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

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 (<9 MPa). UCS strength tests were then conducted for both high rank (black) and low rank (lignite) coals under different saturation conditions; sub-critical and super-critical CO2, N2 and moisture. According to the test results, the UCS strength and Young’s modulus of both types of coals are reduced due to CO2 saturation and N2 saturation does not have much influence on coal strength. The reductions of UCS strength and Young’s modulus in black coal due to super-critical CO2 adsorption are higher than the sub-critical CO2 adsorption by about 40% and 100 %, respectively. Furthermore, with increasing saturation pressure, the reductions of coal UCS strength and Young’s modulus due to super-critical CO2 adsorption become lower. However, the effect of super-critical saturation on the strength of low rank coal could not be investigated due to its low density and strength values. Both laboratory and field-scale model developments were considered in the numerical modelling approach to coal CO2 sequestration. In the case of the laboratory-scale model development, CO2 movement in coal under triaxial test conditions was successfully modelled using the COMET 3 field scale simulator. According to the field-scale models developed using the COMET 3 and COMSOL Multiphysics simulators, CO2 storage capacity in coal increases with increasing injecting pressure and temperature and decreasing bed moisture content. Moreover, pre-estimation of the distance between the wells (injecting and production) is important for the injection of optimum amounts of CO2. On the other hand, CO2 injection causes the coal seam cap rock to be significantly deformed in an upward direction and the amount of deformation is greatly dependent on the injection pressure. In relation to theoretical and analytical approaches to coal CO2 sequestration, a theoretical equation for coal cleat permeability under non-zero lateral strain, triaxial test conditions was developed using basic geotechnical engineering fundamentals and an empirical relationship for gas adsorption capacity in coal as a function of all the effective parameters was developed using basic statistics.