A Look Back on Aliso Canyon

A Look Back on Aliso Canyon

With the long-awaited independent investigation report into the root causes behind the 2015 blowout at the Aliso Canyon gas storage facility being released just recently, it is perhaps timely to take a brief look back on the event that became the largest single natural gas leak in U.S. history.

Most people have, at one time or another, experienced the tell-tale odor of natural gas. The gas used to heat homes and to cook food is composed predominantly of methane (CH4), which itself is odorless, colorless, and highly flammable. That stomach-turning, sulfurous smell is the result of chemical compounds known as mercaptans, which are added to processed natural gas as a safety measure to alert people of a potentially dangerous gas leak. When the residents of Porter Ranch, a small but affluent community nestled in the foothills on the northern edge of the San Fernando Valley, began to notice the familiar stench of mercaptans in late October of 2015, they reported it to the local gas utility and to the State, and began complaining of headaches, nausea, and respiratory symptoms. It was nearly a week later, on October 28, that the presence of a gas leak at the nearby Aliso Canyon facility was announced to the public.

The very foothills upon which Porter Ranch was built was previously owned by the Tidewater Associated Oil Company, and the massive oil reservoir underneath Aliso Canyon produced oil from 1938 until it ran dry in the 1970s. The empty cavity, located over 7,000 feet below ground, was then repurposed to store post-processed (and odorized), saleable natural gas. With a capacity of 84 billion cubic feet, it is the second-largest gas storage facility of its kind in the United States, and acts as a pressurized reservoir of natural gas that can be delivered, via a network of pipelines, to buildings and homes throughout Los Angeles.

It was one of the 115 wellbores tapped into the reservoir, identified as well SS-25, that suffered a severe, 2-foot long rupture of its 7-inch diameter steel casing and began to rapidly release the pressurized gas stored within. For nearly two weeks after its discovery, the full extent and severity of the leak was not known, and residents were given little information to go on as they grew increasingly worried about the health effects of the invisible plume that had descended upon their neighborhood. Amid growing pressure, the California Energy Commission called upon Scientific Aviation (Sci Av) to put their planes to work to find out just how much gas was flowing from the damaged well.

On November 7, atmospheric scientist and Sci Av founder Stephen Conley took off from the Lincoln Regional Airport near Sacramento in his single-engine Mooney bound for Porter Ranch. The aircraft had been modified for atmospheric research with sensors for measuring the air temperature, pressure, humidity, wind speed and direction. In the rear luggage compartment, a vacuum pump pulled outside air through inlets mounted underneath the starboard wing and into a Picarro analyzer that continuously measured and recorded the concentrations of methane and CO2 in the air.

For the next three and a half months, and until the leak was finally plugged in February, Conley based his plane at the Van Nuys airport and conducted 13 flights near the Aliso Canyon site. Over the years, Scientific Aviation had gained experience measuring fugitive methane emissions in oil and gas regions around the country, including in the Uintah Basin and the Barnett and Bakken Shale formations. However, the magnitude of this leak was unlike anything Conley had measured before; at times the levels of methane in the air exceeded 50 parts-per-million (compared to a typical background level of around 1.9 ppm). For several hours each day, Conley flew transects back and forth downwind of leak, intercepting the plume repeatedly at various altitudes. Using the measured methane levels, along with an on-board wind speed and direction measurement, Sci Av was able to quickly calculate and report an emissions rate within 20 minutes after landing the first flight.

What they found was startling. The methane leak rate reached upwards of 60 tons per hour and averaged around 1200 tons per day during the first six weeks of the leak. Along with the methane, came 4.5 tons of ethane per hour. This leak rate effectively doubled the methane emissions of all other sources in the Los Angeles basin combined, and temporarily created the largest known human-caused methane point source in U.S. history (the next largest, an underground coal mine in Alabama, emits at approximately half this rate).


Over the 112-day duration of the leak, Sci Av estimated a total methane release of 97,100 metric tons (and 7300 metric tons of ethane) and published their findings in the peer-reviewed journal Science. Over the next year, Sci Av found themselves having to defend that estimate as NASA claimed it was too low by a factor of 2 while SoCal Gas, the operator of the facility, claimed it to be too high by a factor of 2. A year later, NASA released their estimate of 100,250 metric tons, only 3% higher than the Sci Av estimate, and Aerodyne published their estimate of 86,022 metric tons (11% below the Sci Av estimate) using results from their tracer release experiment. The California Air Resources Board later revised these estimates upward to 109,000 tons for their final report on the incident.

Now, every so often, Sci Av pays a visit to Aliso Canyon to fly a few laps around the facility and make sure that all is still quiet in Porter Ranch. The facility is in use again, albeit now with a more limited volume of gas stored inside. The independent investigation report found that decades of microbial corrosion due to contact of the stainless-steel casing with groundwater ultimately lead to the rupture of the 61-year old wellhead. Thankfully, new construction standards and practices use an exterior concrete casing to prevent such contact with water. However, much of the infrastructure in cities across the U.S. is old and aging. With a population that continues to grow rapidly, and limited funding available to repair or replace the old and overtaxed systems, it may be only a matter of time before another sleeping giant awakes. If that happens, and wherever that happens, Scientific Aviation will be there to answer the call.