Quantification of cracks and the evolution of permeability for reservoir rock under coupled THM: equipment development and experimental research

This study designs and introduces an X-ray-compatible thermo-hydro-mechanical coupling triaxial device. The functional parameters of this device included a confining pressure, axial force, injected fluid pressure, and temperature up to 20 MPa, 400 kN, 10 MPa, and 100 °C, respectively. Triaxial stress loading–unloading cycles and permeability experiments for coal specimens were carried out using this device. Following four pre-set cycles, the specimen was scanned to identify internal cracks, and the relationship between stress, strain, permeability, and crack evolution was analysed. The experimental results showed that new cracks were mainly caused by the expansion of initial cracks as the triaxial stress cycles. The crack area fraction (CAF) increased almost linearly with the stress cycle. With the increase in CAF, the permeability of coal had increased under both high and low deviatoric stress. However, the increment of permeability that was driven by the CAF increment had decreased and eventually became stable. During the initial cracking stage, the increase in permeability from the CAF increase under high deviator stress was always greater. This demonstrates that the contribution of new cracks to increasing permeability under different triaxial stresses was significantly different. The outcomes from this study may provide benefits for coalbed methane extraction and dynamic hazard prevention.