In order to prevent oil and natural leakage, rig workers pump cement down wells after they've finished drilling. This process requires a very particular type of cement, one that must be mixed and stirred in a precise fashion. If the cement is flawed, it can crack or fail to set properly, allowing oil and gas to leak through. If gas escaped through the Gulf rig's cement, it could have shot back up the well -- what's known as a "blowout" -- and ignited the fatal blast.
Halliburton was also responsible for cementing a well off the coast of Australia that blew last August, leaking oil for ten weeks before it was plugged. Though the investigation continues, an official from the U.S. Minerals Management Service testified that a poor cement job probably caused the explosion.
Regardless of whether Halliburton has a particular history of faulty cementing, the process is a risky endeavor. According to the Wall Street Journal, "a 2007 study by three U.S. Minerals Management Service officials found that cementing was a factor in 18 of 39 well blowouts in the Gulf of Mexico over a 14-year period. That was the single largest factor, ahead of equipment failure and pipe failure."
The BP oil spill may spur regulators to reevaluate the basic safety of offshore drilling. Obama has already made clear his intention to suspend expansion of offshore drilling until the Gulf spill has been investigated. If cementing is found to have caused the explosion, look out for new regulation -- or possibly legislation -- that demands safer cementing practices before sanctioning new drilling off domestic coasts.