Well integrity examined
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BP provides testimony to AOGCC on the condition of wells in Prudhoe Bay field
Alan Bailey Petroleum News
Following incidents involving oil and gas leaks at the wellheads of two wells in the Prudhoe Bay field, the Alaska Oil and Gas Conservation Commission is conducting an inquiry into the overall integrity of wells in the field. BP has filed testimony with the commission regarding the well integrity issue. And on Feb. 13 the commission held a public hearing on the issue.
The leaks that triggered the inquiry involved two wells on Drill Site 2 in the field. One of those leaks was from the wellhead of well 02-03 in April 2017. The other was from the wellhead of well 02-02 in December 2018. The commission is concerned that these two incidents may reflect a more general issue relating to the mechanical integrity of wells in the field.
Anchala Klein, BP Alaska vice president for wells, told the commission during the hearing that the actions that BP has taken following the first incident had been insufficient to prevent the second incident.
“BP does regret this outcome and has and will take additional efforts to prevent any recurrence,” Klein said.
Three-casing design Both wells that leaked were old wells with an unusual three-casing design, with the base of the surface casing positioned inside the permafrost. In the case of the 02-03 well BP determined that the subsurface melting and subsidence of permafrost, caused by warm fluids moving up the well, had caused the surface casing to become detached from the deeper casing in the well: The surface casing had subsequently risen and struck the well housing, thus resulting in the leaks. BP told the commission that the wellhead failure of the 02-02 well appears to have resulted from the same casing movement mechanism.
The company has now shut in, plugged and disconnected 14 wells that have the problematic casing design. For two of the wells, BP has drilled new wells from different locations, targeting those wells’ reservoir targets. The company also investigated nine other wells at potential risk because of surface conductor bases in or close to the bottom of the permafrost: The company shut in four of these wells on the basis that it was not possible to be certain of the wells’ safety. BP told the commission that the remaining five wells had designs that did not pose a risk of a separation of the conductor as a consequence of permafrost subsidence.
Doug Cismoski, BP wells manager for intervention and integrity, told the hearing that the well houses have been removed from the 18 shut-in wells. Warning signs have been placed next to the wells, and scaffolding is being erected around them.
In addition, the company commissioned metallurgical testing on the conductor piping of an injector well that had failed a mechanical integrity test. That testing found that the well’s production casing had fractured as a result of overload, and not as a result of imperfections such as corrosion or metal fatigue, or from the use of drilling equipment. The failure of this well does not appear to be related to permafrost subsidence and may have resulted from inward buckling caused by ice, Cismoski said.
Surface subsidence measurements In terms of more general issues relating to the potential subsidence of the permafrost at Prudhoe Bay, the advent of precise elevation measurements using GPS devices enabled BP to start conducting precision elevation measurements of well heads in 2011. This type of surveillance started in the western part of the field and was extended to the eastern operating area in 2014, Ryan Daniel, BP Alaska wells integrity and compliance leader, told the commission hearing. And BP conducted a major survey in 2018, Daniel said.
The company has found that, for the most part, there is minimal surface subsidence in the field’s pads. However, subsidence has been found at six pads, four of them in the western part of the unit. Subsistence rates range from 0.2 feet per year to 0.03 feet per year, Daniel said. The higher rates are in the west. It is not known whether there is any connection between global warming and surface subsidence of the permafrost, he commented.
Daniel emphasized that the well failures under investigation resulted from an old well design in conjunction with subsurface subsidence. That subsidence appears to have pulled the casing apart, releasing the upper part of the casing to float upwards and strike the well housing.
Downward pressure Surface subsidence, on the other hand, would tend to put downward pressure on the well casing. The question of whether the well casing would subsequently sustain damage would depend in particular on how uniformly the movement of the casing would be distributed in the subsurface. Localized strain in the casing could result in buckling - BP can detect buckling of this type in conjunction with well testing, Daniel said.
BP, in its written testimony, said that since 2011 the company has been engaged in engineering work to better understand the impact of permafrost subsidence on well integrity. To date no operated wells have failed pressure integrity tests as a consequence of subsidence.
BP said that in conjunction with its surveillance program it has been categorizing pads, based on their relative risk resulting from wellhead elevation displacement. The company determines the frequency and type of surveying for individual pads, based on each pad’s risk level. Surveillance techniques include wellhead elevations surveys, the use of 3-D calipers for monitoring internal well tubing profiles, and pressure integrity tests. And the interior conditions of wells on pads with increased wellhead displacements are tested using tubing inserted down the wells.
Shut-in wells BP also responded to a series of questions relating to wells that exhibit sustained casing pressures and that are shut in for long periods of time. The company commented that, in alignment with AOGCC regulations, its wells have both primary and secondary barriers, to prevent fluids flowing out of a well either in the subsurface or at the surface. The company also has a well integrity management system involving some 6,000 pressure and temperature transmitters placed in wells, with multiple transmitters placed in individual wells. These transmitters automatically provide minute by minute pressure and temperature reading from well inner and outer annuli.
The company also said that it applies additional well barrier assurance requirements to wells that have been identified as exhibiting sustained casing pressures. A computer application maintains comprehensive well information. Well integrity engineers review the status of wells, and address wells that do not meet the appropriate well barrier operability requirements. A well identified in this way becomes subject to some combination of actions such as further review, well work diagnostics, pressure testing and repair work. The well may be classified for a limited time as under evaluation, or it may remain offline and not operable.
While a well that may be susceptible to sustained casing pressures is monitored daily, experienced well integrity engineers use a number of parameters to assess whether the well can safely continue to operate.
The company also said that it uses a standard engineering methodology to determine the safe limits for well operating pressures, in response to any loss of internal wall thickness, or pitting resulting from corrosion or erosion. Tubing calipers or ultrasonic imaging can provide data about well wall thicknesses.
P&A of obsolete wells When it comes to a well that has remained shut in for a long time, BP assesses whether the well may have any future utility. The Prudhoe Bay owners have voluntarily agreed with the commission to use a risk-based methodology to plug and abandon wells with no future utility. And BP has prepared a list of candidate wells for plugging and abandonment between 2019 and 2021. A number of other defunct wells are already undergoing plugging and abandonment procedures, BP’s testimony said.
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