Page 460 - Practical Well Planning and Drilling Manual

Appendix NEW! revised 11/00/bc 1/30/01 3:29 PM Page 436

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Appendix 1

Method
Once the fracture gradient is known, calculate the maximum gas
influx volume at the next casing point. For this kick situation, the
maximum pressure on the shoe usually occurs when the top of the gas
reaches the shoe (assuming one gas bubble). This may not be the case
if the height of the influx at the shoe is less than the height of the influx
around the drill collars, if the gas expansion is not enough to compen-
sate for the changes in annular capacity.
Assume the driller’s method. This is the worst case and would
apply if the string was plugged and a volumetric kill was required.
Calculation of the exact pressure at the top of a gas kick bubble is
not feasible; there are unknown factors that make little practical differ-
ence. The composition of the gas (compressibility factors) is unknown
and temperature effects can be ignored. The calculations given below
could be slightly more accurate with much greater effort, but this is not
worthwhile since even then the calculated figure cannot account for all
the small factors.
After calculating the kick tolerance, consider whether this is
enough. Kick tolerances may be set by company policy or government
regulation. Account for reaction time of men and equipment, how fast
a kick may occur (related to likely permeabilities), and the level of
training and competence of the rig crews. Extra precautions may be
taken in critical areas, such as restricting ROP, setting fixed drilling
parameters to allow a drilling break to be identified more easily, etc.
Example: Given a casing shoe at 5000 ft with a fracture gradient of
0.75 psi/ft, plan to drill to the next casing point at 8000 ft with a mud
gradient of 0.6 psi/ft in a vertical exploration well. Assume a gas gradi-
ent of 0.1 psi/ft at the casing shoe; 12 /4 in hole, 5 in drillpipe, and 300
1
ft of 8 in drillcollars.
First, calculate the MAASP.

MAASP = 5000 x (0.75 – 0.6) = 750 psi.

Next, calculate the height of a gas influx at the casing shoe where
the pressure at the shoe = formation breakdown pressure, and the for-
mation at casing point has a pore pressure gradient that is 10% greater
than the planned mud gradient.

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