The cement 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. 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.
The well is created by drilling a hole 5 to 30 inches (13 – 76 cm) diameter into the earth with an oil platform which rotates a drill bit. After the hole is drilled, a steel pipe (casing) slightly smaller than the hole is placed in the hole, and secured with cement. The casing provides structural integrity to the newly drilled wellbore in addition to isolating potentially dangerous high pressure zones from each other and from the surface.
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. The problem could have been a faulty cement plug at the bottom of the well. Another possibility would be that cement between the pipe and well walls didn't harden properly and allowed gas to pass through it. A 2007 study by 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. (WSJ, 4/30/2010, TreasurePicks)