In 2015, a leak was discovered in a natural gas storage well located in the Aliso Canyon Oil Field near Los Angeles California. The storage facility was used for natural gas intended for electricity generation in the nearby Southern California market. The leak released large quantities of methane and ethane into the atmosphere, posing serious environmental consequences.
Background and Blowout
The wells within the Aliso Canyon Oil Field were first drilled in the late 1950’s, making these aging wells more vulnerable to deterioration and possible leakages. The problem storage well had corrosion problems in places in the casing that led to leaks. In addition, it was found that the well had not been properly cemented, and the annular space outside the production casing was not sufficiently sealed to give added protection if there were problems with the steel pipes.
When the well was converted from a producing well to a storage well, the casing head had to be replaced, and crew members failed to test the new casing cement job, which could have pointed to warning signs of inadequate well integrity at the time. All of these issues set the stage for corrosion issues that could lead to substantial leaks in the well.
Scientists estimated that almost 100,000 tonnes of methane leaked from the well, making Aliso Canyon one of the worst environmental disasters in terms of natural gas leaks to date. Residents were temporarily relocated for safety while well control operations were under way.
Tertiary Well Control
Significant challenges were presented when it came to mitigating the leak, with the most important being the fact that the leak was in the subsurface and was difficult to locate precisely. It was presumed the leak was in the near subsurface, given the gas had been escaping to the atmosphere from the well near the surface location. The source of the corrosion was presumed to be at the top of the groundwater table.
The first tertiary well control steps taken by the company that operated the storage facility, Southern California Gas Company, involved a top kill. Drilling mud and brine were pumped down the wellbore at the wellhead; however, the methane gas and formation pressure exceeded the pressure of the drilling fluids. Multiple additional attempts were made without success. Given the extended period over which substantial flow was coming up outside the casing in the ground around the well, a 25 ft deep crater had formed. Cables were used to stabilize the casing and wellhead while relief well operations continued.
The first relief well was drilled into the formation offset from the storage well, at a depth of roughly 8,000 feet. This well was used to mitigate the natural gas pressure within the formation itself, lessening the flow within the blowout well. A second relief well was drilled to intersect the base of the initial wellbore, executing a bottom kill by pumping mud and cement. On February 18, 2016, state officials announced that the leak was permanently plugged.
The Aliso Canyonleak likely could have been avoided if standard well integrity testing procedures had been carried out. The company responsible for the methane leak, Southern California Gas Company, received criticism for not having more rapid emergency well control procedures in place. Heavy fines were imposed on the Southern California Gas Company.
Image Credits: By Antandrus at English Wikipedia, CC BY-SA 3.0; By EARTHWORKS – Aliso Canyon leak well pad 4 Credit: Earthworks, CC BY 2.0; By Scott L from Los Angeles, United States of America – 1_D4C1832, CC BY-SA 2.0