 9.2).
Reduction in feed to the centrifuge reduces the output and may change
the cut point.