. "Appendix D: Calculating the Differential Pressure at the Start of the Negative Test and the Quality of Foam Cement." Macondo Well Deepwater Horizon Blowout: Lessons for Improving Offshore Drilling Safety. Washington, DC: The National Academies Press, 2011.
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Δp = po – pi
Δp = pressure differential [pounds per square inch (psi)];
po = pressure outside the casing at the bottom (psi), assumed equal to reservoir pressure of 11,892 psi, which is a pore pressure of 12.57 pounds per gallon (ppg) at the bottom of the reservoir at 18,212 feet (true vertical depth); and
pi = pressure on the inside above the cement (psi).
Here the differential is into the casing. The cement is treated as a solid that does not transmit hydrostatic pressure but that must be strong enough to withstand the pressure differential across it. The top of the cement inside the casing is based on the assumption that 2.8 barrels of foam cement flowed back into the casing when the pressure was bled off at the end of the cement job.
2. Foam quality calculations:
Foam cement: The purpose in this case is to reduce the bottom hole (in situ) density of the slurry from 16.74 ppg to 14.5 ppg. The bottom hole pressure is the hydrostatic pressure of 14 ppg mud or 13,321 pounds per square inch gauge (psig) at 18,304 feet. The static bottom hole temperature is 245°F.