[lentulusgracchus]

They're supposed to, but they had run pipe and "squoze", so perhaps they dropped their guard a bit ..... and the mud may never have given them a chance, if it was gas-cut oil-base mud (like Ixtoc, which was a very sudden, deadly accident).

When a well's being completed behind pipe, sometimes the drilling prognosis will call for cutting the mud weight or changing over to a completion fluid (heavy brine, usually calcium chloride or calcium bromide brine). So instead of, say, 15.0 ppg drilling mud, they may have had 13- or 14-ppg completion fluid in the wellbore, believing it to be isolated from formation pressures.

Also, a "hand" back on the bank said something about a packer suddenly coming loose -- packers are used to isolate one zone from another (usually to restrain bottom-hole pressures, or separate completion zones). If you're relying on a packer to keep your completion sanitary and it suddenly shakes loose for whatever reason, you've got a real headache.

[razorback-bert]

My SWAG after further reading is the block fell. This explains the thud heard first, the damage to the BOP and the release of gas.