HALLIBURTON IMPLICATED IN OIL RIG EXPLOSION

Halliburton also was the cementer on a well that suffered a big blowout last August in the Timor Sea, off Australia. The rig there caught fire and a well leaked tens of thousands of barrels of oil over 10 weeks before it was shut down. The investigation is continuing; Halliburton declined to comment on it.

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An oil-drilling procedure called cementing is coming under scrutiny as a possible cause of the explosion on the Deepwater Horizon rig in the Gulf of Mexico that has led to one of the biggest oil spills in U.S. history, drilling experts said Thursday.

The process is supposed to prevent oil and natural gas from escaping by filling gaps between the outside of the well pipe and the inside of the hole bored into the ocean floor. Cement, pumped down the well from the drilling rig, is also used to plug wells after they have been abandoned or when drilling has finished but production hasn't begun.

In the case of the Deepwater Horizon, workers had finished pumping cement to fill the space between the pipe and the sides of the hole and had begun temporarily plugging the well with cement; it isn't known whether they had completed the plugging process before the blast.

Regulators have previously identified problems in the cementing process as a leading cause of well blowouts, in which oil and natural gas surge out of a well with explosive force. When cement develops cracks or doesn't set properly, oil and gas can escape, ultimately flowing out of control. The gas is highly combustible and prone to ignite, as it appears to have done aboard the Deepwater Horizon, which was leased by BP PLC, the British oil giant.

Concerns about the cementing process—and about whether rigs have enough safeguards to prevent blowouts—raise questions about whether the industry can safely drill in deep water and whether regulators are up to the task of monitoring them.

The scrutiny on cementing will focus attention on Halliburton Co., the oilfield-services firm that was handling the cementing process on the rig, which burned and sank last week. The disaster, which killed 11, has left a gusher of oil streaming into the Gulf from a mile under the surface.

Federal officials declined to comment on their investigation, and Halliburton didn't respond to questions from The Wall Street Journal.

According to Transocean Ltd., the operator of the drilling rig, Halliburton had finished cementing the 18,000-foot well shortly before the explosion. Houston-based Halliburton is the largest company in the global cementing business, which accounted for $1.7 billion, or about 11%, of the company's revenue in 2009, according to consultant Spears & Associates.

Growing worries about potential lawsuits and other costs of the oil spill in the wake of its rapid spread led investors to clobber stocks of companies involved in the Deepwater Horizon well Thursday.

Halliburton fell 5.3% to $31.60 and Cameron International Corp., which built the blowout-prevention equipment that didn't stop the explosion, dropped 13% to $38.70, both at 4 p.m. in New York Stock Exchange composite trading.

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