A London petroleum engineering professor hired by BP says the U.S. government overestimated by 50 percent the amount of oil that spewed from the company's undersea Gulf of Mexico well in 2010.

The lower estimate would cut the maximum Clean Water Act penalties BP faces by up to $7 billion.

Previously sealed court records reviewed by the Houston Chronicle provide for the first time in more than three years a detailed accounting of BP's defense to the government's official flow estimate.

How much oil spilled will be the central issue during the second phase of a civil trial in federal court in New Orleans that is set to begin in September.

BP long has asserted that in a rush to punish it for the worst offshore oil spill in U.S. history, the American government botched its flow rate estimates, which were calculated by a team of scientists from all over the country.

But until now, none of the company's own calculations had become public. BP had only said previously that it believed the government's estimate was at least 20 percent too high.

In a 209-page report last month, Martin Blunt, a professor in Imperial College's Department of Earth Science and Engineering, said he calculated that 3.26 million barrels of oil were released from BP's Macondo well a mile beneath the sea off the coast of Louisiana.

The government estimates that the well discharged 4.9 million barrels.

Both figures include the 800,000 barrels that BP and the U.S. government agree were collected at the wellhead and didn't enter the water. Earlier this year, the government agreed not to count that oil in calculating the penalties it seeks against BP.

By that reckoning, the government estimates that 4.1 million barrels of oil got into the Gulf, and Blunt's estimate is 2.46 million barrels.

Based on the government's estimate, BP would face up to $17.6 billion in Clean Water Act penalties if U.S. District Judge Carl Barbier finds that the company was grossly negligent. He has not yet ruled on that key issue. Based on Blunt's calculation, the maximum penalty would be $10.6 billion.

Blunt said in his report that U.S. government experts switched the method they used to calculate the rate that oil flowed from the well, resulting in the higher, and he believes erroneous, total.

The dispute involves compressibility, which determines how much oil is released from a reservoir as the pressure drops.

Blunt said the main difference between his estimate and those of several U.S. experts is that the government experts “double the compressibility from the value measured on Macondo rock samples.”

Blunt said that is a switch from the approach that one of the U.S. experts used when he first evaluated Macondo oil flow in 2010. Blunt said the U.S. expert initially used the measured compressibility, as Blunt did in his report.

Blunt said that if U.S. experts used that compressibility reading, they would obtain about the same total as he did for the oil spilled during the nearly three months before the runaway well was capped in July 2010.

He said that the U.S. experts have not provided a scientific justification for their decision to double the compressibility from the measured values in making their calculations.

Justice Department lawyers said in a Thursday court filing, which includes a copy of Blunt's report, that they want to call at trial rebuttal experts who will challenge Blunt's conclusions. They said some of BP's own internal experts at the time of the spill calculated higher rock compressibility values than Blunt is relying on now.
