A new study published in the Proceedings of the National Academy of Sciences reveals that after the Deep Water Horizon disaster, engineers and scientists weren't sure that capping the Macondo well was the best idea. In fact, it could have lead to an even larger, harder to control spill.
Ars Technica has a nice post on the study, which explains the work of experts after the disaster. A Well Integrity Team was assembled to analyze the composition of the sea floor around the well to figure out if a cap would be enough to contain all of the oil in the Macondo reservoir.
The concern wasn't the strength of the cap—experts were fairly sure it would hold—but rather the strength of sea floor around it. You probably recall the horrifying images of oil relentlessly spewing into the ocean. The concern was that capping the well would merely disperse that pressure beneath the sea floor from which it could spring up in countless different locations.
In other words, capping one out-of-control well could have lead to a much bigger, much harder to control deep water problem.The key would be to carefully monitor the pressure of the well after it was capped so that the solution could be aborted if it turned out it was only making things worse. According to Ars:
Armed with model simulations of how the well would behave in various scenarios, they carefully watched the pressure inside the well slowly build after the cap was put on. If it climbed to 7,500psi within six hours, they knew the well was intact and would hold. If the pressured stayed below 6,000 psi, they knew the well was leaking and they would need to abort to avoid a blowout.