Effect of gas pressure fracturing on anisotropic effective stress coefficient and permeability enhancement of coal
摘要
In the deep high-stress and low-permeability coal seam extraction, the compaction and expansion of coal, the deterioration of mechanical properties, the transformation effect of the fracture network, and the permeability evolution are closely related to the effective stress of coal under true triaxial mining environment (σeij=σij−αijp, where σeij is the effective stress, σij is the principal stress, αij is the effective stress coefficient, and p is the pore pressure), and the key to quantify the effect of the effective stress is to determine the anisotropic properties of αij. Anisotropic effective stress coefficient and permeability evolution characteristics of coal during CO2 fracturing under different shallow and deep stress states are analyzed in this study. The results indicated that the αij of coal revealed anisotropy during the CO2 fracturing induced damage process, showed a uniform increase with the increase in damage degree, the anisotropy of αij subsequently increased as well, and always exhibited α1 > α3 > α2. The elevation of true triaxial initial stresses constrained the propagation of gas fracture-inducing cracks. αij in a state of high stress was lower than that in a state of low stress under initial fracturing conditions. For different fracturing directions, during the initial damage stage, it is observed that αij shows αF > αB > αD (αF is the effective stress coefficient in face cleat direction, αB is the effective stress coefficient in butt cleat direction, and αD is the effective stress coefficient in bedding direction.). As the fracturing progresses, αD demonstrated a more significant increase compared to αB and αF, indicating that natural fractures distributed along bedding planes are more prone to propagation under the driving force of high-pressure gas. The tensile stress generated by CO2 fracturing of coal could stimulate the tip fissures to combine with beddings and cleats to generate tilted fissures, and form a composite network of shear or