Experimental study on guided hydraulic fracturing of low-permeability coal rock reservoirs under true triaxial conditions

The exploitation of coalbed methane in low-permeability coal rock reservoirs is a major research focus in energy exploitation. This study constructed a theoretical model for calculating the circumferential stress at the fracture tip by taking into account the hydraulic fracture Ⅰ-Ⅱ composite stress intensity factor so as to investigate the expansion patterns of hydraulic fracture under low filtration loss in low-permeability reservoirs. We took the low-permeability rock samples containing prefabricated cracks as the research object and conducted a true triaxial guided hydraulic fracturing test to explore the expansion patterns of cracks under low filtration loss with different horizontal stress differences and fluid injection rates. Results show that, in hydraulic fracturing of low-permeability rock formations, increasing fluid injection rate tended to cause a single straight fracture. The effect on crack length was significantly greater than the effect on crack width. Both the hydraulic fracture initiation pressure and sustained expansion pressure increased with increasing fluid injection rate. When the maximum horizontal principal stress remains unchanged and the minimum horizontal principal stress decreases, the initiation pressure of the crack decreased. The hydraulic force will make the crack propagation path change from inclined curvature to a gentle one, and the width of the crack under water pressure shows a trend of first increasing and then decreasing. In addition, increasing injection rate and horizontal principal stress difference would lead to increasing total injection volume of rock damage, i.e., the volume of the fracture increased under low leaching losses.