Methane Source Attribution Challenges in the Surat Basin, Australia

One of the case study sites for the Climate and Clean Air Coalition (CCAC) Methane Science Studies is the coal seam gas (CSG) field in the Surat Basin, Queensland, Australia, where there are over 6000 CSG wells and associated gas and water processing infrastructure. Previous bottom-up estimates suggest that the major source of methane in the region is cattle, not CSG (Katestone, 2018, Luhar et al. 2018).In September 2018, an airborne measurement campaign was undertaken to provide a top-down estimate of regional methane emissions. Modelling of the airborne methane mole fraction data has produced a defensible total methane emissions estimate. However, there are challenges with proportioning the top-down estimates provided by the airborne data, because of adjacent sources with similar d13C-CH4 isotopic chemistry, rapid mixing of adjacent sources and substantial dilution of the plumes at the airborne measurement sampling height. We present how we will overcome these challenges.At each gas production well, tens of thousands of litres of water are produced daily in association with the methane extracted from the coal measures. This water is stored in ponds and is also used as a water supply for cattle feedlots, which are located throughout and adjacent to the CSG wells and processing facilities. Power stations are also located within the CSG field. This arrangement makes it challenging to obtain clean top-down estimates of the emissions from CSG production. Quantifying methane emissions associated with CSG production is further complicated by numerous other sources of methane in the region immediately adjacent to the CSG field. These sources include grazing cattle, abattoirs, more power production facilities, coal mines, wetlands, natural gas seeps, and small urban centres with associated sewage treatment plants and landfills. Grab bag air samples were collected at each of these sources and analysed for d13C-CH4, d13C-CO2 and dD-CH4.The airborne measurement campaign was undertaken under warm daytime spring conditions. This caused rapid uplift and mixing of the methane plumes. The maximum difference between the lowest and highest methane mole fraction from 90 airborne collected grab bag air samples was only 0.03 ppm. Even at this low mole fraction, by implementing quality management protocols we were able to extract trends in the isotope data sets. This presentation will outline the quality management procedures and how the measurements of d13C-CH4, d13C-CO2 and dD-CH4 will be used to assist with methane source attribution.ReferenceKatestone Environmental Pty Ltd (2018) Surat Basin Methane Inventory 2015 - Summary Report. Prepared for CSIRO March 2017 (D15193-11).Luhar, A., Etheridge, D., Loh, Z., Noonan, N., Spencer, D., Day, S. (2018). Characterisation of Regional Fluxes of Methane in the Surat Basin, Queensland. Final report on Task 3: Broad scale application of methane detection, and Task 4: Methane emissions enhanced modelling. Report to the Gas Industry Social and Environmental Research Alliance (GISERA). Report No. EP185211, October 2018. CSIRO Australia.