Macondo Well Deepwater Horizon Blowout: Lessons for Improving Offshore Drilling Safety(2012)

Chapter: Appendix D: Calculating the Differential Pressure at the Start of the Negative Test and the Quality of Foam Cement

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Page 166
Suggested Citation:"Appendix D: Calculating the Differential Pressure at the Start of the Negative Test and the Quality of Foam Cement." National Academy of Engineering and National Research Council. 2012. Macondo Well Deepwater Horizon Blowout: Lessons for Improving Offshore Drilling Safety. Washington, DC: The National Academies Press. doi: 10.17226/13273.
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Appendix D

Calculating the Differential Pressure at the Start of the Negative Test and the Quality of Foam Cement

See the well diagram in Figure D-1.

1. Pressure differential at the start of the negative test:

Page 167
Suggested Citation:"Appendix D: Calculating the Differential Pressure at the Start of the Negative Test and the Quality of Foam Cement." National Academy of Engineering and National Research Council. 2012. Macondo Well Deepwater Horizon Blowout: Lessons for Improving Offshore Drilling Safety. Washington, DC: The National Academies Press. doi: 10.17226/13273.
×

Δp = po – pi

where

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.

where

Page 168
Suggested Citation:"Appendix D: Calculating the Differential Pressure at the Start of the Negative Test and the Quality of Foam Cement." National Academy of Engineering and National Research Council. 2012. Macondo Well Deepwater Horizon Blowout: Lessons for Improving Offshore Drilling Safety. Washington, DC: The National Academies Press. doi: 10.17226/13273.
×

where

γN = specific gravity of nitrogen (compared with air),

p = pressure (pounds per square inch absolute),

z = gas deviation factor (dimensionless), and

T = temperature (degrees Rankine = 460 + degrees Fahrenheit).

So, for every in situ gallon of slurry there will be 0.174 gallon of nitrogen mixed with 0.826 gallon of base 16.74-ppg cement slurry. Thus, the in situ foam quality is 17.4 percent. Note that the Chevron tests used a 13 percent quality foam, which corresponds to the weight fraction of nitrogen necessary to create a 14.5 ppg density foam at atmospheric conditions. Therefore, more nitrogen is required to create the same density foam at the much higher pressure and temperature of the bottom of the Macondo well.

At the mixer at the surface, the slurry is blended and pumped at about 600 psig. The volume of nitrogen introduced to 0.826 gallons of base cement is the in situ volume increased through the real gas law.

This is added to 0.826 gallon of base cement. Thus, for every 1 gallon of base cement, 1.94 gallons of N2 at 600 psig is required. This is a 66 percent quality foam.

The density of the foam slurry at the mixer will be as follows:

The previous equations and results can be combined to obtain an equation for the density of the slurry at any depth with a corresponding pressure, temperature, and gas deviation factor.

Page 169
Suggested Citation:"Appendix D: Calculating the Differential Pressure at the Start of the Negative Test and the Quality of Foam Cement." National Academy of Engineering and National Research Council. 2012. Macondo Well Deepwater Horizon Blowout: Lessons for Improving Offshore Drilling Safety. Washington, DC: The National Academies Press. doi: 10.17226/13273.
×

Page 166
Suggested Citation:"Appendix D: Calculating the Differential Pressure at the Start of the Negative Test and the Quality of Foam Cement." National Academy of Engineering and National Research Council. 2012. Macondo Well Deepwater Horizon Blowout: Lessons for Improving Offshore Drilling Safety. Washington, DC: The National Academies Press. doi: 10.17226/13273.
×
Page 167
Suggested Citation:"Appendix D: Calculating the Differential Pressure at the Start of the Negative Test and the Quality of Foam Cement." National Academy of Engineering and National Research Council. 2012. Macondo Well Deepwater Horizon Blowout: Lessons for Improving Offshore Drilling Safety. Washington, DC: The National Academies Press. doi: 10.17226/13273.
×
Page 168
Suggested Citation:"Appendix D: Calculating the Differential Pressure at the Start of the Negative Test and the Quality of Foam Cement." National Academy of Engineering and National Research Council. 2012. Macondo Well Deepwater Horizon Blowout: Lessons for Improving Offshore Drilling Safety. Washington, DC: The National Academies Press. doi: 10.17226/13273.
×
Page 169
Suggested Citation:"Appendix D: Calculating the Differential Pressure at the Start of the Negative Test and the Quality of Foam Cement." National Academy of Engineering and National Research Council. 2012. Macondo Well Deepwater Horizon Blowout: Lessons for Improving Offshore Drilling Safety. Washington, DC: The National Academies Press. doi: 10.17226/13273.
×
Next: Study Committee Biographical Information »
Macondo Well Deepwater Horizon Blowout: Lessons for Improving Offshore Drilling Safety Get This Book
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The blowout of the Macondo well on April 20, 2010, led to enormous consequences for the individuals involved in the drilling operations, and for their families. Eleven workers on the Deepwater Horizon drilling rig lost their lives and 16 others were seriously injured. There were also enormous consequences for the companies involved in the drilling operations, to the Gulf of Mexico environment, and to the economy of the region and beyond. The flow continued for nearly 3 months before the well could be completely killed, during which time, nearly 5 million barrels of oil spilled into the gulf.

Macondo Well-Deepwater Horizon Blowout examines the causes of the blowout and provides a series of recommendations, for both the oil and gas industry and government regulators, intended to reduce the likelihood and impact of any future losses of well control during offshore drilling. According to this report, companies involved in offshore drilling should take a "system safety" approach to anticipating and managing possible dangers at every level of operation -- from ensuring the integrity of wells to designing blowout preventers that function under all foreseeable conditions-- in order to reduce the risk of another accident as catastrophic as the Deepwater Horizon explosion and oil spill. In addition, an enhanced regulatory approach should combine strong industry safety goals with mandatory oversight at critical points during drilling operations.

Macondo Well-Deepwater Horizon Blowout discusses ultimate responsibility and accountability for well integrity and safety of offshore equipment, formal system safety education and training of personnel engaged in offshore drilling, and guidelines that should be established so that well designs incorporate protection against the various credible risks associated with the drilling and abandonment process. This book will be of interest to professionals in the oil and gas industry, government decision makers, environmental advocacy groups, and others who seek an understanding of the processes involved in order to ensure safety in undertakings of this nature.

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