The New York Times takes a look into the cement used at Deepwater Horizon. From that initial problem, a cascade of failures followed.
Workers on the rig somehow missed the buildup of pressure inside the well, even though keeping a close eye on that is the most important safety requirement of their jobs.
… Once the well began leaking, the pressure buildup should have been evident to workers monitoring the drilling mud that was being circulated through the pipes. An oil well blowout is almost always preceded by such warning signs, but a crew must be alert to catch them.
If the higher pressure had been detected in time, workers should have been able to close off the well with valves and dissipate the pressure safely. A central question is whether the crew or managers on the Horizon were in too much of a hurry to wrap up the work and move to the next job.
Believing they were virtually done with the well, managers on the rig made a fateful decision. They decided to begin pumping seawater into the drilling pipes, displacing the heavy mud that usually fills them.
Once some mud had been removed, the pressure at the bottom of the well was apparently able to overcome the counterweight of the fluid in the pipes. Seawater and drilling mud suddenly began spewing upward, followed by a tremendous rush of natural gas.