CO2 enhanced flow characteristics of naturally-fractured bituminous coals with N2 injection at different reservoir depths

Coal matrix swelling caused by the adsorption of CO2 during the CO2-ECBM process is the major reason for the decline of coal permeability and CO2 injectivity. Measures to recover this permeability reduction are of significant importance for the field projects. Many studies to date have established the responses of the coal matrix upon the adsorption of CO2 under various laboratory conditions, and some have revealed that the injection of N-2 into swelled coal can recover coal permeability to some extent. This study therefore examines the effect of various N-2 saturation durations on the permeability alteration of the swelled coals subjected to the adsorption of sub-and super-critical CO2 at different reservoir depths. A series of permeability tests was performed on a triaxial test rig, and the results show that high adsorption of super-critical CO2 in coal produces greater permeability reduction; the average CO2 permeability recovery rates are 25.08% and 37.31% for 6 MPa and 14 MPa N-2 saturation, respectively, as the higher N-2 permeating pressure favours greater permeability recovery; longer N-2 saturation duration also produces higher permeability recovery, but the permeability recovery process eases with the saturation duration; coal permeability is very susceptible to the poro-elastic effect in high confining environments and the effect of matrix responses on the interaction with gas sorption is therefore negligible.