Hydro-mechanical property variation of deep coal seams with CO2 injection is a critical issue in the CO2 sequestration process as this causes unpredictable CO2 injectibilities and significant safety issues. CO2 adsorption-induced coal matrix swelling is the main identified cause, where super-critical CO2 is expected to create a greater influence compared to sub-critical CO2, due to its highly chemically reactive nature. Therefore, this study aimed to distinguish the effects of sub- and super-critical CO2 injections on coal flow and strength properties.

A high-pressure rig was first developed to conduct experiments under in-situ conditions and the developed rig was then used to conduct coal permeability tests. The test results exhibit a much greater swelling effect with super-critical CO2 adsorption, which therefore produces lower permeability values in coal compared to sub-critical CO2. Temperature also appears to create a significant influence on CO2 flow behaviour in coal, and CO2 permeability clearly increases with increasing temperature for high CO2 injection pressure (> 10MPa). Interestingly, N2 has the potential to reverse CO2-induced swelling effect to some extent, which is favourable for field projects. The results of UCS strength tests show that both strength and Young's modulus of any type of coal are significantly reduced due to CO2 adsorption, and the reduction is higher for super-critical CO2 adsorption than sub-critical CO2. CO2 adsorption-induced strength reduction increases with increasing coal rank, which is due to the well-developed natural cleat system in mature high rank coals.

The extensive time and cost of laboratory experiments can be minimised by the use of appropriate numerical modelling tools and this study therefore provides a step-by-step guide to the development of such a modelling tool using a user-friendly field-scale simulator (COMET 3). This model can be effectively used to predict the CO2 movements in coal under tri-axial laboratory conditions. Theoretical models play an important role in speeding the identification of coal mass properties and are believed to be more accurate. Therefore, a theoretical equation for coal cleat permeability under non-zero lateral strain tri-axial test condition was also developed. These models are expected to be highly useful in future, laboratory-scale CO2 sequestration studies to establish the required tri-axial test conditions.

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