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* )

*Bottom-Hole Pressure Reduction Due to Gas Cutting *

*Calculation of bottom-hole pressure reduction in a static mode*

*is expressed by the iterating Strong-White equation [Strong],*

*a simplistic model of which is given as:*

*△p=n*2.3×log10×p*

*where*

*. △p = reduction in bottom-hole pressure, atm*

*. n = ratio of gas to mud*

*. p = hydrostatic pressure, atm*

*. n = (1 – x)/x*

*. x = weight of cut mud/weight of uncut mud*

*Or, the equation can be rearranged as:*

*△p=((w1-w2)/w2)×33.81×log（p/14.7）*

*where*

*. △p= bottom-hole pressure reduction, psi*

*. w1 = weight of uncut mud, lb/gal*

*. w2 = weight of cut mud, lb/gal*

*. p = hydrostatic pressure of mud, atm*

*The addition of flow to the static mode is an iterating set of equations*

*that generally are accepted to introduce errors of >100%. These*

*equations are most fully developed in the underbalance models of*

*the various service and engineering companies.*

*The simplest mathematical solution for a drilling operation is*

*to calculate height in the annulus of the mud displaced by the*

*gas cutting and reduce that to a pressure loss:*

（-1）*△p=（△v×Av）×p×0.052*

*where*

*. （-1）△p = bottom-hole pressure decrease, psi*

*. V = pit volume increase due to gas cutting, bbl*

*. Av = annular volume behind drill pipe, bbl/ft*

*. P = density of the (uncut) drilling fluid, ppg*

*. 0.052 = units constant*

Trip gas and connection gas are indications of the swabbing effect of

upward pipe movement. The swabbing force reduces the pressure exerted

by the drilling fluid to below the formation pressure. Generally this

problem is temporary and gas cutting can be handled by a gas buster and

a degasser.

Increasing mud density only because of gas-cut mud is not generally

a good solution. If the gas cut is the result of drilling a gas-bearing

formation, increasing mud density may lead to lost circulation.

However, increasing connection gas may be an indication of increasing

formation pressure.

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