Gridded maps of geological methane emissions and their isotopic signature

Methane (CH4) is a powerful greenhouse gas, whose natural and anthropogenic emissions contribute ∼20&thinsp;% to global radiative forcing. Its atmospheric budget (sources and sinks), however, has large uncertainties. Inverse modelling, using atmospheric CH4 trends, spatial gradients and isotopic source signatures, has recently improved the major source estimates and their spatial–temporal variation. Nevertheless, isotopic data lack CH4 source representativeness for many sources, and their isotopic signatures are affected by incomplete knowledge of the spatial distribution of some sources, especially those related to fossil (radiocarbon-free) and microbial gas. This gap is particularly wide for geological CH4 (geo-CH4) seepage, i.e. the natural degassing of hydrocarbons from the Earth's crust. While geological seepage is widely considered a major source of atmospheric CH4, it has been largely neglected in 3-D inverse CH4 budget studies given the lack of detailed a priori gridded emission maps. Here, we report for the first time global gridded maps of geological CH4 sources, including emission and isotopic data. The 1∘×1∘ maps include the four main categories of natural geo-CH4 emission: (a) onshore hydrocarbon macro-seeps, including mud volcanoes, (b) submarine (offshore) seeps, (c) diffuse microseepage and (d) geothermal manifestations. An inventory of point sources and area sources was developed for each category, defining areal distribution (activity), CH4 fluxes (emission factors) and its stable C isotope composition (δ13C-CH4). These parameters were determined considering geological factors that control methane origin and seepage (e.g. petroleum fields, sedimentary basins, high heat flow regions, faults, seismicity). The global geo-source map reveals that the regions with the highest CH4 emissions are all located in the Northern Hemisphere, in North America, in the Caspian region, in Europe and in the East Siberian Arctic Shelf. The globally gridded CH4 emission estimate (37&thinsp;Tg&thinsp;yr−1 exclusively based on data and modelling specifically targeted for gridding, and 43–50&thinsp;Tg&thinsp;yr−1 when extrapolated to also account for onshore and submarine seeps with no location specific measurements available) is compatible with published ranges derived using top-down and bottom-up procedures. Improved activity and emission factor data allowed previously published mud volcanoes and microseepage emission estimates to be refined. The emission-weighted global mean δ13C-CH4 source signature of all geo-CH4 source categories is about −49&thinsp;‰. This value is significantly lower than those attributed so far in inverse studies to fossil fuel sources (−44&thinsp;‰) and geological seepage (−38&thinsp;‰). It is expected that using this updated, more 13C-depleted, isotopic signature in atmospheric modelling will increase the top-down estimate of the geological CH4 source. The geo-CH4 emission grid maps can now be used to improve atmospheric CH4 modelling, thereby improving the accuracy of the fossil fuel and microbial components. Grid csv (comma-separated values) files are available at https://doi.org/10.25925/4j3f-he27.</p

Application of Gauss's theorem to quantify localized surface emissions from airborne measurements of wind and trace gases

Airborne estimates of greenhouse gas emissions are becoming more prevalent with the advent of rapid commercial development of trace gas instrumentation featuring increased measurement accuracy, precision, and frequency, and the swelling interest in the verification of current emission inventories. Multiple airborne studies have indicated that emission inventories may underestimate some hydrocarbon emission sources in US oil- and gas-producing basins. Consequently, a proper assessment of the accuracy of these airborne methods is crucial to interpreting the meaning of such discrepancies. We present a new method of sampling surface sources of any trace gas for which fast and precise measurements can be made and apply it to methane, ethane, and carbon dioxide on spatial scales of  ∼&thinsp;1000 m, where consecutive loops are flown around a targeted source region at multiple altitudes. Using Reynolds decomposition for the scalar concentrations, along with Gauss's theorem, we show that the method accurately accounts for the smaller-scale turbulent dispersion of the local plume, which is often ignored in other average mass balance methods. With the help of large eddy simulations (LES) we further show how the circling radius can be optimized for the micrometeorological conditions encountered during any flight. Furthermore, by sampling controlled releases of methane and ethane on the ground we can ascertain that the accuracy of the method, in appropriate meteorological conditions, is often better than 10 %, with limits of detection below 5 kg h−1 for both methane and ethane. Because of the FAA-mandated minimum flight safe altitude of 150 m, placement of the aircraft is critical to preventing a large portion of the emission plume from flowing underneath the lowest aircraft sampling altitude, which is generally the leading source of uncertainty in these measurements. Finally, we show how the accuracy of the method is strongly dependent on the number of sampling loops and/or time spent sampling the source plume

Conflicting estimates of natural geologic methane emissions

Global bottom-up and top-down estimates of natural, geologic methane (CH4) emissions (average approximately 45 Tg yr–1) have recently been questioned by near-zero (approximately 1.6 Tg yr–1) estimates based on measurements of 14CH4 trapped in ice cores, which imply that current fossil fuel industries' CH4 emissions are underestimated by 25%–40%. As we show here, such a global near-zero geologic CH4 emission estimate is incompatible with multiple independent, bottom-up emission estimates from individual natural geologic seepage areas, each of which is of the order of 0.1–3 Tg yr–1. Further research is urgently needed to resolve the conundrum before rejecting either method or associated emission estimates in global CH4 accounting

Intercomparison of detection and quantification methods for methane emissions from the natural gas distribution network in Hamburg, Germany

In August and September 2020, three different measurement methods for quantifying methane (CH4) emissions from leaks in urban gas distribution networks were applied and compared in Hamburg, Germany: the “mobile”, “tracer release”, and “suction” methods. The mobile and tracer release methods determine emission rates to the atmosphere from measurements of CH4 mole fractions in the ambient air, and the tracer release method also includes measurement of a gaseous tracer. The suction method determines emission rates by pumping air out of the ground using soil probes that are placed above the suspected leak location. The quantitative intercomparison of the emission rates from the three methods at a small number of locations is challenging because of limitations of the different methods at different types of leak locations. The mobile method was designed to rapidly quantify the average or total emission rate of many gas leaks in a city, but it yields a large emission rate uncertainty for individual leak locations. Emission rates determined for individual leak locations with the tracer release technique are more precise because the simultaneous measurement of the tracer released at a known rate at the emission source eliminates many of the uncertainties encountered with the mobile method. Nevertheless, care must be taken to properly collocate the tracer release and the leak emission points to avoid biases in emission rate estimates. The suction method could not be completed or applied at locations with widespread subsurface CH4 accumulation or due to safety measures. While the number of gas leak locations in this study is small, we observe a correlation between leak emission rate and subsurface accumulation. Wide accumulation places leaks into a safety category that requires immediate repair so that the suction method cannot be applied to these larger leaks in routine operation. This introduces a sampling bias for the suction method in this study towards the low-emission leaks, which do not require immediate repair measures. Given that this study is based on random sampling, such a sampling bias may also exist for the suction method outside of this study. While an investigation of the causal relationship between safety category and leak size is beyond the scope of this study, on average higher emission rates were observed from all three measurement-based quantification methods for leaks with higher safety priority compared to the leaks with lower safety concern. The leak locations where the suction method could not be applied were the biggest emitters, as confirmed by the emission rate quantifications using mobile and tracer methods and an engineering method based on the leak's diameter, pipeline overpressure, and depth at which the pipeline is buried. The corresponding sampling bias for the suction technique led to a low bias in derived emission rates in this study. It is important that future studies using the suction method account for any leaks not quantifiable with this method in order to avoid biases, especially when used to inform emission inventories.</p

Flaring efficiencies and NOx emission ratios measured for offshore oil and gas facilities in the North Sea

Gas flaring is a substantial global source of carbon emissions to atmosphere and is targeted as a route to mitigating the oil and gas sector carbon footprint due to the waste of resources involved. However, quantifying carbon emissions from flaring is resource-intensive, and no studies have yet assessed flaring emissions for offshore regions. In this work, we present carbon dioxide (CO2), methane (CH4), ethane (C2H6), and NOx (nitrogen oxide) data from 58 emission plumes identified as gas flaring, measured during aircraft campaigns over the North Sea (UK and Norway) in 2018 and 2019. Median combustion efficiency, the efficiency with which carbon in the flared gas is converted to CO2 in the emission plume, was 98.4% when accounting for C2H6 or 98.7% when only accounting for CH4. Higher combustion efficiencies were measured in the Norwegian sector of the North Sea compared with the UK sector. Destruction removal efficiencies (DREs), the efficiency with which an individual species is combusted, were 98.5% for CH4 and 97.9% for C2H6. Median NOx emission ratios were measured to be 0.003ppmppm-1CO2 and 0.26ppmppm-1CH4, and the median C2H6:CH4 ratio was measured to be 0.11ppmppm-1. The highest NOx emission ratios were observed from floating production storage and offloading (FPSO) vessels, although this could potentially be due to the presence of alternative NOx sources on board, such as diesel generators. The measurements in this work were used to estimate total emissions from the North Sea from gas flaring of 1.4Tgyr-1 CO2, 6.3Ggyr-1 CH4, 1.7Ggyr-1 C2H6 and 3.9Ggyr-1 NOx

Aircraft-based mass balance estimate of methane emissions from offshore gas facilities in the southern North Sea

Atmospheric methane (CH4) concentrations have more than doubled since the beginning of the industrial age, making CH4 the second most important anthropogenic greenhouse gas after carbon dioxide (CO2). The oil and gas sector represents one of the major anthropogenic CH4 emitters as it is estimated to account for 22 % of global anthropogenic CH4 emissions. An airborne field campaign was conducted in April–May 2019 to study CH4 emissions from offshore gas facilities in the southern North Sea with the aim of deriving emission estimates using a top-down (measurement-led) approach. We present CH4 fluxes for six UK and five Dutch offshore platforms or platform complexes using the well-established mass balance flux method. We identify specific gas production emissions and emission processes (venting and fugitive or flaring and combustion) using observations of co-emitted ethane (C2H6) and CO2. We compare our top-down estimated fluxes with a ship-based top-down study in the Dutch sector and with bottom-up estimates from a globally gridded annual inventory, UK national annual point-source inventories, and operator-based reporting for individual Dutch facilities. In this study, we find that all the inventories, except for the operator-based facility-level reporting, underestimate measured emissions, with the largest discrepancy observed with the globally gridded inventory. Individual facility reporting, as available for Dutch sites for the specific survey date, shows better agreement with our measurement-based estimates. For all the sampled Dutch installations together, we find that our estimated flux of (122.9 ± 36.8) kg h−1 deviates by a factor of 0.64 (0.33–12) from reported values (192.8 kg h−1). Comparisons with aircraft observations in two other offshore regions (the Norwegian Sea and the Gulf of Mexico) show that measured, absolute facility-level emission rates agree with the general distribution found in other offshore basins despite different production types (oil, gas) and gas production rates, which vary by 2 orders of magnitude. Therefore, mitigation is warranted equally across geographies.</p

Methane Clumped Isotopes: Progress and Potential for a New Isotopic Tracer

The isotopic composition of methane is of longstanding geochemical interest, with important implications for understanding petroleum systems, atmospheric greenhouse gas concentrations, the global carbon cycle, and life in extreme environments. Recent analytical developments focusing on multiply substituted isotopologues (‘clumped isotopes’) are opening a valuable new window into methane geochemistry. When methane forms in internal isotopic equilibrium, clumped isotopes can provide a direct record of formation temperature, making this property particularly valuable for identifying different methane origins. However, it has also become clear that in certain settings methane clumped isotope measurements record kinetic rather than equilibrium isotope effects. Here we present a substantially expanded dataset of methane clumped isotope analyses, and provide a synthesis of the current interpretive framework for this parameter. In general, clumped isotope measurements indicate plausible formation temperatures for abiotic, thermogenic, and microbial methane in many geological environments, which is encouraging for the further development of this measurement as a geothermometer, and as a tracer for the source of natural gas reservoirs and emissions. We also highlight, however, instances where clumped isotope derived temperatures are higher than expected, and discuss possible factors that could distort equilibrium formation temperature signals. In microbial methane from freshwater ecosystems, in particular, clumped isotope values appear to be controlled by kinetic effects, and may ultimately be useful to study methanogen metabolism

Global Inventory of Gas Geochemistry Data from Fossil Fuel, Microbial and Burning Sources, version 2017

The concentration of atmospheric methane (CH4) has more than doubled over the industrial era. To help constrain global and regional CH4 budgets, inverse (top-down) models incorporate data on the concentration and stable carbon (δ13C) and hydrogen (δ2H) isotopic ratios of atmospheric CH4. These models depend on accurate δ13C and δ2H end-member source signatures for each of the main emissions categories. Compared with meticulous measurement and calibration of isotopic CH4 in the atmosphere, there has been relatively less effort to characterize globally representative isotopic source signatures, particularly for fossil fuel sources. Most global CH4 budget models have so far relied on outdated source signature values derived from globally nonrepresentative data. To correct this deficiency, we present a comprehensive, globally representative end-member database of the δ13C and δ2H of CH4 from fossil fuel (conventional natural gas, shale gas, and coal), modern microbial (wetlands, rice paddies, ruminants, termites, and landfills and/or waste) and biomass burning sources. Gas molecular compositional data for fossil fuel categories are also included with the database. The database comprises 10 706 samples (8734 fossil fuel, 1972 non-fossil) from 190 published references. Mean (unweighted) δ13C signatures for fossil fuel CH4 are significantly lighter than values commonly used in CH4 budget models, thus highlighting potential underestimation of fossil fuel CH4 emissions in previous CH4 budget models. This living database will be updated every 2–3 years to provide the atmospheric modeling community with the most complete CH4 source signature data possible. Database digital object identifier (DOI): https://doi.org/10.15138/G3201T

Application of Gauss's theorem to quantify localized surface emissions from airborne measurements of wind and trace gases

Airborne estimates of greenhouse gas emissions are becoming more prevalent with the advent of rapid commercial development of trace gas instrumentation featuring increased measurement accuracy, precision, and frequency, and the swelling interest in the verification of current emission inventories. Multiple airborne studies have indicated that emission inventories may underestimate some hydrocarbon emission sources in US oil-and gas-producing basins. Consequently, a proper assessment of the accuracy of these airborne methods is crucial to interpreting the meaning of such discrepancies. We present a new method of sampling surface sources of any trace gas for which fast and precise measurements can be made and apply it to methane, ethane, and carbon dioxide on spatial scales of ĝ1/4 1000ĝ€m, where consecutive loops are flown around a targeted source region at multiple altitudes. Using Reynolds decomposition for the scalar concentrations, along with Gauss's theorem, we show that the method accurately accounts for the smaller-scale turbulent dispersion of the local plume, which is often ignored in other average mass balance methods. With the help of large eddy simulations (LES) we further show how the circling radius can be optimized for the micrometeorological conditions encountered during any flight. Furthermore, by sampling controlled releases of methane and ethane on the ground we can ascertain that the accuracy of the method, in appropriate meteorological conditions, is often better than 10ĝ€%, with limits of detection below 5ĝ€kgĝ€hĝ'1 for both methane and ethane. Because of the FAA-mandated minimum flight safe altitude of 150ĝ€m, placement of the aircraft is critical to preventing a large portion of the emission plume from flowing underneath the lowest aircraft sampling altitude, which is generally the leading source of uncertainty in these measurements. Finally, we show how the accuracy of the method is strongly dependent on the number of sampling loops and/or time spent sampling the source plume

Direct Measurement of Coal Seam Gas and Agricultural Methane Emissions in the Surat Basin, Australia

Direct Measurement of Coal Seam Gas and Agricultural MethaneEmissions in the Surat Basin, AustraliaBryce F.J. Kelly (1), Xinyi Lu (1), Stephen J. Harris (1), Rebecca E. Fisher (2), Dave Lowry (2), James L. France(2,3), Jorg Hacker (4), Bruno Neininger (5), Thomas Röckmann (6), and Stefan Schwietzke (7)(1) UNSW Sydney, School of Biological, Earth and Environmental Sciences, UNSW Sydney, Australia([email protected]), (2) Royal Holloway, University of London, Egham Hill, Egham, Surrey TW20 0EX, UnitedKingdom, (3) British Antarctic Survey, High Cross, Madingley Rd, Cambridge CB3 0ET, United Kingdom, (4) AirborneResearch Australia and Flinders University, PO Box 335, Salisbury South, 5106, Australia, (5) METAIR AG, Airfield Hausenam Albis, 8915 Hausen am Albis, Switzerland, (6) Institute for Marine and Atmospheric Research, Utrecht University,Netherlands, (7) Environmental Defense Fund, GermanyThe Surat Basin, Queensland, Australia, is a hot-spot of methane emissions for Australia. Within the Surat Basinthere are over 6000 coal seam gas wells with extensive supporting pipeline networks and processing plants. Theregion also supports a multi-billion-dollar agricultural industry, and at times accounts for over half of Australia’sbeef production, with over 500,000 cattle; both grazing and housed in feedlots. Individual feedlots may hold10,000 or more cattle at any one time. The top 6 emitters in the region and their bottom-up estimated percentagecontribution towards regional methane emissions include: cattle (78.1%), CSG processing (8.4%), coal extraction(8.3%), piggeries (1.4%), CSG production (1.1%), and landfill (1.0%) (Luhar et al. 2018). Because there are manymethane sources within the Surat Basin it can be difficult to make a top-down assessment of methane emissionsthat can be attributed to an individual source. Source attribution is particularly challenging because many of thecattle feedlots are co-located with both CSG production wells and processing plants.This presentation will discuss what activities have been undertaken to date in the Surat Basin, Australia, and the future research aims. In brief, in September 2018 a joint airborne and ground measurement survey campaignwas undertaken to make a top-down assessment of methane emissions in the region and investigate methane source attribution. The portion of the Surat Basin that was surveyed in September 2018 extends from Miles to Toowoomba, covering an area of approximately 200 km (NW to SE) by 100 km (NE to SW). Continuous measurements of the methane mole fraction in the atmosphere were recorded during 13 flights over 15 days using an aircraft-mounted LGR greenhouse gas analyser. These flights had a number of objectives aimed at assessing the regional methane flux, with more detailed surveys above major CSG processing and power generation plants. Detailed airborne measurements were also undertaken above the large-scale cattle feedlots. To support the insights from the airborne measurements, continuous methane mole fraction measurements in the ground level atmosphere were recorded along more than 1000 km of main roads using a car-mounted LGR greenhouse gas analyser or a Picarro 2201-i analyser. To assist with source attribution, over 200 grab bag samples of air were collected, and the stable carbon and hydrogen isotope ratios of methane measured ( 13C-CH4 and D-CH4). The presentation will discuss how thesurveys complement each other and potentially constrain how we will interpret the results.ReferenceLuhar, A., Etheridge, D., Loh, Z., Noonan, N., Spencer, D., Day, S. 2018. Characterisation of Regional Fluxes ofMethane in the Surat Basin, Queensland. Final report on Task 3: Broad scale application of methane detection, andTask 4: Methane emissions enhanced modelling. Report to the Gas Industry Social and Environmental ResearchAlliance (GISERA). Report No. EP185211, October 2018. CSIRO Australi