(EnergyAsia, November 22 2011, Tuesday) — US major Chevron has accepted “full responsibility” for an ongoing oil spill off the Brazilian coast caused by its drilling operations 370 km off the northeastern coast of Rio de Janeiro.

As a result of under-estimating the pressure in an underwater reservoir, Chevron said an appraisal well sprung a leak on November 7, leaking between 5,000 and 8,000 barrels of crude oil as estimated by the state agency, Brazils National Petroleum Agency (ANP). The leak has caused an oil sheen, estimated at around 18 barrels, to form about 120 km off the Brazilian coast.

The company’s Brazil country manager, George Buck, said it has deployed up to 18 vessels to support well plugging operations at the Frade field, and sheen cleanup.

“The vessels are using oil response methods approved by the Brazilian government, such as deployment of containment booms and surface skimming. At no time has sand or chemical dispersants been used in the process,” it said.

In declaring that Chevron accepted full responsibility for the incident, Mr Buck said:

“We were drilling toward a targeted reservoir and encountered an unexpected pressure spike or kick. We believe the kick caused fluid to move into the wellbore and increased pressure opened a segment of the wellbore. Fluid then escaped to reach thin fissures and migrate to the ocean floor.

“We moved quickly to begin plugging and cementing procedures to seal the source. Our combined efforts greatly diminished the size of the sheen and stopped the source of the seep flow within only four days of first detection. We believe no new oil is seeping from the reservoir. Seep lines continue to drain in the form of droplets.

“The well was safely shut in, following standard operating procedures. All equipment performed as expected. A complete investigation is underway to understand the cause of the incident.

“Findings will be shared with Brazilian government agencies and Chevrons global operations to prevent this from happening again.”

Subsidiary Chevron Brasil Upstream Frade Ltda has suspended all drilling operations at the Frade field, which it jointly owns with Petrobras and the Frade Japan consortium. The field’s producing wells are maintaining output at 79,000 barrels of oil equivalent per day.