An equipment failure at a deepwater Gulf of Mexico drilling platform contracted by a unit of Royal Dutch Shell plc released about 319 barrels of biodegradable drilling mud and water last weekend before being contained, according to a report filed with the National Response Center (NRC).

The leak was from Transocean Ltd.’s Deepwater Nautilus rig, which has been drilling Shell Offshore Inc.’s Appomattox No. 5 sidetrack well in Mississippi Canyon Block 348. The well, in more than 7,000 feet of water, is at a site about 72 miles off the coast of Louisiana.

“Shell can confirm it has a loss of 319 barrels of drilling fluid from a booster line (which provides additional drilling fluids and is separate from the wellbore itself),” a Shell spokesperson said.”Once there were indications of a leak, a source was identified and at no time were there safety or well control issues. The integrity of this well was also not compromised.”

No injuries were reported and the well has been temporarily abandoned. Officials from the Bureau of Safety and Environmental Enforcement (BSEE) flew to the site on Sunday to investigate.

“The leak was isolated, stopped and remedial action has been approved by the regulator (BSEE), and this includes temporarily abandoning the well, pulling the riser and making appropriate repairs,” said the Shell spokesperson. “The drilling mud used is synthetic and biodegradable.”

According to Shell, the leaked fluid contained 180.2 barrels of base oil, which is the synthetic material in the drilling mud. The fluid used in the mud at Appomattox is supplied by M-I LLC, a unit of Schlumberger Ltd., which is a Shell contractor.

Shell in May received approval from federal regulators to drill up to eight exploration wells at the Appomattox prospect, which was discovered in early 2010 (see Daily GPI, April 29). Prior to the deepwater drilling moratorium, Shell had drilled two wells in the prospect to more than 25,000 feet below the seabed, the first encountering 530 feet of oil pay, the next 380 feet.

Shell has an 80% operating interest in the prospect; Nexen Inc. holds the remaining stake. Earlier this year Nexen CEO Marvin Romanow told investors that Appomattox “has the potential to be our best discovery” in the GOM (see Daily GPI, July 16).

Shell’s approved Appomattox drilling plan was challenged in federal court earlier this year by conservation groups that argued that the government acted prematurely in approving the plan without completing an environmental study of the GOM. The Center for Biological Diversity said the leak at the drilling rig solidifies the risks to drilling in the offshore.

The leak “demonstrates once again that no matter what Shell says, oil spills will continue to happen so long as Interior continues to rubber stamp drilling plans,” said Miyoko Sakashita, a senior attorney at the center.

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