The Macondo well blowout spilled much less oil than the U.S. government estimates of 4.9 million bbl, according to a reservoir engineering expert hired by operator BP plc.
In May, Martin J. Blunt submitted a previously confidential report to the U.S. District Court for the Eastern District of Louisiana. Determining the amount of oil spilled will determine BP’s penalties under the U.S. Clean Water Act (CWA). If overseeing Judge Carl Barbier were to find that BP was grossly negligent in the Macondo blowout — and the higher government spill figure is used — the CWA penalty could be close to $18 billion.
The government had calculated that the total amount of oil spilled from the blown well was 4.9 million bbl. BP has argued that government spill estimates are too high. Among other things, BP claimed that it captured 810,000 bbl from the flowing well at the time of the incident, which would reduce the penalty by $891 million to $3.5 billion, depending on whether BP is found grossly negligent.
If Blunt’s oil spill calculations were used to determine the CWA fine, the penalty could be reduced by much more.
Barbier is overseeing BP’s multi-district litigation (MDL) No. 2179 to determine, among many other items, how much compensation under the CWA is required. Barbier’s colleague, U.S. District Court Judge Sarah S. Vance, earlier this year approved an agreement for BP to plead guilty to 14 Department of Justice charges, including manslaughter, and to pay more than $4 billion in criminal fines (see Daily GPI, Jan. 30).
Blunt’s findings were prepared on behalf of BP Exploration & Production Inc. and minority Macondo partner Anadarko Petroleum Corp. (10%).
“I calculate that the volume of oil released from the Macondo reservoir was 3.26 million stock tank barrels (MMstb),” Blunt stated. “I have used conservative assumptions to avoid understatement of the volume. I find a range of oil released between 2.9 MMstb and 3.7 MMstb. This is the total volume of all the oil that left the reservoir, including any oil burnt or collected, converted to a volume at surface (stock tank) conditions of 60 degrees F and 1 atmosphere pressure.”
Blunt, a distinguished member of the Society of Petroleum Engineers, noted that he has been teaching fluid flow principles for more than 20 years at Imperial College in London. He also disclosed that his first job after graduating from college was with BP, where he worked for four years.
Blunt said he used “a well established method from my field called material balance.” In material balance, three quantities — the oil volume connected to the well, compressibility and pressure depletion — are multiplied together to calculate cumulative oil flow.
“Compressibility determines how much oil is released from the rock as the pressure drops,” he noted. “The main difference between my estimate” and those of the U.S. government and others “is that they double the compressibility from the value measured on Macondo rock samples at an independent service laboratory,” which he said was a “switch” from an approach first used by the government and others to evaluate the Macondo oil flow.
“We will see that this has been a repeated problem in the work of the government experts,” Blunt noted. “In order to obtain their estimates of 5 MMstb oil released, they had to make assumptions that disagree significantly with direct measurements of the Macondo rock and fluid properties.
“The government investigators each disregarded vital pieces of experimental evidence without justification; not all of their errors were identical, yet they arrived at the same final answer. There is a choice: either accept their calculation of 5 MMstb, despite the lack of any scientific explanation of why the measurements are wrong, or perform a calculation consistent with the data and arrive at a lower value. I have chosen the latter approach.”
Blunt claimed in his analysis that experts who did not use the material balance approach “took an estimate of the final flow rate before shutâ€in, approximately 50,000 stb/day, and assumed that the flow rate during the preceding 86 days was even higher. Of course, simple mathematics dictates that the outcome of a cumulative-flow calculation based on this assumption will exceed 4.5 MMstb.
“They justified this approach by assuming that there were no impediments to oil flow in the wellbore, blowout preventer or tubing that might have caused flow to be lower in those preceding days. They did not prove this assumption; indeed, they hardly discussed it. By contrast, the material balance method used here is not tied to an assumption about historical flow rates…”
Government experts, said Blunt, also “assumed the reservoir oil was completely connected to the Macondo well, omitting to analyze geological features that the government’s nonlitigation expert consultant said would limit connectivity. None of the government experts analyzed the evidence from the Macondo geology and seismic analysis that indicates that the oil reservoir was not completely connected to the well,” even though a government consultant from the University of Texas at Austin had said “geological evidence pointed to a significant probability of poor connectivity…” in the reservoir.
“The government estimates therefore have an implicit upward bias from ignoring evidence that some of the oil was likely compartmentalized and hence cut off from flowing to the well.” The government’s team also “overestimated flow by overstating the pressure depletion in the reservoir,” Blunt claimed.
“The government experts based their calculation on measurements of capping stack pressure increasing slowly over time, but the reservoir pressure was rising faster than they suggest: there was an extra factor — the weight of oil — that was also increasing…By assuming the oil was hotter and lighter than it actually was, indeed unfeasibly hot, they end up overestimating pressure depletion, and hence the oil flow.”
Blunt’s findings, issued in two parts, may be downloaded here and here.
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