BP defends its 2010 Gulf spill estimate in federal court

By Susan Buchanan
Contributing Writer

Barrel counting continued at the quantification trial for the 2010 Macondo spill in U.S. District Court on Poydras St. last week. With Judge Carl Barbier presiding, BP and Anadarko called their witnesses after the U.S. Justice Department rested its case the previous week. According to BP’s defense, 3.26 million barrels spewed into the Gulf three years ago while the United States says it was five million barrels, including what was collected. BP rested its case late last week. Data presented in the quantification trial will determine how much BP is fined per barrel under the Clean Water Act.

Petroleum engineering professor Curtis Whitson at the Norwegian University of Science and Techno­logy at Trondheim, under questioning from Hariklia Karis for BP Tuesday, said he and others built a model for the Macondo reservoir’s fluids. Whitson also said that his group evaluated a model by thermodynamics expert Aaron Zick and saw a couple of serious flaws in it concerning shrin­kage. From his group’s modeling, Whitson calculated a shrinkage factor of 42 to 44, depending on how solubility was treated. That meant 42 to 44 barrels reached the ocean’s surface for every 100 barrels that rose from the Macondo formation below. Liquid barrels at the surface are called stock-tank barrels in the oil industry.

Justice Dept. witness Aaron Zick, who testified the week before, had estimated that 50.6 to 50.7 barrels of liquid were left for every 100 barrels in the formation when oil floated slowly to the surface. And under a different measure, Zick put shrinkage at 49.4 barrels, using a four-stage, separator-process method that’s based on the way companies extract oil. BP maintains that less liquid was left in barrels that surfaced than the feds estimate.

Questioned Tuesday by Mike Brock for BP and Anadarko, rock mechanics Professor Robert Zim­mer­man at Imperial College in London said he’d analyzed Macon­do reservoir data, collected by Houston-based Weatherford Labo­­ra­tories, to gauge pore-volume compressibility. He said the Ma­condo rock was weakly consolidated sandstone. “My estimate of the average compressibility of the rocks in the reservoir was 6.35 micro­sips,” Zimmerman said. Microsips are used by the oil industry to mea­sure compressibility.

The higher the microsips, the greater the amount of oil thought to be in a rock formation, meaning more of it could escape. For awhile during the spill, BP scientists recommended using 12 microsips for rock compression at the Macondo site. But BP’s defense this fall has focused on a figure of around 6 microsips, suggesting less oil was in the formation.

Petroleum engineering professor Alain Gringarten at Imperial College in London, questioned Tuesday by Martin Boles for BP and Anadarko, said he evaluated total discharge from the Macondo well in two steps. He calculated permeability from pre-spill data at the reservoir level. Then he used that permeability, along with pressure measured during the spill and the well’s shut-in afterwards, to calculate discharge. “I found that the permeability of the reservoir is 238 millidarcies and that the cumulative discharge of oil is between 2.4 and 3 million stock-tank barrels,” Gringarten said. Millidarcies are a permeability unit used in petroleum engineering. Gringarten also estimated that 810,000 barrels had been collected at the Macondo site.

In the quantification trial, BP examined a number of witnesses from Imperial College in London, with which it has research ties.

Reservoir engineering director Robert Clifford Merrill, Jr. at BP Exploration in Houston was questioned Wednesday by Martin Boles. Merrill said his Macondo spill work included estimating pressures during flow-rate uncertainty. In modeling with his team early in the spill, he used six microsips for the Macondo’s rock compressibility. But in early July, the team also experimented with 12 microsips in its calculations. After the well’s July 15 shut-in, Merrill used six microsips for rock compressibility, and said he supports that number now because it was the measured value.

Multiphase flow expert Michael Zaldivar, president and founder of Evoleap, LLC in Houston was questioned Wednesday by Barry Fields for BP and Anadarko. Zaldivar said the well’s sunken riser pipe in late April caused a “slug flow,” or alternating exodus of oil and gas from the Macondo reservoir from May 13 to 20. The riser pipe had a kink with holes in it. Based on riser-end-flow and kink-leak-flow modeling, Zaldivar calculated a well flow rate of 24,900 to 35,900 stock-tank barrels per day from May 13 to 20, 2010, with a best estimate for that period of 30,000 bpd.

Under the Clean Water Act, fines against BP could range from $1,100 per barrel spilled if simple negligence is found to as much as $4,300 a barrel if the company is considered grossly negligent. Judge Barbier is hearing the trial without a jury and must decide how many barrels spewed. Eighty percent of CWA fines will be directed to economic and ecological restoration along the Gulf Coast.

This article originally published in the October 21, 2013 print edition of The Louisiana Weekly newspaper.