Starting from mid June, Congress made multiple requests to BP to explain condition of the leaking well and provide facts about possible oil leak into rock formation surrounding the wellbore (read letters here).

All along since the Macondo Well ruptured on April 20 in the Gulf of Mexico, BP was designing a well cap from “scratch” with two objectives in mind: 1) to contain all leaking oil and 2) to have possibility to shut-in the well. As we learned from Congress, Steven Chu, Admiral Thad Allen and independent sources BP knew about sea floor oil leaks they discovered during Top Kill operation, but was not forthcoming about oil sipping from sea floor.

It is not surprising after Top Kill procedure BP’s press briefings did not mentioned anything about designing an impermeable well cap. The first week of July BP installed mini Blowout Preventer, a cap with three RAM valves that serves dual purpose: to contain all leaking oil and to shut-in well if wellbore can hold oil reservoir pressure.

Admiral Thad Allen said it took so long to design this cap because BP had to manufacture and test this specially designed cap from scratch.

Kent Wells of BP mentioned in one of BP’s briefings that Macondo well will be considered intact, or unbroken, if wellbore can hold 8,000 – 9,000 psi pressure during well integrity test. If pressure is found significantly lower than 8,000 psi that would imply oil sips into surrounding rock formation. Oil leaking into rock formation poses a great risk of sea bed erosion and all measures are taken to avoid such outcome.

Congress Letters to BP, source link:
http://globalwarming.house.gov/files/LTTR/2010-06-13_BPWellIntegrityFollowup.pdf