A theoretical and experimental investigation of gas adsorption-dependent bulk modulus of fractured coal

Abstract

Changes in the bulk modulus of coal induced by sorbing gases occur during gas production and CO2 sequestration processes in coal seams. Understanding these changes, however, remains a challenge due to the complexity of coupled sorptive poromechanical processes. Some studies report that the bulk modulus of fractured coal reduces with gas adsorption, others state otherwise. In this study, we perform a set of triaxial experiments to investigate the bulk modulus variation in fractured coals with non-sorbing (helium) and sorbing (carbon dioxide) gases. We then present a theoretical model developed based on energy conservation and poroelasticity to predict the bulk modulus of fractured coal and its variation with sorbing gases. The model performance in predicting the bulk modulus is then assessed using the experimental results where a good performance is observed. A parametric study is further conducted to investigate the effect of key model parameters on the bulk modulus. The experimental results demonstrate that bulk modulus variations are predominantly caused by two counteracting effects: a hydromechanical effect resulting from a decrease in the effective stress due to pore pressure buildup and a sorptive-mechanical effect due to adsorption-induced matrix swelling. The experimental and theoretical results also show that these effects can lead to an increase or decrease in the bulk modulus of fractured coal. The parametric study using the theoretical model reveals that the initial fracture porosity and the adsorbed gas content are important factors controlling the variations in bulk modulus.