Aliso Canyon gas leak

Southern California Gas Company
Industry	Natural gas
Founded	1867
Headquarters	Los Angeles, United States
Area served	Los Angeles basin and surrounding areas
Products	Natural gas distribution and storage
Parent	Sempra Energy

The Aliso Canyon gas leak—also referred to as the Porter Ranch gas blowout or Porter Ranch gas leak—was a significant methane leak occurring near Porter Ranch, Los Angeles within the Santa Susana Mountains. The leak was first identified on October 23, 2015, when methane began escaping from a well located in the Aliso Canyon Oil Field. This facility is the United States' second-largest underground natural gas storage site and is operated by Southern California Gas Company (SoCal Gas), a branch of Sempra Energy. On January 6, 2016, California Governor Jerry Brown officially declared a state of emergency in response to the incident. The leak was reportedly brought under control by February 11, with permanent sealing completed on February 18.

It is estimated that approximately 100,000 metric tons of methane and 2,500 metric tons of ethane were emitted during the event, causing an increase in atmospheric methane concentration of roughly 0.002%, which is projected to decline by half over a period of six to eight years. This leak stands as the most severe single natural gas release in U.S. history in terms of environmental impact.

The South Coast Air Basin, home to nearly 18 million people, typically emits around 413,000 tonnes of methane and 23,000 tonnes of ethane annually. The carbon emissions resulting from the Aliso Canyon incident may surpass those generated by the Deepwater Horizon oil spill in the Gulf of Mexico.

In comparison, satellite remote sensing has detected that the Raspadskaya coal mine emits over 760,000 metric tons of methane each year without any current plans for mitigation.

Facility

Oil was first discovered at Aliso Canyon in 1938, with production carried out by Tidewater Associated Oil Company, owned by J. Paul Getty, until depletion of the Sesnon-Frew reservoir—the largest within the field—in the early 1970s. On December 18, 1968, a blowout and subsequent fire occurred during valve removal operations; this incident destroyed equipment but resulted in no injuries. Getty later sold his stake to Pacific Lighting Company, which converted the site into a natural gas storage facility in 1972. Unlike modern practices where wells are sealed with cement from surface to bottom to secure casings and prevent water intrusion, older wells at this site lacked such comprehensive sealing. The Aliso Canyon storage complex comprises 115 wells accessing a reservoir capable of holding up to 86 billion cubic feet of natural gas. This gas is supplied to homes, businesses, and utilities throughout the Los Angeles basin, making it the second-largest facility of its kind nationwide.

Discovery of leak

The leak was detected by Southern California Gas Company on October 23 in well SS-25 during routine monitoring activities. On that same day, residents near Porter Ranch reported noticing a substantial gas leak; however, SoCal Gas initially denied its existence until approximately October 28. Many locals suspected that the blowout had started earlier and reported unexplained health issues prior to official acknowledgment. Authorities including California Attorney General Xavier Becerra have formally classified the event as a blowout. Both media coverage and scientific analyses have confirmed significant methane emissions associated with this incident.

An article published in February 2016 suggested that failure of the well casing under high pressure allowed uncontrolled gas release—a scenario consistent with OSHA’s definition of a blowout as an uncontrolled discharge of gas or fluids from a well.

By February 1, 2016, well SS-25 had emitted an estimated total of 91,000 metric tons of methane. The Huffington Post compared this environmental disaster to other major incidents such as BP’s Gulf oil spill and Exxon Valdez spill, underscoring its profound ecological consequences.

Causes

The source of the leak was traced to a compromised metal pipe within the damaged 7-inch casing of the injection well known as "Standard Sesnon 25" (SS 25), situated at a depth of 8,750 ft. Initially, SoCal Gas estimated the leak originated within 500 feet of the gas column. Well SS 25, drilled in 1953, had its safety valves removed in 1979 due to deterioration and leakage; these valves were never replaced because the well was not classified as "critical"—meaning it was not close to populated areas such as roads, parks, or residential zones. A congressional inquiry confirmed that several safety valves on pipes dating from the 1950s at Aliso Canyon/Porter Ranch were similarly removed and left unreplaced. Atmospheric scientist Steve Conley remarked that failure of this 61-year-old wellhead was to be expected. A specialist in injection wells from Texas A&M University proposed that casing failure might have been caused by gas flowing through both tubing and casing to satisfy customer demand.

On December 7, 2015, an anonymous source released an infrared video capturing a methane cloud hovering over Porter Ranch, making visible the otherwise unseen leak plume.

On May 17, 2019, the California Public Utilities Commission announced that Blade Energy Partners had finalized a root cause investigation into the October 23, 2015 leak. Their principal conclusions included:

The breach in the outer 7-inch casing resulted from microbial corrosion triggered by groundwater exposure.
SoCal Gas neglected to perform thorough inspections or investigations following earlier leaks; more than 60 casing leaks at Aliso Canyon since the 1970s lacked any failure analysis.
The company did not implement risk assessments for maintaining well integrity, nor did it have systematic external corrosion protection or real-time pressure monitoring systems for ongoing surveillance.
Revised safety protocols and regulations instituted by the California Geologic Energy Management Division now address most of the root causes identified.

Additionally, on May 17, 2019, the CPUC released a video explaining the details of their leak investigation.

In response to these findings, SoCal Gas stated that Blade’s report confirmed their adherence to existing gas storage regulations at the time of the incident and emphasized that their updated operational practices, alongside new state regulations, mitigate most of the identified causes.

Emissions

Natural gas consists predominantly of methane, a powerful greenhouse gas with a global warming potential approximately 86 times that of carbon dioxide over a 20-year span, decreasing to about 29 times over a century. The Aliso Canyon leak initially emitted roughly 44,000 kilograms of methane per hour (equivalent to about 1,200 tons daily), comparable to the yearly emissions produced by approximately 200,000 automobiles. By January 2016, emission rates had dropped from a peak estimated at 58,000±12,000 kg/h down to around 30,300 kg/h—still comparable to annual emissions from over 1.4 million vehicles. The carbon footprint from this leak surpassed that of the Deepwater Horizon disaster and accounted for nearly 1.5% of annual methane and CO2 emissions in the SoCAB region.

Atmospheric scientist Steve Conley from UC Davis independently assessed emissions by conducting flights over the site in a specialized single-engine aircraft equipped with analyzers—one of only a few such planes in the United States. He highlighted both the unprecedented scale and extreme complexity involved in these measurements. His previous contract with the California Energy Commission enabled him to respond swiftly; without this arrangement, initial detection of the leak’s magnitude might have been delayed.

The TCCON at Caltech detected elevated methane levels within one day but only later pinpointed Aliso Canyon as their origin. TCCON data also facilitated an estimate of ethane emissions at approximately 7,700 ± 1,700 tonnes alongside Conley’s methane estimate totaling about 97,100 tonnes.

Beyond methane, the leak discharged odorants including tert-butyl mercaptan, tetrahydrothiophene, and methyl mercaptan, which are responsible for the characteristic rotten-egg odor and can induce nausea. The released gas also contained carcinogenic compounds such as benzene. These pollutants present potential long-term environmental and health hazards extending beyond immediate local impacts.

Impact on the local population

Residents in the vicinity experienced symptoms such as headaches, nausea, skin irritation, and severe nosebleeds, with approximately 50 children visiting school nurses daily due to nosebleeds. There was a noticeable rise in infections affecting the eyes, ears, and throat. By December 25, 2015, more than 2,200 families in Porter Ranch had been temporarily relocated, while over 2,500 additional households were in the process of relocation. As of January 7, 2016, SoCal Gas had moved 2,824 households—roughly 11,300 individuals—and over 6,500 families had applied for assistance. Two local schools were also relocated during January. To support the community, SoCal Gas established a resource center at Porter Ranch Town Center, distributed weekly updates via mail and email, provided notifications through calls and texts for critical developments, and formed a community advisory committee.

In Eight Mile, Alabama, a natural gas odorant spill in 2008 exposed residents to prolonged mercaptan exposure. Although residents continue to report persistent health issues, they received significantly less assistance compared to those in Porter Ranch. While methyl mercaptan is classified as toxic when inhaled according to its MSDS, government and industry representatives generally regard it as relatively harmless. The extent to which this compound contributed to health problems in Porter Ranch and Eight Mile remains a subject of ongoing debate.

Well closure

By late November 2015, SoCal Gas had conducted six attempts to kill the well by injecting mud and brine to halt gas emissions; the final attempt occurred on November 25. These efforts were unsuccessful due to ice buildup and high upward pressure averaging .

On December 4, 2015, SoCal Gas commenced drilling a relief well targeting the caprock layer approximately deep, with assistance from Boots & Coots, a Halliburton subsidiary. This approach mirrored the relief well technique used during the Deepwater Horizon oil spill in 2010. The strategy involved pumping liquids and cement into the primary well once it was safe to vent the gas. The company anticipated completing the initial relief well by February 24, 2016, with plans for a secondary relief well and repairs extending through March.

Following a seventh attempt to plug the well with slurry on December 22—which resulted in a crater about 25 feet deep around the wellhead—the risk of a blowout increased significantly. Stabilization of the wellhead became necessary using tension cables, which halted further above-ground plugging efforts. This situation raised concerns among state regulators regarding the structural integrity of the wellhead.

In January, SoCal Gas sought approval to capture and burn off the escaping gas; however, state authorities rejected this request citing safety issues.

On February 11, 2016, the relief well reached the base of the leaking well, enabling SoCal Gas to inject heavy fluids to control gas flow. By February 18, officials declared that the leak had been permanently sealed.

Criticism

Steve Conley, an atmospheric scientist overseeing emission monitoring during the leak, noted the absence of an effective rapid response system for incidents of this scale: "No protocols currently exist to measure massive leaks like this or monitor them effectively for resolution." He underscored the importance of establishing agreements that would enable immediate measurement within hours after detection.

Los Angeles City Councilman Mitchell Englander condemned SoCal Gas for operating a facility serving some 20 million customers without a contingency plan. He pointed out delays in deploying necessary equipment and addressing contamination affecting residences and vehicles.

The Center for Biological Diversity criticized Governor Brown’s sluggish reaction and described his regulatory approach as hands-off, neglecting risks posed by aging natural gas infrastructure.

Experts in petroleum engineering reproached the six-week delay before initiating relief well drilling and questioned why only one relief well was drilled instead of two.

The Associated Press reported that SoCal Gas had underreported benzene concentrations near the site of the leak.

Regulatory responses

Within a span of two days, more than a dozen state and local agencies became involved. The California Air Resources Board conducted methane measurements using a combination of ground stations, towers, satellites, and aircraft, sharing the collected data with both the California Energy Commission and SoCal Gas. The Board raised in-depth inquiries about the characteristics and behavior of the leak. Meanwhile, the Federal Aviation Administration established a temporary flight restriction over the affected area until March 2016. On December 7, 2015, Los Angeles City Attorney Mike Feuer filed a lawsuit against SoCal Gas concerning its management of the incident. Subsequently, on December 15, the County of Los Angeles proclaimed a state of emergency.

By December 23, Feuer’s office had obtained a court order mandating SoCal Gas to expedite relocation efforts for those residents impacted by the leak.

Community members appealed to Governor Jerry Brown—noting that his sister Kathleen Brown serves on Sempra Energy’s board—to take action. The Governor visited the site on January 4, 2016, declared a state of emergency two days later, and required stricter inspections and safety protocols at all natural gas storage facilities across California; injections at Aliso Canyon had already been prohibited.

Health authorities cautioned that extended exposure to certain trace chemicals might result in adverse health effects.

In January 2016, California senators called upon federal agencies to assess both legal and technical aspects of the event and advocated for an accelerated reduction of gas volumes at the facility. Following this, four state senators introduced legislation imposing an immediate halt on new gas injections and the utilization of older wells (SB 875), ensuring SoCal Gas is responsible for relocation and emergency expenses without transferring costs to consumers (SB 876), and mandating comprehensive inspections of all 14 underground storage sites with upgraded safety requirements including advanced leak detection systems and stringent emergency response plans (SB 887).

The disruption at Aliso Canyon placed significant pressure on gas supply for power generation and the electrical grid. In response, the CPUC directed Southern California Edison to deploy an 80 MWh lithium-ion battery storage system at its Mira Loma substation near San Bernardino to help prevent winter power outages.

Political response

On March 31, 2016, The New York Times published "The Invisible Catastrophe," which underscored the political ramifications of the leak during an election cycle. Although Governor Jerry Brown refrained from politicizing the matter, freshman Congressman Steve Knight—who had received campaign contributions from Sempra Energy, SoCal Gas’ parent company—initially defended the corporation. Knight later advocated for congressional hearings; however, his Democratic opponent Bryan Caforio made addressing the environmental crisis a key issue in their closely fought 25th Congressional District campaign.

According to the Los Angeles Times, Caforio emerged as the most vocal politician on this matter, appearing alongside activist Erin Brockovich at Porter Ranch and criticizing Knight’s delayed reaction. While Knight sought to keep the issue out of partisan politics, Republican Los Angeles County supervisor Michael Antonovich pushed for accountability from SoCal Gas.

Aftermath

Key figures such as Congressman Brad Sherman and Senator Henry Stern urged that no steps be taken to reactivate the facility until comprehensive health assessments and leak investigations were finalized. SoCal Gas pursued regulatory clearance to recommence natural gas injection into the depleted oil reservoir at Porter Ranch, a vital storage hub for gas-fired power plants. Discussions with regulators proceeded concurrently with ongoing inquiries.

Sherman stressed the necessity of exercising utmost caution before allowing Aliso Canyon to resume full operations, with officials and lawmakers agreeing that the site was not yet ready for standard use. He recommended restricting SoCal Gas’s activities to only those essential for avoiding disruptions in electricity supply. Experts have identified the leak as the most significant in U.S. history.

Legal consequences

On November 23, 2015, residents who had been displaced initiated legal action in Los Angeles Superior Court, demanding that SoCal Gas provide disclosure of health risk data related to the leaked gases and seeking compensation. By January 2016, reports indicated that nearly one thousand lawsuits had been filed. On February 2, 2016, criminal charges were brought by Los Angeles County against SoCal Gas for failing to promptly report the leak, including three counts of delayed notification and one count of releasing air contaminants.

On September 13, 2016, SoCal Gas entered a plea agreement, pleading no contest to a single misdemeanor charge for not immediately reporting the October 2015 leak and consenting to pay a $4 million penalty. This misdemeanor was the least severe charge possible and could be expunged. The fine amounted to only a small portion of SoCal Gas’s earnings during an average half-day in fiscal year 2015. The Porter Ranch incident remains recognized as the largest gas leak in U.S. history.

In 2018, SoCal Gas consented to a $119.5 million settlement paid to multiple government agencies connected to the event.

External links

CalOES.gov: official Aliso Canyon Natural Gas Leak website – California Governor's Office of Emergency Services (CalOES)
SoCalGas.com: official Aliso Canyon gas leak website
CalEPA.gov: California Aliso Canyon natural gas leak information
CPUC.gov: Aliso Canyon Well Failure – California Public Utilities Commission (CPUC)
CalOEHHA.gov: Aliso Canyon Underground Storage Field – Office of Environmental Health Hazard Assessment (OEHHA), January 15, 2016