Experimental verification of a fully coupled thmc model for the evolution of coal permeability under the influence of gas adsorption

Significant laboratory and modelling efforts have been made to investigate the evolution of coal permeability during CO2 sequestration. However, experimental results normally cannot match the modelling results very well. Theoretically, coal permeability should not change at the final steady state under the constant stress conditions due to the uniform swelling of the coal sample, or it should slightly increase due to the expansion of the coal matrix bridge which connects the coal matrix faces in the fracture. However, the laboratory observations are contradicted to these predictions. At laboratory, permeability is usually measured when gas adsorption reaches equilibrium, and calculated as a function of porosity, pressure and temperature. Through this procedure, the effect of matrix swelling cannot be manifest, since permeability cannot be monitored with the dynamic gas adsorption. We have measured coal permeability evolutions during CO2 injection in laboratory. The aim of this study is to verify our numerical model against our experimental permeability measurements and the reason for their discrepancy was analyzed. The results show that the measured permeability values at the laboratory interestingly lie between the lowest permeability value at the switching point and the final value at the equilibrium, which supports our speculations that permeability was measured before the final steady state was reached in the laboratory. These observations and conclusions were related to the sorption behavior of the adsorbing CO2 during its injection into coal samples. This study discovers the constraint of the current experiment method of measuring permeability at the equilibrium state because the equilibrium state was seldom reached at laboratory.