Microfracturing of Coal Related to Adsorption-Induced Swelling and Matrix-Scale Heterogeneity

We report dilatometry experiments conducted on unconfined mm-scale coal samples. The samples were exposed to CO₂ at pressures up to 100 MPa, at a temperature of 40 ° C. Cycling the CO₂ pressure between 0 and 100 MPa resulted in reversible and irreversible strains. Substantial hysteresis was observed in strain versus pressure response in the first and second pressurization cycles. Only after two or three cycles, the measured strain response became fully reversible, and equilibration was four times faster compared to the first cycle. SEM imaging showed that microfractures had formed in our samples during the tests. We infer that these were caused by heterogeneous swelling of the coal macerals, and were responsible for the strain hysteresis and fast CO₂ penetration into the samples under the experimental conditions used here. Under in situ effective stresses, microfractures are unlikely to form or open. We argue that, therefore, Enhanced Coalbed Methane (ECBM) production can be achieved only by active reservoir stimulation.