Investigation of the effect of carbon dioxide sequestration on the hydro - mechanical properties of coal

Abstract

The process of carbon dioxide (CO2) sequestration in deep coal seam causes both coal seam permeability and strength to be significantly reduced due to CO2 adsorption-induced coal matrix swelling. In addition, in deep coal seams CO2 exists in its super-critical state, which has quite different chemical and physical properties compared to sub-critical CO2. However, to date, there has been a lack of understanding regarding the effect of super-critical CO2 injection on coal flow and strength. The main objective of this study is to understand the effects of sub-critical and super-critical CO2 injections on coal flow and strength properties through experimental, numerical, theoretical and analytical investigations. A high pressure triaxial set-up was first developed to conduct permeability tests under high injecting and confining pressures, axial load and temperature conditions. The developed set-up was then used to conduct permeability tests for naturally fractured black coal samples taken from the Appin coal mine of the Bulli coal seam, Southern Sydney basin to identify the effects of sub-critical and super-critical CO2 injections on coal permeability. According to the experimental results, the amount of swelling due to CO2 adsorption depends on the CO2 phase state and confining and injecting pressures, and super-critical CO2 adsorption creates approximately double the swelling effect compared to sub-critical CO2. In addition, super-critical CO2 exhibits somewhat lower permeability values compared to sub-critical CO2, and this permeability reduction increases with increasing injecting pressure. Interestingly, N2 has the potential to reverse the CO2 induced swelling areas to some extent. If the temperature effect on permeability is considered, temperature has a positive effect on CO2 permeability. The CO2 permeability increment with increasing temperature increases with increasing CO2 pressure, and the effect of temperature on coal permeability is negligible at low CO2 pressures (