Viability of Underground Natural Gas Storage in California: An Independent Review of Scientific and Technical Information.

At the time of the Aliso Canyon incident in 2015, there was no reported quantitative operational monitoring program for ambient methane or other trace gases at Aliso Canyon (or any other UGS facility in California). A variety of methane measurement methods was deployed in the months that followed to improve confidence in the SS-25 well leak rate as it evolved in response to efforts to control the well and reduce reservoir pressure by gas withdrawal. These methods include complementary airborne surveys using low-altitude in situ sampling and high-altitude remote sensing, as follows: (1) total methane emissions were determined using an aircraft equipped with a Picarro in situ methane analyzer flying cylindrical patterns around the facility; and (2) spatially resolved emissions from individual infrastructure components were estimated using an aircraft equipped with JPL’s Airborne Visible/Infrared Imaging Spectrometer (AVIRIS-NG). Both airborne methods have since been applied to other UGS facilities in California: Total facility methane emissions were measured at selected facilities roughly 40 times, from June 2014 through August 2017. Local methane emissions were measured roughly 80 times from January 2016 through August 2017 with the AVIRIS-NG method. UGS facilities are also subjected to daily surveys of all wellheads with hand-held gas analyzers, offering the ability to find small concentration anomalies at wellheads. Together, these measurements provide relevant information on current UGS facility emissions, discussed below in the context of greenhouse gas (GHG) emissions as well with regards to integrity implications. In general, methane (CH₄) emissions from UGS facilities are a potential concern for climate change because methane is a powerful GHG. Methane emissions from the total California natural gas supply chain from production to combustion should be carefully controlled below ~3% of the total amount used if short-term (~20 yr) climate impacts are to be minimized. We compared the recent airborne measurements of methane emissions from gas storage facilities with annual GHG reporting by the UGS operators to the California Air Resources Board. Taken together, the mean emissions of roughly 1,060 kg/hr (~9.3 GgCH₄ (~0.5 Bcf annually) from the active UGS facilities in California are ~7.8% of total natural gas-related methane emission estimated by the California Air Resources Board (CARB) and ~2.6 times the CARB estimate for gas storage-related methane emissions. Those emissions are dominated by three facilities: Honor Rancho, Aliso Canyon (after the SS-25 leak repair), and McDonald Island, which contribute 45%, 16% and 14%, respectively, to the UGS total. We conclude that UGS-related methane emissions appear to be a small part of both California’s methane and total GHG emission inventories. However, the ongoing methane emissions from California UGS facilities are roughly equivalent to having a 2015 Aliso Canyon incident every 10 years. This, combined with super-emitter (defined as anomalous emissions relative to expectation) activity at three facilities, suggests a mitigation opportunity for meeting the state’s short-lived climate pollutant mitigation targets in the natural gas sector. 

Measurements of natural gas emissions at UGS facilities also provide an atmospheric tracer that can enable eforts to monitor the integrity of surface and subsurface infrastructure— potentially ofering early warning to minimize the impact of leaks and avoid LOC and other hazardous situations for some failure modes. Methane in particular is both the primary constituent of natural gas and can be measured by a variety of methods to identify, diagnose, and guide responses to integrity issues. Methane emissions are also qualitatively indicative of emissions of toxic compounds (e.g., benzene), though relationships vary with reservoir. There are many methane measurement methods that can be applied to UGS leak detection; however, they have difering capabilities and limitations. Several of these methods have been successfully demonstrated in operational field conditions at Aliso Canyon, Honor Rancho, and other facilities, including several examples that illustrate the potential for coordinated application of multiple synergistic observing system “tiers.”