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BP reports spill cause Shuts in 14 Prudhoe Bay wells pending a risk analysis for problem well design Alan Bailey Petroleum News
In a report to the Alaska Oil and Gas Conservation Commission, BP has attributed a wellhead leak in the Prudhoe Bay oil field in April to permafrost subsidence and a resulting failure of the surface well casing. BP has now shut in 14 wells that have the same casing design and is conducting an assessment of the wells, to determine the risk level associated with using these wells for production.
The commission has accepted BP’s findings and has agreed with the follow-up action that the company proposes.
“BP has completed its investigation, they have shared the results with AOGCC, and AOGCC agrees with their conclusions and forward plans,” AOGCC Executive Secretary Samantha Carlisle told Petroleum News in a July 18 email.
“BP is committed to operating Prudhoe Bay in a safe, reliable and compliant manner,” BP spokeswoman Dawn Patience has told Petroleum News. “The report identified the most likely cause of the incident was a mechanical failure related to permafrost subsidence specific to the design of this well. BP quickly took appropriate steps to evaluate other similar wells, five of which were in production. Mechanical plugs were put in place and those wells remain shut-in today.”
Vented gas and oil The wellhead incident began on April 14, when gas started venting from the wellhead of well 02-03 on Drill Site 2 of the Prudhoe Bay field. The gas leak generated a spray of oil that impacted the well pad. It turned out that there were two leaks in the wellhead: an oil leak at the pressure gauge assembly at the top of the wellhead structure, and a gas leak farther down the wellhead. In response to the leaks, the well was eventually killed on April 17. An investigation showed that the crude oil spray plume did not leave the pad and that the leaked oil only impacted the pad’s reserve pit - no tundra was impacted.
It was apparent that the leak had happened as a consequence of the wellhead structure rising upward by three to four feet. The upper leak resulted from the pressure gauge at the top of the wellhead hitting the roof of the well house and breaking off. A subsequent investigation showed that the lower leak resulted from a valve handle striking the side of the well house, with the consequent stress on the wellhead structure causing a flange gasket to lose its integrity, according to BP’s report to AOGCC.
Three-casing design The well, drilled by ARCO Alaska in 1970, is one of a few old Prudhoe Bay wells that incorporate a design involving three sections of casing: a surface casing, an intermediate casing and a production casing, according to the BP report. Most Prudhoe Bay wells have just two sections of casing, the report says. The well casing consists of steel tubing that provides structural integrity to the well. And in the case of well 02-03 the bottom of the surface casing is located within the deep layer of permafrost that lies under the surface of the North Slope.
The BP investigation determined that the upward movement of the well 02-03 wellhead that damaged the wellhead resulted from a tensile failure of the surface casing: With the upper section of the casing becoming detached from the lower section, pressures within the well had caused the upper part of the well to move upwards, with the wellhead subsequently hitting the wellhead housing. And it appears that surface casing had broken under a load resulting from subsidence of the permafrost in the area of the well.
A further analysis of the incident indicated that the breakage of the well casing was a consequence of the rigid three-casing design of the well, with the location of the base of the upper casing within the permafrost. The two-casing design used for most Prudhoe Bay wells does not present the same problem because the well tubing in this design is inherently more flexible than in the three-casing design, the BP investigation found.
14 wells shut in As a follow-up to its investigation, BP has now shut in all 14 Prudhoe Bay wells that use the three-casing design with the upper casing terminating inside the permafrost. The company is conducting modeling of the failure potential of these wells and developing ways of predicting potential subsidence loading failures. The company is also applying this analysis to nine other wells which have a three-casing design but where the surface casing terminates in rigid rock below the permafrost: The company views these wells to be at less risk than the other 14 three-casing wells, but wants to better assess the risk level.
At the conclusion of its follow-up analysis, BP plans to risk assess all 23 three-casing wells, to determine whether to resume or continue production from these well. The company will also update its design parameters for new wells, to take account of its findings. BP has also been surveying levels of subsidence at drill site 2 and is enhancing its subsidence monitoring program to confirm the program’s effectiveness in predicting potential permafrost subsidence problems, the BP report says.
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