The tricky process of sealing an offshore oil well with cement – suspected as a major contributor to the Gulf of Mexico disaster – has failed dozens of times in the past, according to an Associated Press investigation.

Yet federal regulators give drillers a free hand in this crucial safety step – another example of lax regulation regarding events leading up to the April 20 explosion on the Deepwater Horizon oil rig.

Federal regulators don’t regulate what type of cement is used, leaving it up to oil and gas companies. The drillers are urged to simply follow guidelines of the American Petroleum Institute, an industry trade group.

Far more stringent federal and state standards and controls exist on cement work for roads, bridges and buildings.
While the chain of failures on the Deepwater Horizon is under investigation, rig owner Transocean has singled out cement work as one likely fundamental cause of the blowout.

Even before Transocean pointed to cementing, independent experts suspected it, partly because faulty cement work – either badly mixed or poorly placed against well walls – is so prevalent at offshore wells.

An AP review of federal accident and incident reports on offshore wells shows that the cementing process has been implicated at least 34 times since 1978. Many of the reports, available from the U.S. Minerals Management Service that regulates offshore wells, identify the cause simply as “poor cement job.”

• In a November 2005 accident where the Deepwater Horizon was positioned above another well in the Gulf, faulty cement work allowed wall-supporting steel casing to come apart. Almost 15,000 gallons of drilling fluid spilled into the Gulf.

• Just a week later in a nearby well at another platform, cement improperly seeped through drilling fluid. As a result of an additive meant to quicken setting time, the cement then failed to block a gas influx into the well. When the crew finally replaced heavy drilling fluid with lighter seawater, as they also did last month before the blowout at Deepwater Horizon, the well flowed out of control and much of the crew had to be evacuated.

• Cementing was identified by federal investigators as a glaring cause of an August 2007 blowout, also off Louisiana. They said: “The cement quality is very poor, showing what looks like large areas of no cement.”

Reports by MMS, a branch of the Interior Department, also provide evidence of the role bad cement work has played in accidents. One study named cementing as a factor in 18 of 39 well blowouts at Gulf rigs from 1992 to 2006. Another attributed five of nine out-of-control wells in 2000 to cementing problems.

SPECIAL CHALLENGES

Cementing in the oil rig business is a sensitive, involved process. Well cement constitutes an essential barrier that is difficult to install and control, said Gene Beck, a petroleum engineer at Texas A&M University at College Station.

Deep-water wells pose special challenges: severe pressures and temperatures, as well as the need for specialized equipment and lots of cement. The wellhead of the Deepwater Horizon operation sat on the ocean floor, nearly a mile from the surface. The drill hole itself went another 13,000 feet into rock.

All cement begins as a slurry with cement flakes and water. Contractors then add ingredients to make the cement set at the right time and to keep out gas and oil.

There are three major U.S. cementing companies: Halliburton, Schlumberger and BJ Services. Cementing is typically performed by such rig contractors as part of a broad range of drilling services that they supply.

Halliburton, which had the Deepwater Horizon job, mixes in nitrogen to make its slurry more elastic. The nitrogen helps create a lightweight cement that resembles a gray foamy mousse and bonds better to the casing.

But the recipe also depends on the job, because cement must respond to varying pressures and temperatures. Cement contractors work closely with oil and gas companies on the formulas for individual wells. The oil and gas companies have the final say on what is used.

Once the consistency of the mix is decided on, it is pumped deep into the well, where it first sinks to the bottom and then oozes upward to fill the narrow spaces between the steel casing pipe and rock walls. When the cement sets, the casing and cement are supposed to form an impenetrable wall to keep gas or oil from pushing into the hole anywhere but the bottom, where its flow up the pipe can be controlled.

But if gas bubbles invade the setting cement, they can form a channel for pressurized gas and oil to surge uncontrollably up the well, usually around the casing. The cement must be strong enough to withstand up to 5,000 pounds of pressure per square inch, to keep the well walls from collapsing.

“Cement is cheap, and it fixes a lot of problems, but it’s not a good place to cut corners,” Beck said. Many oil and gas companies will scrimp, though, if they don’t think they need all the ingredients in the cement, he said. Cement is often squeezed in later to try to fill gaps, but Beck said the success rate of this remedial work is low.

SIGN OF TROUBLE

Halliburton was completing the final cement work on the exploratory well beneath Deepwater Horizon in the wee hours of April 20. It added an initial cement plug to the well to act as a cap until a later production phase.

Workers started running a series of tests to check if the cement and casing could stand up to sufficient pressure. The first tests of outward, positive pressure showed no problems.

In the first sign of trouble, though, the well then failed a negative pressure test, where internal fluid pressure is reduced, according to congressional testimony from a BP PLC executive. It showed different pressures in two areas, indicating an unseen leak somewhere in the well.

Despite the test, managers eventually decided to replace drilling fluid with seawater and set a final cement plug so the well could be mothballed pending a decision to possibly begin production drilling.

And while it is not yet clear what sections of the casing or cement may have failed, it is known that the blowout ignited and exploded before the last plug was set.

In the aftermath of the blowout, questions have been raised about the safety of nitrogen-laced cement foam. But several cementing experts told the AP it is a sound technique.

Halliburton says it has used such a mix on scores of wells and told a congressional committee that the cementing on the Deepwater Horizon job was successful.

But in the wake of the accident, some experts support mandatory uniform cement standards for underwater wells.