Methane emissions from underground gas storage in California

The observational strategies applied in this study address many of the aforementioned challenges for measuring UGS methane emissions including the ability to efficiently and reliably sample the total emissions of facilities and to attribute emissions to individual components and activity within those facilities. We conducted multiple airborne surveys of the 12 active California UGS facilities using a combination of advanced remote-sensing (infrared imaging spectroscopy) over-flights and in situ sampling of atmospheric boundary layer methane plumes from those facilities. This integrated approach offers three primary advantages: (a) denser temporal sampling to better constrain variability and minimize sample bias (e.g. 229 over flights in this study compared to 35 in the Fischer et al 2017 study), (b) spatially resolved measurements of individual emission sources for improved attribution, and (c) cross-validation between independent estimation methods for improved confidence.

The locations and key attributions of our study population of UGS facilities are described in figure S1 which is available online at stacks.iop.org/ERL/15/045005/mmedia. Spatially, each survey was designed to address all surface infrastructure in a given facility including well heads, gas lines, compressors, vent stacks, and dehydrators, including several where the compressor station is located several kilometers distant from the storage field (e.g. Wild Goose and Lodi). This degree of spatial completeness was possible given the combination of high spatial resolution (typically < 3 m) from the remote sensing instrument and the unfettered access offered by both aircraft methods. Temporally, the survey flights were conducted between January 2016 and November 2017 with most facilities sampled across at least three seasons to assess potential seasonal variability. Intensive campaigns were also conducted with revisit intervals ranging from minutes to days, particularly during Fall 2016 and Fall 2017. The study spanned a range of normal operations, malfunctions and maintenance activity from multiple facilities including the active phase of the Aliso Canyon blowout incident in 2016 and subsequent return to injection operations in summer 2017.

The classic version of the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS-C, Green et al 1998) and the next generation instrument (AVIRIS-NG, Hamlin et al 2011) measure ground-reflected solar radiation from the visible to infrared spectral regions (380–2500 nm). Both instruments have a 34° field of view while the spectral resolution and sampling of AVIRIS-C and AVIRIS-NG is approximately 10 nm and 5 nm respectively. With the exception of a few flights during the Aliso Canyon blowout incident in early 2016 with AVIRIS-C in the NASA ER-2, the remote sensing portion of this study was conducted with AVIRIS-NG in a King Air B-200—typically at flight altitudes of 3 km above ground level, equivalent to 1.8 km swath width and 3 m pixel size. The AVIRIS-C and AVIRIS-NG methane retrieval is based on absorption spectroscopy between 2100 and 2500 nm and uses a linearized matched filter to calculate a mixing ratio length in units of ppm-m representing the thickness and concentration within a volume of equivalent absorption (Thompson et al 2015). This and related techniques have been demonstrated in a number of previous airborne campaigns (Thompson et al 2015, Frankenberg et al 2016, Thompson et al 2016, Thorpe et al 2017, Krautwurst et al 2017, Duren et al 2019). Thorpe et al (2016) demonstrated that plumes for controlled releases as low as 10 kgCH₄ h⁻¹ were consistently observed by AVIRIS-NG across multiple flight altitudes and wind conditions and a minimum detection limit of 2 kgCH₄ h⁻¹. Frankenberg et al (2016) quantified emission rates ranging from 2 to 5000 kgCH₄ h⁻¹ for the Four Corners region primarily from natural gas production and Duren et al (2019) reported methane emissions between 9 and 2600 kgCH₄ h⁻¹ for methane plumes observed from the landfill, agriculture, and waste management sectors of California.

Two methods were used to estimate methane emission rates: a Large Eddy Simulation (LES) scaling applied to AVIRIS-C data during the Aliso Canyon blowout and a mass balance approach for the AVIRIS-NG results. A LES model was run using an initial prescribed emission flux during the Aliso Canyon blowout (Conley et al 2016) and a linear scalar was applied iteratively until the Integrated Methane Enhancement (IME, kg CH₄) matched the retrieved AVIRIS-C IME. This scalar was then multiplied by the initial LES flux to estimate the flux for each observed plume. Given LES modelling is computationally expensive it was not feasible to generate LES model runs for the 135 remaining plumes observed with AVIRIS-NG. Instead, a mass balance method was used to estimate the emission flux by combining AVIRIS-NG IME values and contemporaneous wind speed measurements to estimate the emission flux. Building upon previous studies that have reported emission uncertainties solely derived from wind measurements (Frankenberg et al 2016), this study provides a more realistic uncertainty estimate derived from both the IME calculation and wind measurements. In section S1.2, the Large Eddy Simulation (LES) scaling and mass balance approach are discussed in further detail.

The in situ air sampling approach used by Scientific Aviation provides accurate methane flux estimates at the scale of individual facilities. This mass balance approach involves executing a cylindrical flight pattern (i.e. stacked, approximately constant altitude circles at altitudes from the minimum safe flight altitude to the top of the emissions plume) while measuring methane concentrations and wind speeds and the application of Gauss's Theorem to estimate the flux divergence through the cylinder (Conley et al 2014, Conley et al 2017). The airborne system is flown on a fixed wing single engine mooney aircraft, extensively modified for research as described in (Conley et al 2014). Ambient air is collected through ∼5 m of tubing that protrudes out of backward-facing inlets mounted below the right wing. In situ methane, carbon dioxide, and water vapor are measured with a Picarro 2301 f cavity ring down spectrometer (Crosson 2008) operated in its precision mode at 1 Hz.

The horizontal wind speed and direction was derived from measurements of true airspeed and GPS-derived ground speed. Winds were sampled at 1 Hz using a dual GPS compass that determines aircraft heading and ground speed with sufficient accuracy to resolve the horizontal wind components to about 0.2 m s⁻¹ accuracy (Conley et al 2014). The horizontal wind is calibrated periodically by flying ∼5 km L-patterns in the free troposphere; a heading rotation and airspeed adjustment is made to the wind calculation to minimize the dependence of the wind on aircraft heading (Conley et al 2014). This method of estimating emissions has been demonstrated across multiple flight campaigns (Smith et al 2015, Conley et al 2017, Schwietzke et al 2018) and uncertainties typically range from 10% to 30% depending on factors such as number of laps conducted and wind variability (Conley et al 2017).

Facility total methane emission estimates were derived on a daily basis—either using a single Scientific Aviation mass-balance estimate when available and/or the daily average of emission estimates derived from AVIRIS-NG data. These daily estimates are plotted as time-series for a number of facilities that were revisited at high frequency, like Aliso Canyon, Honor Rancho, and McDonald Island (figures 1, 4, 6 and 8). The combination of Scientific Aviation and AVIRIS-NG emission estimates provide a time-series with denser temporal sampling than available with a single data set. Additionally, results were cross-validated on dates where both Scientific Aviation and AVIRIS-NG were flown as part of coordinated intensive campaigns (e.g. January 12, 2016 and September 16, 2017 for Aliso Canyon, October 6, 2017 for McDonald Island, and October 16, 2017 for Honor Rancho). An average emission rate was also calculated from all observations at a given facility, which was scaled by the source persistence (ratio of the number of observed plumes to the total number of overflights) to estimate annual emissions for gas storage facilities surveyed by AVIRIS-NG and Scientific Aviation (see section S1.3). Annual emission estimates are shown in table 3 and figure 9 and compared with reported emissions from EPA and CARB databases (California Air Resources Board CARB 2018, Environmental Protection Agency EPA 2018).

Zoom In Zoom Out Reset image size

The largest reported loss of containment at a UGS facility (and indeed the largest reported single methane point source to date) was due to a blowout of storage well Standard Sesnon 25 (SS25) at Aliso Canyon on October 23, 2015 (Department of Energy DOE 2016). Natural gas was released from the wellhead and a subsurface breach in the annular casing for nearly four months. In an effort to quantify the leak rate, Scientific Aviation conducted 11 mass balance flights from early November 2015 through early February 2016 (Conley et al 2016). AVIRIS-C also conducted 21 overflights of Aliso Canyon during the active phase of the leak on January 12, January 14, and February 9, 2016. On January 12, 2016 the two airborne systems conducted simultaneous overflights and reported similar emission rates of 17 564 ± 1086 kgCH₄ h⁻¹ derived from AVIRIS-C and 20 700 ± 4140 kgCH₄ h⁻¹ from Scientific Aviation (figure 1). As expected, AVIRIS-C detected the central methane plume associated with gas venting at the well head itself, however it also detected two secondary emission locations from the hillsides below the wellhead (figure 2). The secondary emission locations were subsequently confirmed by onroad methane surveys by South Coast Air Quality Management District (SC-AQMD) personnel.

Zoom In Zoom Out Reset image size

After the storage well was successfully plugged on February 11, 2016, a dramatic reduction in emission estimates was observed (figure 1) for both AVIRIS-C (220 ± 88 kgCH₄ h⁻¹ for February 19) and Scientific Aviation (310 ± 62 kgCH₄ h⁻¹ for February 21). The February 19, 2016 AVIRIS-C flights confirmed the presence of residual methane outgassing from soil at both the well head and one of the secondary emission locations (figure 2(d)). Serendipitously, AVIRIS-C detected a relatively small methane plume at an oil well several kilometers to the northwest. That source was also confirmed by a SC-AQMD ground survey and attributed to associated gas venting. While technically this was leakage from the shallower oil formation rather than the deeper gas storage reservoir, the operator reported that the venting was in part a response to the temporary termination of injection into the storage reservoir during the blowout.

After over a year of well repairs, testing, and inspections, gas injection resumed at the Aliso Canyon UGS facility in late July 2017. Scientific Aviation conducted a series of overflights in August and September 2017 to assess the possibility of additional leakage. Those flights led to mixed results including highly variable emissions (∼50–500 kgCH₄ h⁻¹) that could not be readily explained.

Source attribution in Aliso Canyon can be challenging given the limited surface access, steep terrain, and the co-location of hundreds of gas storage and oil production wells (figure 3). AVIRIS-NG overflights began in late August 2017 on a Dynamic Aviation King Air B200 aircraft with typical flight altitudes of 3 km above ground level, resulting in a 1.8 km swath width and 3 m pixel size. Maps of plumes observed with AVIRIS-NG provided source attribution by identifying intermittent venting from a blowdown stack near the main compressor facility (figure 3(d)), an oil well (figure 3(a)), tank (figure 3(b)), and drilling rig (figure 3(c)). These results suggest that the large variability in Aliso Canyon methane emissions following return to operations could be a combination of reservoir gas vented from the blowdown stack and emissions associated from the shallower oil reservoir present at the oil well, tank, and drilling rig.

Zoom In Zoom Out Reset image size

Figure 4 shows the time series of emission estimates derived by both AVIRIS-NG and Scientific Aviation from March 2016 through October 2017 ranging from ∼50 to 500 kgCH₄ h⁻¹. Daily emission estimates are derived from AVIRIS-NG either using a single observation or the average of multiple observations in a given day (see section S.1.2.2). Coincident flights occurred on September 16, 2017 and emission estimates varied between 221 ± 68 kgCH₄ h⁻¹ for AVIRIS-NG and 193 ± 108 kgCH₄ h⁻¹ for Scientific Aviation.

Zoom In Zoom Out Reset image size

AVIRIS-NG surveys of all 12 active UGS facilities conducted in 2016 (fall) and 2017 (spring, summer, fall) provided a unique opportunity to quantify emissions from a range of emission modes. Here we focus primarily on the Honor Rancho and McDonald Island facilities given the diversity of source type and variability observed there. The most commonly observed methane plumes at Honor Rancho were associated with a persistent source to the west of the compressor station (later confirmed by the operator to be a leaking bypass/isolation valve in the facility's emergency shutdown stack) and plumes at 1 or 2 of the 5 large reciprocating compressors on the east side of the station (figure 5). The bypass valve leakage at Honor Rancho persisted through 2017 and represents an emission mode not observed at any other California UGS facility. The plumes that were observed episodically at two of the compressors suggest rod pack venting that is higher than reported for those two units (no plumes were observed at the other 3 units). The compressor plumes at Honor Rancho were observed 29% of the time—consistent with activity data for these two units reported to EPA in 2015 (e.g. they were reported to be in the operating state 16% and 28% of the time, respectively). Other sources observed at Honor Rancho include a dehydrator and two unknown sources associated with visible infrastructure (table 1).

Zoom In Zoom Out Reset image size

The time series for Honor Rancho is shown in figure 6 and both instruments indicate highly variable emission estimates between January 2016 and November 2017. On October 16, 2017 coincident flights indicated higher estimated emissions from AVIRIS-NG than Scientific Aviation. Figure 7(a) shows October 16 results from both instruments with the center of the cylindrical flight path of Scientific Aviation shown at location 1 and location 2 indicating those regions where higher in situ CH₄ concentrations were observed. At location 3, two AVIRIS-NG CH₄ plumes were observed with plume shapes consistent with local winds towards the south and the Scientific Aviation enhancements (location 2). The AVIRIS-NG CH₄ plumes are close to the cylindrical flight path and figure 7(b) indicates that even the lowest altitude Scientific Aviation flight paths were around 168 m above the main facility where the AVIRIS-NG plumes were observed for an unknown source (location 4) and an emergency shutdown stack (location 5). As a result, the cylindrical flight path of Scientific Aviation resolved a portion of the enhancements at greater than 168 m above the ground (location 6) but missed the enhancements below 168 m, which suggests an underestimate. AVIRIS-NG results from 2016 and 2017 were shared with the Honor Rancho facility operators and subsequent flights performed in the fall of 2018 indicated that the persistent emissions from the emergency shutdown stack (figures 5(a) and (c)) was likely mitigated while emissions from the compressor plume continued to be observed intermittently (figures 5(b) and (d)).

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

McDonald Island UGS exhibited two primary methane sources as confirmed by facility operators that are associated with both compressor stations from blowdown stack venting associated with maintenance activities (figure S4(c)) and compressor loss (figure S4(d)). Emission estimates indicate high variability between February 2016 and October 2017 that range between 83.7 and 763.0 kgCH₄ h⁻¹ (figure 8). On October 6, 2017 coincident AVIRIS-NG and Scientific Aviation flights resulted in significantly different emission estimates. The Scientific Aviation aircraft measures wind speed according to Conley et al (2014), using true airspeed and GPS-derived ground speeds to derive an average wind speed of 2.3 m s⁻¹ for the period of measurement between 19:24 and 19:41 UTC (corresponding to the duration of 4 AVIRIS-NG scenes), significantly higher than wind speeds of 0.7 m s⁻¹ derived from HRRR (Benjamin et al 2016) that were used in the AVIRIS-NG mass balance analysis (Duren et al 2019). Local meteorological station wind data (CIMIS Holt #248 station) indicated a higher wind speed (1.7 m s⁻¹) compared to HRRR, and the resulting emission estimate (see figure 8, orange) showed improved agreement with results from Scientific Aviation on October 6, 2017.

Zoom In Zoom Out Reset image size

Additional examples of different emission modes at other UGS facilities are summarized in figure S4 and table 1 including: blowdown stack venting at Gill Ranch, hydrator venting at Lodi, blowdown stack venting at Kirby Island, and compressor loss at Wild Goose. Note that source attribution to component scale was not possible for dates that only had Scientific Aviation flights hence in cases where AVIRIS-NG only detected one plume at a given facility (see table 1, Kirby Island and Wild Goose) there is lower confidence in the dominant emission mode.

Annual emissions estimates were calculated using the same approach for both AVIRIS-NG and Scientific Aviation. An average emission rate was calculated from all observations at a given source or facility and scaled based on the frequency of observed emissions from a given source or facility (section S1.3). Next, the average emission rate across observed plumes was scaled by the source persistence to estimate per-hour methane emissions, which was multiplied by 8760 h to generate an annual emissions estimate. Additional details on these calculations are provided in section S1.3 and Scientific Aviation frequency values are presented in table 2. Because AVIRIS-NG often observed a number of sources for a given facility, annual emission estimates are calculated by source (table 1) before the results are aggregated by facility to permit direct comparison with Scientific Aviation (table 3). For AVIRIS-NG, the 2018 flights were used only to improve the calculation of the frequency of observed emissions.

The estimated annual methane emissions for each facility are summarized in figure 9 and table 3. The Honor Rancho and McDonald Island UGS facilities have the highest measured annual emissions of the gas storage facilities where emissions were observed. We estimate that the total annual methane emissions from the UGS sector in California averaged between 11.0 ± 3.8 GgCH₄ yr⁻¹ (AVIRIS-NG) and 12.3 ± 3.8 GgCH₄ yr⁻¹ (Scientific Aviation). Total annual methane emissions for the 7 facilities that reported emissions in 2016 were estimated between 9.0 ± 3.2 GgCH₄ yr⁻¹ (AVIRIS-NG) and 9.5 ± 3.2 GgCH₄ yr⁻¹ (Scientific Aviation), in both cases around 5 times higher than reported (see table 3; California Air Resources Board CARB (2018)). Figure 9 shows a comparison of annual methane emissions estimates from AVIRIS-NG (orange), Scientific Aviation (blue), as well as reported emissions from the EPA (dark red, Environmental Protection Agency EPA (2018)) and CARB (light red, California Air Resources Board CARB (2018)). Under-reporting of emissions appears most pronounced for Honor Rancho and McDonald Island, consistent with evidence from AVIRIS-NG and discussions with facility operators to determine that malfunctions and leakage associated with maintenance activity were present at these facilities.

Zoom In Zoom Out Reset image size

For further comparison, annual emission rates assuming 0.5% loss based on DOGGR annual mean natural gas injected are plotted in light gray, indicating that measured emissions are typically below a 0.5% loss rate. Figure 10 compares only those gas storage facilities where Scientific Aviation, AVIRIS-NG, and reported emissions are available. In all cases, Scientific Aviation and AVIRIS-NG results agree within the reported errors and are significantly higher than the reported emissions.

Zoom In Zoom Out Reset image size

A comprehensive comparison between AVIRIS-NG and Scientific Aviation measurements acquired both simultaneously and time-averaged over several months of repeat observations was presented in Duren et al (2019). As shown in figure 3(a) and S.12b from Duren et al (2019), there was good agreement between coincident AVIRIS-NG and Scientific Aviation measurements and greater divergence for measurements acquired at different times due to the highly variable nature of emissions.

For this study, a direct comparison between Scientific Aviation and AVIRIS-NG results is shown in figure 11 with a linear fit with a R² of 0.45. This emphasizes the challenge in comparing results that were derived largely from measurements acquired at different times due to the highly intermittent nature of these emissions (table 1), consistent with findings reported in Duren et al (2019). Comparing simultaneous measurements from four same day sets of flights (figure 12) using a linear regression results in improved agreement with a R² value of 0.99.

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size