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 )

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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