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The science behind Macondo well capping
After the worst oil spill in U.S. history, BP’s Macondo well was finally capped on July 15, 2010. But the decision process leading to that well capping proved far from easy, given the potential for the capping operation to result in a seafloor oil leak that would have proved much more difficult to deal with than the leak from the well itself.
A paper published in early December in the Proceedings of the National Academy of Science by scientists involved in the well capping decision describes the science behind the decision that stopped the flow of oil from the well.
Leading contender From quite early in the response effort the use of a device known as a capping stack to close off the well became a leading contender for controlling the spill. But, with the well penetrating poorly consolidated sediments beneath the seafloor, the possibility of the downhole pressure buildup following a capping operation causing a catastrophic seafloor oil leak became a major concern. The exceptional subsurface pressure gradient in the sediments at the well site made the sediments particularly susceptible to fracturing, potentially enabling a seafloor oil leak to occur following a well breach, the paper says.
In May 2010 an operation called “top kill” involved pumping drilling mud down the well. And when this operation failed to stop the blowout BP engineers postulated the possibility of a wellbore breach having allowed mud to escape from the well — the existence of such a breach would likely provide a route for oil to escape from the well following a capping operation, thus increasing the probability of a catastrophic leak.
To evaluate the possibility of an subsurface oil blowout, BP and a well integrity team consisting of scientists and engineers from government agencies and academia agreed on a test involving the temporary capping of the well to enable well pressure monitoring, with a procedure to re-open the well within a fairly short time if pressures remained below a specified level.
In the event, when in mid-July the cap was applied, the oil pressure in the well did climb above a level below which there would have been a clear indication of a well breach. But unfortunately when the pressure subsequently stopped climbing the pressure was still too low to completely rule out the possibility of a breach being present.
So, to ensure that no subsurface blowout would occur, government regulators ordered that the wellbore should be re-opened after 24 hours, the paper says.
Meantime, to account for the observed maximum well pressure attained after the capping operation, scientists plugged reservoir data supplied by BP into a U.S. Geological Survey computer model originally designed to simulate the flow of groundwater through subsurface aquifer rocks. By simulating what would happen were there no well breach, given estimated oil flow rates from the well, the scientists were able to determine that the observed lower-than-expected well pressures following capping were likely to have resulted from oil depletion in the subsurface oil reservoir following the well blowout.
And, given this explanation for the observed pressures, the government allowed the capping operation to continue beyond 24 hours, but with continuous monitoring of well pressure and geophysical surveillance data, and with a re-evaluation of the well capping decision at regular intervals.
The geophysical surveillance included the use of seismic surveys, conducted as frequently as four times per day, to seek early evidence of any flow of oil and gas from the well bore through the surrounding rocks.
With the shut-in of the well extending over several days, new pressure data from the well enabled the near-continuous updating of the reservoir model used to assess the possibility of a well breach. And, with the scientists also refining the assumed reservoir geometry used in the model, the well pressures predicted from the model turned out to be a close match with the pressures measured in the well. It appeared that the well had maintained its integrity following the capping operation.
It subsequently became possible to keep the stacking cap in place until Aug. 2, at which time the use of a relief well enabled the Macondo well to be fully sealed off and cemented.
Success in the capping operation can be attributed to collaboration between the many scientists, engineers and emergency response officials involved; clear protocols for data requests through a well-defined chain of command; the very rapid analysis of diverse datasets; the co-location of government scientists with BP staff; continuous access to required expertise and training; and excellent access to BP’s data and mitigation plans, the paper says.
—Alan Bailey
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