BAKER HUGHES - Drilling Fluids Reference Manual

PRESSURE PREDICTION AND CONTROL
where,
R = ft/hr
N = rpm
W = lb
D = diameter in inches.
As predicted, plotting “d” as calculated from this equation versus depth does establish
recognizable trends in normally pressured formations and recognizable deviation from these
trends as abnormal pressures are encountered.
In 1971, Rehm and McClendon proposed that if it were possible to correct for fluid weight, the
“d” value could be related directly to formation pressure rather than to differential pressure.
They suggest making a correction by multiplying the basic “d” value by the ratio of the normal
fluid weight pressure gradient for the area to the actual fluid weight being used.
⎛ Normal Fluid Weight
d xc = d
⎟ ⎞
⎜
⎝ Fluid Weight in Use ⎠
Use of this correction factor gives excellent results and permits graphic determination of
pressure gradients in abnormally pressured zones.
Gas Content of Drilling Fluid
Increases in the gas content of a drilling fluid was recommended as an indicator for detecting
abnormal pressure zones as early as 1945 (Pixler). Since experience has shown that gas cutting
is not always a result of an under-balance condition, correct interpretation of gas cutting trends
is imperative.
Gas may be entrained in the fluid column as a result of the following conditions.
• When a formation containing gas is drilled, cuttings and cavings containing gas are circulated up
the hole. Hydrostatic pressure on these particles is reduced. Gas in the particles expands and is
released to the drilling fluid system, cutting the weight. In such cases, increasing fluid weight will
not stop the gas cutting. This condition can be verified by reducing drilling rate or by stopping
drilling and circulating bottoms up.
• While drilling a pressured formation, the differential between the hydrostatic fluid column
pressure and formation pressure is reduced. During a connection or trip, the piston effect of
upward pipe movement can swab formation gases and fluids into the wellbore. As the fluid or gas
is circulated up the annulus, gas (if present) will expand. When detected, the gas is generally
concentrated and reaches the surface at the same time, thereby showing a significant gas cut.
When using gas in the drilling fluid as an indicator of abnormal pressure, a gas detector unit of
some type is required. A trend of background gas and connection gas can be noted as drilling
progresses. Both background gas and connection gas normally increase slowly with depth. Any
sudden increase in background or connection gas should be watched carefully and taken as a
possible warning of increasing pore pressure.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 12-14