ROBERT SIEGEL, host:

Now, what may have caused this massive oil spill. One focus of the investigation is a cementing job done hours before the explosion. Cementing has been blamed in previous oil rig blowouts. And some drilling experts say in this case, it's hard to imagine a scenario where cementing did not play a role.

NPR's Elizabeth Shogren has the story.

ELIZABETH SHOGREN: Twenty hours before the accident, a team from Halliburton was pumping cement into the Deepwater Horizon well. There was already a pipe in the well for the oil to flow through, but that wasn't happening yet. Cementing is one of the last steps in the construction of a well. It's done to seal the crack between the pipe and the wall of rock.

Mr. BRUCE BULLOCK (Director, Maguire Energy Institute, Southern Methodist University): They're pumping it down the middle of the pipe, and you want it to come back up between the pipe and the crack of the wall.

SHOGREN: Oil industry veteran Bruce Bullock now heads the Maguire Energy Institute at Southern Methodist University. He says it probably will take a long time before investigators know what went wrong, but he says there are several ways to botch a cement job. For instance, you could mix it wrong, or not pump enough of it into the well.

Mr. BULLOCK: Let's say, for instance, there was an area that the cement did not fill in.

SHOGREN: Bullock says oil and gas could be pushing through the rock into the drill hole. If that was the case, operators should have known about it because they would have had cement left over. It's kind of like when you're baking. If the batter doesn't fit in the pan, that's a clue something is amiss. When you're drilling a well, it's a signal that you're getting pressure back up the hole, probably from a mixture of oil, gas and other stuff.

Mr. BULLOCK: If some oil and gas were to invade the well, kind of get around the cement, if you will, before it dried and then work its way up the well then you could have a problem.

SHOGREN: Oil and gas could surge out of the well. And if there's enough of it, it will explode, like Deepwater Horizon did. A blowout preventer should stop a surge from causing a leak, but in this case, it didn't.

Still, Bud Danenberger, who recently retired as the chief federal regulator of offshore production, says the cementing job shouldn't have created the problem to begin with.

Mr. BUD DANENBERGER (Former Chief, Offshore Regulatory Programs, Offshore Minerals Management): It's pretty established technology if you follow the right precautions and you monitor your information, that should be straightforward.

SHOGREN: But in recent years, cementing problems have become more prevalent. Danenberger was a co-author of a report that showed cementing contributed to half of the 39 blowouts on U.S. rigs over 15 years. After that, government pushed to change cementing standards but let industry write them.

A cementing job done by Halliburton is also implicated in an Australian oil rig disaster last year that gushed oil into the Timor Sea for 74 days. Halliburton refused to talk on tape about cementing or the accident in the Gulf, but the company's press release says its crew performed the Deepwater Horizon cementing job correctly.

That's also what the company told members of Congress yesterday, including Democrat Ed Markey from Massachusetts.

Representative ED MARKEY (Democrat, Massachusetts): I am suspicious that the cementing job played a role in this.

SHOGREN: But Markey says he's going to withhold judgment until more is known about the accident.

Rep. MARKEY: This is going to be the biggest investigation that's taken place since Three Mile Island. We will have to go back and ensure that whatever happened never happens again.

SHOGREN: Even if the cementing job was faulty, experts say it's likely it was only one of a series of failures responsible for spewing millions of gallons of crude into the Gulf.

Elizabeth Shogren, NPR News, Washington.

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