Assessment of the CO2 sequestration potential of coal seams in the Hunter Coalfield, Sydney Basin

Abstract

The extensive coal deposits of the Hunter Coalfield of the Sydney Basin provide an ideal natural analogue to assess the potential of CO2 storage in coal. Following on from a collaborative study supported by the CSIRO's Energy Transformed Flagship and the CO2CRC on the CO2 storage potential of the Hunter Coalfield, five coal seam gas ‘compartments’ (where gas content and composition show similar distributions) were identified based on structural and gas data. The seam gas composition varies between predominantly CH4 to predominantly CO2, with up to 100% CO2 in some areas. The magmatic origin of the CO2 in these coals is supported by isotopic data as well as the proximity of igneous dykes and sills, but the present-day distribution may also be related to gas migration through geological structures and dissolution in groundwater. This study has refined a previous assessment of the CO2 storage potential of the Hunter coals. Using a simplistic approach the theoretical total storage capacity of the coalfield is estimated to about 9512 Mt of CO2 between 300 and 800 m depth, with an average coal permeability of ∼1.5 mD between these depths. Permeability is believed to be an order of magnitude lower at depths greater than 800 m. The southern region of the coalfield shows the greatest potential for CO2 storage given its high permeability, proximity to potential CO2 sources, and thick sequence of coal seams, including deeper seams as possible storage targets. The actual available storage capacity for the coalfield, however, could be considerably less owing to factors such as high existing CO2 concentrations in some areas, low permeability in many areas, accessibility issues owing to the presence of national parks, and existing and proposed coal mines.