Experimental Evaluation of Permeability of Coal in Supercritical CO2 and N2 Injection Under Stress and Strain Restricted Conditions

A micro pilot test of CO2-enhanced coal bed methane (CO2-ECBM), which is a carbon capture storage (CCS) technique for global warming prevention, was carried out in Yubari. CO2 was injected into a coal seam, which consisted of a cleat and a coal matrix, and the injection rate of CO2 was decreased with time. The matrix swelled as CO2 was adsorbed. Depending upon the constraint conditions, the cleat closed with CO2 injection, and the permeability decreased. In this study, the strain and stress constraint conditions were created in-laboratory and N2 and supercritical CO2 were injected repeatedly in a coal specimen for observation of the permeability, Vp, circumferential strain and confining pressure. In order to achieve the strain constraint condition, a circumferential extensometer was installed in a cylindrical specimen and the output was used to servo-control the confining pressure intensifier. In order for the matrix to adsorb CO2 and swell, supercritical CO2 was injected into the coal seam under the strain constraint conditions, the cleat (passage of flow) was closed, and infiltration of supercritical CO2 was restrained for a short time, and flow was then suspended. A swelling pressure of approximately 6 MPa was observed at this time with adsorption of supercritical CO2 onto coal. When N2 was re-injected into the coal specimen in which supercritical CO2 was injected under the strain constraint conditions, no immediate flow was generated. However, several hours later, when permeation began, the flow was progressively increased. Although the matrix might have adsorbed CO2 and swelled when supercritical CO2 was re-injected into the coal under stress constraint conditions, the cleat was not closed; therefore, infiltration of supercritical CO2 was continued and the flow was not stopped. A swelling strain of approximately 0.6% with adsorption of supercritical CO2 on coal was observed at this time.