Saturday, May 22, 2010

What caused the Deepwater Horizon disaster?

See the full post on The Oil Drum: http://www.theoildrum.com/node/6493#more

The blowout and oil spill on the Deepwater Horizon in the Gulf of Mexico was caused by a flawed well plan that did not include enough cement between the 7-inch production casing and the 9 7/8-inch protection casing. The presumed blowout preventer (BOP) failure is an important but secondary issue. Although the resulting oil spill has potentially grave environmental implications, recent efforts to limit the flow with an insertion tube have apparently been effective. Continuous efforts to slow or stop the flow include drilling two nearby relief wells that may intersect the MC 252 wellbore within 60-90 days.


11 comments:

Anonymous said...

Thanks for posting this.

However, my question is that you feel that the cause of the blow-out was a poor cement job up the annulus, due to a lack of cement, correct?

My own calculations indicate that unless the hole was very badly washed out, more than enough cement was pumped to ensure a decent amount was in the 9 7/8" x 7" annulus.

That doesn't mean to say that it wasn't channeled, allowing gas (or condensate?) to enter the annulus.

However, that charts you present would not show anything awry if, as you theorise, all this was happening in the annulus as the annulus pack-off (seal) was in place and pressure tested - so there was no communication route between the well-bore and the annulus at that time.

In addition, I doubt your explanation of the rapid increase in pit volume is correct, for two reasons.

Firstly, the trip tank volume does not change until 20:30 hrs (when it actually drops) and secondly, the gas reading only goes to a maximum of about 75 units (only about 15,000 ppm) - if pure gas was coming up, there would be much, much higher readings.

In addition, the casing diagrams have seen indicate that the next casing at surface was 16", the burst of which is much less that the collapse of the 9 7/8" of the tapered string.

So wouldn't any gas pressure in the annulus rupture the 16" and therefore the blow-out would be much closer to the BOP Stack rather than very close to surface, which is what the photos indicate?

If a poor cement job is the basic cause, wouldn't it be possible that the inflow test which apparently failed, failed bacause of gas / condensate entering through the shoe track?

Best Regards

Scott
(britinhouston@yahoo.com)

TBD said...

Scott is pretty much spot on. There was adequate cement. The neg test that brought the well in would not have done so even if there were no cement. The well was logged for 5 days without flowing with 14 ppg mud in hole. When the long string was run, cemented, and landed everything outside the production string should have been isolated cement or not. I have two thoughts but I pitch my best shot. The auto fill float equipment did not convert (would not seal from bottom up). This would allow a positive test (plug bump), but not a negative test. The pay zone being within a few bbls of the shoe allow HC influx to shoe (inside production pipe). Once the flow started inside the pipe it unloaded very fast (I've personally seen this happen). The production pipe became bouyant (due to gas inside instead of 14 ppg mud) enough to lift landing ring up and created a second flow path. By then they were trying to close in on 9-7/8" casing or the landing head. I have questioned why they have not cut the riser connector off and put a new blind on top. The 16" is not rated to withstand the SICP...no to mention the rupture disc in it. So shutting it in may present the risk of a breech below wellhead which is not acceptable. One other item of interes..who in there right mind would do a neg test 16 hours after cementing a production string. Also; a CBL run that soon may be of little conclusive value. I have run CBL's at 24 and 72 hours. I've had the ones at 24 show no bond and 72 hours show perfect bond...same well same borehole fluids same hydrostatic. Temperature cures cement and BHT goes up significantly once circulating is suspended. Bottomline it was the failure to honor the failed neg test data, being in a rush, failed float equip, inadaquate BOP's shear rams, and poor casing design. IMHO

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