Tegan N. Lavoie

Aircraft-Based Estimate of Total Methane Emissions from the Barnett Shale Region

We present estimates of regional methane (CH4) emissions from oil and natural gas operations in the Barnett Shale, Texas, using airborne atmospheric measurements. Using a mass balance approach on eight different flight days in March and October 2013, the total CH4 emissions for the region are estimated to be 76 ± 13 × 10(3) kg hr(-1) (equivalent to 0.66 ± 0.11 Tg CH4 yr(-1); 95% confidence interval (CI)). We estimate that 60 ± 11 × 10(3) kg CH4 hr(-1) (95% CI) are emitted by natural gas and oil operations, including production, processing, and distribution in the urban areas of Dallas and Fort Worth. This estimate agrees with the U.S. Environmental Protection Agency (EPA) estimate for nationwide CH4 emissions from the natural gas sector when scaled by natural gas production, but it is higher than emissions reported by the EDGAR inventory or by industry to EPAs Greenhouse Gas Reporting Program. This study is the first to show consistency between mass balance results on so many different days and in two different seasons, enabling better quantification of the related uncertainty. The Barnett is one of the largest production basins in the United States, with 8% of total U.S. natural gas production, and thus, our results represent a crucial step toward determining the greenhouse gas footprint of U.S. onshore natural gas production.

Constructing a Spatially Resolved Methane Emission Inventory for the Barnett Shale Region

Methane emissions from the oil and gas industry (O&G) and other sources in the Barnett Shale region were estimated by constructing a spatially resolved emission inventory. Eighteen source categories were estimated using multiple data sets, including new empirical measurements at regional O&G sites and a national study of gathering and processing facilities. Spatially referenced activity data were compiled from federal and state databases and combined with O&G facility emission factors calculated using Monte Carlo simulations that account for high emission sites representing the very upper portion, or fat-tail, in the observed emissions distributions. Total methane emissions in the 25-county Barnett Shale region in October 2013 were estimated to be 72,300 (63,400-82,400) kg CH4 h(-1). O&G emissions were estimated to be 46,200 (40,000-54,100) kg CH4 h(-1) with 19% of emissions from fat-tail sites representing less than 2% of sites. Our estimate of O&G emissions in the Barnett Shale region was higher than alternative inventories based on the United States Environmental Protection Agency (EPA) Greenhouse Gas Inventory, EPA Greenhouse Gas Reporting Program, and Emissions Database for Global Atmospheric Research by factors of 1.5, 2.7, and 4.3, respectively. Gathering compressor stations, which accounted for 40% of O&G emissions in our inventory, had the largest difference from emission estimates based on EPA data sources. Our inventorys higher O&G emission estimate was due primarily to its more comprehensive activity factors and inclusion of emissions from fat-tail sites.

Direct and Indirect Measurements and Modeling of Methane Emissions in Indianapolis, Indiana

This paper describes process-based estimation of CH4 emissions from sources in Indianapolis, IN and compares these with atmospheric inferences of whole city emissions. Emissions from the natural gas distribution system were estimated from measurements at metering and regulating stations and from pipeline leaks. Tracer methods and inverse plume modeling were used to estimate emissions from the major landfill and wastewater treatment plant. These direct source measurements informed the compilation of a methane emission inventory for the city equal to 29 Gg/yr (5% to 95% confidence limits, 15 to 54 Gg/yr). Emission estimates for the whole city based on an aircraft mass balance method and from inverse modeling of CH4 tower observations were 41 ± 12 Gg/yr and 81 ± 11 Gg/yr, respectively. Footprint modeling using 11 days of ethane/methane tower data indicated that landfills, wastewater treatment, wetlands, and other biological sources contribute 48% while natural gas usage and other fossil fuel sources contribute 52% of the city total. With the biogenic CH4 emissions omitted, the top-down estimates are 3.5-6.9 times the nonbiogenic city inventory. Mobile mapping of CH4 concentrations showed low level enhancement of CH4 throughout the city reflecting diffuse natural gas leakage and downstream usage as possible sources for the missing residual in the inventory.

Assessing the Methane Emissions from Natural Gas-Fired Power Plants and Oil Refineries

Presently, there is high uncertainty in estimates of methane (CH4) emissions from natural gas-fired power plants (NGPP) and oil refineries, two major end users of natural gas. Therefore, we measured CH4 and CO2 emissions at three NGPPs and three refineries using an aircraft-based mass balance technique. Average CH4 emission rates (NGPPs: 140 ± 70 kg/h; refineries: 580 ± 220 kg/h, 95% CL) were larger than facility-reported estimates by factors of 21-120 (NGPPs) and 11-90 (refineries). At NGPPs, the percentage of unburned CH4 emitted from stacks (0.01-0.08%) was much lower than respective facility-scale losses (0.09-0.34%), and CH4 emissions from both NGPPs and refineries were more strongly correlated with enhanced H2O concentrations (R2avg = 0.65) than with CO2 (R2avg = 0.21), suggesting noncombustion-related equipment as potential CH4 sources. Additionally, calculated throughput-based emission factors (EF) derived from the NGPP measurements made in this study were, on average, a factor of 4.4 (stacks) and 37 (facility-scale) larger than industry-used EFs. Subsequently, throughput-based EFs for both the NGPPs and refineries were used to estimate total U.S. emissions from these facility-types. Results indicate that NGPPs and oil refineries may be large sources of CH4 emissions and could contribute significantly (0.61 ± 0.18 Tg CH4/yr, 95% CL) to U.S. emissions.

Correction to Assessing the Methane Emissions from Natural Gas-Fired Power Plants and Oil Refineries

In our article, the throughput estimate for the three natural gasfired power plants, P1−3, was calculated incorrectly. The throughput estimate in Table 3 of the main article incorrectly shows the methane (CH4) throughput for the duration of the experiment (kg CH4/experiment), not the hourly throughput estimate (kg CH4/h), as intended. Correcting this error changed the CH4 loss rates for these three power plants, shown in Table 3. Please note that, except in one case (e.g., P2, 9/19, stack loss rate), the corrected CH4 loss rates are not statistically different from the numbers reported in the original article (at a 95% confidence level, note that numbers in Table 3 are reported ±1σ). This also results in adjustment of several in-text references to the corrected values, described here. Within the Abstract and the Results and Discussion section, the reported range of CH4 loss rates for stacks changes from 0.01−0.14% to 0.01−0.08%, and the range of CH4 loss rates for the entire facilities changes from 0.10−0.42% to 0.09−0.34%. Within the Results and Discussion section, the description of the factor difference between the stack and facility-scale losses should be updated to reflect the new numbers in Table 3. The original sentence “The percentage of unburned CH4 emitted from stacks at the three NGPPs (0.01−0.14%) was lower than respective facility-scale losses (0.10−0.42%) in all cases, by factors of 3 (P1, 9/20), 15 (P1, 9/21), 10 (P2, 9/21), and 13 (P3, 9/25), again suggesting that more CH4 is lost from noncombustion-related equipment than from combustion processes (Table 3)”., should be updated to “The percentage of unburned CH4 emitted from stacks at the three NGPPs (0.01−0.08%) was lower than respective facility-scale losses (0.09−0.34%) in all cases, by factors of 4 (P1, 9/20), 12 (P1, 9/21), 9 (P2, 9/21), and 17 (P3, 9/25)...”. Additionally, in the original Abstract we note that the calculated throughput-based emission factor (EF) derived from the NGPP measurements made in this study were, on average, a factor of 4.4 (stacks) and 42 (facility-scale) larger than industry-used EFs. This sentence should be changed to read “a factor of 4.4 (stacks) and 37 (facility-scale) larger than industry-used EFs”. Note that this change is not related to the corrected calculation of throughput estimation described above, but due to an unrelated miscalculation of the average factor difference, as the error did not carry through to the throughputbased EF calculation shown in the last two columns of Table 3. To reflect the correction of this calculation, an updated version of eq S4 from the Supporting Information is provided below, which applies a correction factor to eq S4 ((60 min/50 min) in this example) to obtain an hourly heat input, instead of the total heat input per length of the experiment as used previously. Please refer to the original Supporting Information text for a description of the example used in eq S4 below. The hourly average heat inputs obtained using eq S4 are then used to calculate the corrected CH4 throughputs reported here, using eq 3 in the main text, with corrected version shown below.