Hydraulic fracturing simulation test and fracture propagation analysis of large-scale coal rock under true triaxial conditions

Hydraulic fracturing is a pressure relief and permeability enhancement technology widely applied in low permeable coal seam. To study the impacts of in-situ stress, natural fractures and flow rates on fracture propagation and fracture network, the true triaxial hydraulic fracturing experiments of large-scale coal seam were conducted on large-scale true triaxial hydraulic fracturing experimental system and tracer technique in hydraulic fluid. The propagation and geometry of hydraulic fractures were described by cutting the fractured specimen. The relationship between fracture width and in-situ stress was analyzed and fracture network mechanism in coal rock was studied. The experimental results indicate that: 1) Fracture network forms when hydraulic fractures propagate along cleats after initiation. 2) Hydraulic fracture is greatly affected by in-situ stress. Complex fracture morphology easily forms when maximum horizontal stress is close to vertical stress and both of them are much bigger than minimum horizontal stress. 3) Natural fractures in coal rock are the foundation of fracture network, which is also affected by flow rate. 4) Hydraulic fractures may steer and branch under the impacts of natural fractures at local, but they finally adjust the propagation to the direction of maximum horizontal stress. 5) During hydraulic fracturing process, the changes of fracture width are affected by factors include in-situ stress, flow rates, natural fractures in coal rock, etc. The results could provide technical support for understanding fracture network mechanism and determination of field fracturing parameters in CBM reservoir.