Friday, December 16, 2011

Deepwater Horizon - NAE Report

FIGURE 2-7 Uncontaminated cement compressive strength tests (DP = differential pressure). Source: Committee.

The most definitive report yet on the Gulf oil spill has just been released. This long awaited report by the National Academy of Engineering (NAE) on the causes of the Gulf oil spill has been buried like a document dump on the Friday before Christmas. This is the final report by the NAE. It was initially empowered to do this investigation by Interior Secretary Ken Salazar, who then promptly ignored its preliminary findings when he imposed his unilateral drilling moratorium in May 2010. Combined with the juicy news that BP has alleged in federal court that its cement contractor, Halliburton, has engaged in the destruction of physical evidence, a claim Halliburton has denied, this is guaranteed to become a hot button issue. The NAE report provides support for BP's claim and a motive for Halliburton's alleged improprieties. The most important new information is a detailed examination of the nitrified cement that was used. A careful reading of the discussion of Compressive Strength of Foamed Cement and Un-Foamed Cement on pages 22-23 of the report is quite revealing.

The properties of Class H cement are well known. The properties of foamed cement are not well known and not easy to measure because of the compressibility of the foam. In principle, the compressive strength of foamed cement should be less than the compressive strength of unfoamed Class H cement, given the same curing conditions and additives. Testing has shown this to be true. The compressive strength of foamed cement has been shown to be approximately 35 percent of that Class H base cement under the same curing conditions (Gardner 2010). Testing done by Chandler Engineering (Sabens and Maki, Jr 2002) has shown that foamed cement begins to establish compressive strength at about the same time as the base cement (Class H in this case), but the strength of the foam continuously lags that of the base cement as curing time increases. Accepting these trends as representative, the committee created Figure 2-7 to show the compressive strengths of the various cement slurries. The Chevron (protocol 1) and Halliburton base slurry curves are taken from the laboratory testing done on those two un-foamed slurries. The curves for the two foamed cements are not from direct measurement, but assume the foamed cement compressive strength is reduced according to the foam protocol used in the Chevron test software (by a factor of approximately 35 percent). [SNIP]
Figure 2-7 shows the time at which the negative pressure test was started after cement slurries were pumped into the Macondo well. The figure also shows a differential pressure of about 999 psi that was created between the reservoir pressure and the reduced hydrostatic pressure inside the casing during the negative test (see Appendix C for the calculation). Figure 2-7 indicates that the foamed cement using the Chevron data would have just barely established the strength required to resist crushing under the differential pressure imposed by the negative test, assuming that the cement was not contaminated or altered by other events. The foamed cement using the Halliburton base data and the foam algorithm would not have achieved sufficient compressive strength.

In simple English, when the crew of the Deepwater Horizon replaced the drilling mud in the riser above the blowout preventer and upper section of the production immediately below (down to a depth of about 8,300 feet) with lighter sea water, they created a hydrostatic pressure at the very bottom of the well (in the shoe track) that was 999 pounds per square inch less than the pressure of the formation. As you can see in figure 2.7, the Halliburton foam algorithm (HAL Foam Algorithm) line is below that value at the time the test took place (about 900 psi at 16.4 hours after the cement was pumped into place). It is also apparent that allowing more time for the cement to cure had little additional effect. Contrast that to the curve for the un-foamed Halliburton cement (HAL base Cmt). At 16.4 hours the foamed cement has a compressive strength of about 900 psi and is steady but the un-foamed cement is about 2700 psi and rising, peaking at about 23 hours. So experience would have made them expect a compressive strength of 2700 psi when all they had was 900 psi.

It was the failure of the crew to recognize this dramatic reduction in compressive strength that led directly to the blowout. The cement was not strong enough to withstand the 999 psi pressure differential and broke during the negative pressure test. While the transcripts of the Joint Investigative team (JIT) have been hidden behind a firewall making linking impossible, the gist of the testimony by the various witnesses was that none of them had used foamed cement at the depth of this well, 18,000 feet. Combined with an abnormally high percentage of nitrogen in the mix, which resulted in a lower compressive strength, they were in totally uncharted territory. Add in that very few of the men had ever performed a negative pressure test because the standard practice was to wait until the well was being completed, at a later stage of construction, before going to an “underbalanced’ condition and it is no surprise that the crew was confused. They did try to close the blowout preventer after mud was spewing onto the deck, but the gas had risen above the blowout preventer so it had no way to stop the gas that had already flowed past it. The crew’s fate was sealed when the diesel engines sucked in the flammable natural gas/air mixture, over-revved and exploded.

The most obvious lessons learned ought to be:

1) If no one has done the procedure before, get qualified technical experts to teach you how before attempting to do it! Ask questions!!
2) Never use a nitrified cement with such a high percentage of nitrogen in it a pressure barrier!
3) If you have never done a negative pressure test before, you have never actually confirmed the quality of the cement job in the shoe track. This is why the prevalent cement failure mode is an annular blowout, negative pressure tests were quite rare and by the time you unbalance a completed well you want the gas to flow, so it matters not whether the flow is through perforations in the production casing or U-tubing up through the shoe track.
4) A huge amount of effort was wasted on the expectation that this was an annular blowout, when it was a “wet shoe” blowout. A timely attempt at a “top kill” would have succeeded and stopped the flow of oil into the Gulf in mid-May.


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