Adsorption and mineral trapping of CO2 in coal systems

High-CO2 coal seams occur in many coal basins world-wide and provide natural analogues of the processes likely to occur as a result of CO2 injection and storage in coal systems. CO2 storage in coal seams is an attractive geosequestration option when coupled with enhanced production of coal bed methane (ECBM) that depends on coal’s higher adsorption capacity for CO2 relative to CH4. CO2 is stored in coal predominantly as adsorbed molecules on micropore surfaces (adsorption trapping) that allow higher densities and greater volumes at shallower depths than in sandstone and carbonate reservoirs where CO2 is stored initially as a free phase (structural/stratigraphic trapping). In the longer term CO2 will dissolve in formation water and react with minerals in the host formation (solution/ ionic trapping) and may be precipitated as carbonate minerals (mineral trapping). Recent studies suggest that solution trapping is the predominant CO2 sink in natural gas fields, whereas our natural analogue studies in the Bowen and Gunnedah Basins indicate that magmatic CO2 has been stored in coal and sandstone formations since the Mesozoic through a combination of adsorption and mineral carbonation reactions. Gas stable isotopes confirm generation of secondary biogenic methane in CO2-rich coal seams by reduction of CO2 that suggests methanogenesis may provide an additional sequestration mechanism for CO2 in coal seams. These observations compare well with those seen previously in the Sydney Basin. © 2009 CO2CRC