Coal swelling strain and permeability change with injecting liquid/supercritical CO2 and N2 at stress-constrained conditions

►In order to understand the phenomena observed in the CO2 Enhanced Coalbed Methane Recovery field trials at Yubari, two laboratory experiments were conducted under stress-constrained conditions. ►In Test I, liquid CO2 was injected into a water-saturated coal specimen and then heated and injected as supercritical CO2. This was to simulate the initial stage of CO2 injection at Yubari when the coal seam was saturated with water. A volumetric swelling strain of 0.25 to 0.5% was observed after injecting liquid CO2. ►In Test II, supercritical CO2 was injected into a coal specimen saturated with N2, and then N2 and CO2 were repeatedly injected. This test was to simulate the case of N2 injection and CO2 re-injection at Yubari. The swelling strain was about 0.5 to 0.8% after injecting supercritical CO2. ►Following further injection of N2 in Test II, slow strain recovery was observed in the coal. At an effective stress of 2 MPa, the permeability of the water-saturated coal specimen was 2 × 10− 6 darcy. In contrast, the permeability of the N2-saturated coal specimen was originally 5 × 10− 4 to 9 × 10− 4 darcy, and after injection of supercritical CO2 it decreased to 2 × 10− 4 darcy. Further injections of N2 and supercritical CO2 caused little subsequent change in permeability.