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March 2019

Vol. 24, No.10 Week of March 10, 2019

Wells need checking

AOGCC orders BP to recover well casings from at least 4 Prudhoe Bay wells

Alan Bailey

Petroleum News

Following a review of well integrity issues relating to permafrost subsidence in the Prudhoe Bay field, the Alaska Oil and Gas Conservation Commission has ordered BP, the field operator, to recover the casings and production tubing from at least four wells in the field linked to recent well integrity incidents. In a Feb. 28 order the commission told the company that it must use the recovered steel well tubing to further assess the causes of the incidents and any impacts of permafrost subsidence on the condition of the wells. In a March 5 email, BP spokeswoman Megan Baldino told Petroleum News that BP is working with AOGCC to comply with the agency’s order.

Three incidents

The AOGCC inquiry follows three recent loss of well integrity incidents in the field. Two of these incidents, in April 2017 and December 2018, involved wells on drill site 2, in which the wellheads rose, striking the structure of the wellhead housing. In both cases the resulting damage to the wellheads caused surface leakage of oil and gas. In the third incident, in March 2017, an injection well failed during mechanical integrity testing, with permanent damage to the casing string and, again, the release of fluids from the well to the surface.

During an AOGCC hearing on Feb. 13 BP presented evidence regarding the nature of the well failures and reviewed how the company’s well integrity program operates.

Subsidence induced stress

BP told the commission that the April 2017 incident, which involved the 02-03B well, resulted from the subsidence of the permafrost around the well bore, caused by the melting effect of warm fluids in the well. The well is of an unusual three-casing design, with the base of the surface casing in the permafrost. The subsidence-induced stress on the casing had caused the surface casing to detach and then rise upwards. At the time of the hearing the company had not completed its investigation of the December 2018 incident, but this incident, involving the 02-02A well, a well of the same design as the 02-03 well, appears to have the same cause and effect.

But although the 02-03 well was in use at the time of its wellhead failure incident, the 02-02 well had been shut-in for more than 12 years.

Retrieval and examination of the failed casing string from the March 2017 incident indicated that ice plugs had deformed and broken the well tubing, BP told the commission.

Subsidence monitoring

BP also told the commission that, with the advent of high-precision GPS equipment, the company had been monitoring the exact surface elevations of well heads in the field since 2011. This monitoring had revealed some surface subsidence at six pads, four of them in the western part of the field.

Following its investigation of the April 2017 incident, BP shut in 14 wells that have the same three-casing design as the well that failed.

“The small number of wells with this design are monitored in real-time, remain shut-in and their flowlines and wellhouse structures have been removed to prevent recurrence,” Baldino told Petroleum News. “BP remains committed to operating Prudhoe Bay in a safe, reliable and compliant manner.”

The company also investigated nine other wells with surface conductor bases in or close to the base of the permafrost, opting to shut in four of these wells on the basis that it was not possible to be certain of the wells’ safety.

The AOGCC said that BP is conducting studies into permafrost thaw behaviors on its well pads and plans a geotechnical study of drill site 2, to gain a better understanding of permafrost loading on well casings.

Well integrity management

As part of its findings, the AOGCC told BP that that the company’s well integrity management may not be sufficient to identify two-casing wells at risk from subsidence. Essentially, although the company has found no evidence that wells of this type are at risk of failure, the company has no evidence, either, that permafrost subsidence will not cause catastrophic failure, the commission wrote.

Moreover, although BP has developed a model for the failure of the 02-03 well, that model includes untested hypotheses and assumptions. For example, there are uncertainties regarding the significance of the placement of surface casing relative to the permafrost, the comparison of subsidence loads on different casing diameters, and the impact of different rock sequences with different rock properties in the permafrost zone, the commission wrote.

The AOGCC also wrote that the fact that the 02-02 well failed, despite having been shut in, demonstrates the shortcomings of BP’s model for the well failures.

Evidence needed

And, although BP’s understanding of the 02-02 well failure is incomplete, the planned abandonment procedures for this well will prevent the company from recovering further evidence for gaining more insights into the well failure mechanism, the AOGCC wrote.

The AOGCC also said that, although there are slip joints in the surface casings of the two drill site 2 wells that failed, to accommodate some level of thaw related subsidence around the wells, there were no good engineering records regarding the design capabilities of these joints - the wells in question are old wells drilled during early development of the field. The commission also commented that, in the absence of wellhead subsidence measurements prior to 2011, there is the possibility of unknown, earlier subsidence that could have placed unknown stresses on the Prudhoe Bay well casings.

Well interventions ordered

In the light of its findings, the AOGCC has ordered BP to conduct some well interventions in 2019, to recover production tubing and various casings and conductors from at least two of the three-casing wells, including the 02-02 well and other wells that will be determined through negotiations between the commission and BP. The company must also, within the 2019-20 well plugging and abandonment schedule, conduct rig interventions on at least two wells with two-casing designs, to enable an understanding of the effects of well surface casing subsidence. The target wells for this work will be selected by AOGCC in consultation with BP.

Permit applications for the well work must include specifications of data acquisition and analysis to be performed - there are deadlines for reporting on the results of the work. And the commission wants BP to issue its report on its findings from the well 02-02 incident by March 15.

The commission has also spelled out specific plugging techniques to be used for the 14 wells flagged for plugging and abandonment following the Prudhoe Bay well integrity incidents. Further well abandonment requirements may also be issued after a review of the results of the mandated well interventions, the AOGCC wrote.






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