CD1 leak causes traced
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ConocoPhillips provides analysis of March 4 gas release to regulators, Congress
Kristen Nelson Petroleum News
ConocoPhillips Alaska has determined two causal factors and four conditions which led to the natural gas release at the Colville River unit CD1 pad on March 4 and to initial releases from multiple wells across the pad.
The company provided its final report on the natural gas release at CD1 to the Alaska Oil and Gas Conservation Commission on May 3. On May 10, the company answered questions from Congress on the incident.
ConocoPhillips said in its report to the commission that gas from what was later determined to be the C10-Halo interval in the WD-03 disposal well, then being drilled at CD1, was observed March 4 as “intermittent, low pressure natural gas releases at CD1-05, which is approximately 450 feet away from WD-03.”
On March 4 the release was reported to AOGCC and an incident management team activated.
ConocoPhillips clarified the timeline in response to questions from members of Congress.
“It took four days from when the natural gas release was first observed to identify the C10-Halo subsurface interval at the WD-03 waste disposal well as a potential source and to capture the flow and route it to facilities,” the company said.
“By March 8, ConocoPhillips had secured the location, determined the most probable gas source, and established a controlled flow path for the gas up the outer annulus of the WD-03 well into the Alpine Central Facility,” the company said in its May 10 letter.
The company told the commission April 1 that an estimated 7.2 million cubic feet of natural gas was not captured and said it believes most of that gas escaped to the atmosphere between March 4 and March 8. On March 8 “gas flow was routed through the WD-03 outer annulus” into the Alpine Central Facility.
In its May 3 final report to AOGCC ConocoPhillips said an estimated 24 million cubic feet was recovered, based on metering data from the flow directed to the ACF from the WD-03 outer annulus from March 8 through March 29, when source control was achieved and WD-03 stopped producing gas after circulation of kill weight fluid.
Causal factors ConocoPhillips identified two causal factors.
In its letter to Congress, the company said its incident investigation report to AOGCC identified two causal factors, “neither of which relate to well design. The investigation team determined that (1) the cause of the natural gas release was a surface casing shoe in WD-03 that broke down when pressure limits were exceeded during freeze protect operations and that (2) subsequent pressure increases in the WD-03 outer annulus were not recognized or address, which could have led to more immediate investigation or remedial action.”
“Pressure limits were exceeded while performing Annular Leak-Off Test, LOT, and Freeze Protect operations on the 13 3/8” x 7 5/8” annulus,” the company told the commission.
A maximum test pressure of 14.9 ppg, pounds per gallon, is specified for the LOT and freeze protect operation, but “actual WD-03 Annular LOT results were 17.2 ppg.”
That excessive pressure during the annular LOT and freeze protect operations on Feb. 27 “most likely broke down the casing shoe and provided an initial pathway for gas migration around the outside of the WD-03 surface casing,” the company said, and the subsequent injection of some 300 barrels of water to displace the mud in the outer annulus as part of the freeze protection “likely expanded the pathway.”
The second causal factor, ConocoPhillips told the commission, was that the pressure increases during post-annular Lot and freeze protect operations were not recognized. “The pressure increases in WD-03’s OA from March 1 to March 3 were not recognized and/or addressed and, accordingly, did not lead to investigation or remedial action during that period. The volume of gas released from the C10/Halo in WD-03 could have been reduced if actions to address the elevated OA pressures had been taken earlier.”
ConocoPhillips told the commission there was also a potential missed indicator. “A well in proximity to WD-03’s well path potentially had indications of gas from shallower zones than the Qannik. Further review into the source of this gas may have informed WD-03 well planning.”
CD1-48 was identified on the pre-drill anti-collision review “as a well proximity risk to the planned WD-03 well path.” While the Qannik interval was cemented off in CD1-48, “efforts to bleed off pressure within the CD1-48 OA were unsuccessful, indicating a potential source of gas shallower than the Qannik formation. Further review of the source of this gas might have informed WD-03 well planning,” the company said.
Cementing wells In its letter of the members of Congress, ConocoPhillips said the WD-03 was “constructed in accordance with regulatory requirements, including cementing and casing regulations.”
It is standard industry practice in Alaska - consistent with regulations - “not to fully cement wells. Keeping the outer annulus of the intermediate section of a well uncemented allows for freeze protection and well diagnostics to be carried out during the life of the well, which enhances well integrity.”
Wells crossing intervals deemed significant hydrocarbon zones or “abnormally geo-pressured strata” require cement isolation, the company said, but the C10-Halo interval was determined not to be a significant hydrocarbon zone and not abnormally geo-pressured in the area of the WD-03.
Conditions In its final report to the commission, ConocoPhillips said: “Based on historical evaluation methods used to successfully drill 49 other CD1 wells, the C10/Halo at the WD-03 well path was determined not to be a ‘significant hydrocarbon zone’ or ‘abnormally geo-pressured strata’ during pre-drill planning and/or during drilling operations,” so no cement isolation was deemed necessary.
“While cement bond/USIT logs are not typically run to evaluate surface casing cement in production and injection wells, the USIT log run in WD-03 provided some relevant data related to identifying a potential gas migration pathway,” the company told AOGCC.
Another condition cited by the company relates to freeze protection.
“Displacing water with diesel in WD-03’s OA for freeze protection purposes reduced the hydrostatic pressure in the OA, causing it to become hydrostatically underbalanced relative to the C10/Halo pore pressure,” ConocoPhillips said.
While the diesel was necessary for freeze protection, the displacement allowed “gas to flow out of the C10/Halo and into the WD-03 OA.”
There is a permafrost thaw bulb under the CD1 pad, the company said, “that provides little to no resistance, both vertically and horizontally, for any gas that escaped from the WD-03 wellbore due to the compromised surface casing shoe and that also reached the base of the permafrost. The condition manifested as surface releases hundreds of feet from WD-03 along the well row and across the CD1 pad.”
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