Directional hydraulic fracturing by prefabricating longitudinal (axial) cracks on the sides of boreholes is an effective technique to control hard roof and ensure mining safety. In the present work, a true triaxial hydraulic fracturing experimental device was used to study the influence of prefabricated crack (pre-crack) parameters (including inclination angle and crack length), fracturing fluid viscosity, and mechanical characteristics of rocks on directional hydraulic fracturing with axial pre-cracks. The results show that as the pre-crack inclination angle increases, the fracture initiation pressure gradually increases, and the fracture extension length increases firstly and then decreases. With the increase of the pre-crack length, the fracture initiation pressure declines and the crack propagates over a shorter distance. As the fracturing fluid viscosity increases, the fracture initiation pressure grows. When the viscosity reaches 40 mPa·s, the crack extension length increases remarkably. Moreover, larger fracture toughness results in a higher fracture initiation pressure and shorter fracture extension length. Our research results can help optimize parameters of angle and length of pre-cracks and the viscosity of fracturing fluids based on the characteristics of the surrounding rocks for coal seam roof control.

通过在钻孔侧面预制纵向（轴向）裂缝进行定向水力压裂是控制硬顶并确保采矿安全的有效技术。在本工作中，使用了真正的三轴水力压裂实验装置来研究预制裂缝（裂缝）参数（包括倾角和裂缝长度），压裂液粘度和岩石力学特性对定向水力压裂的影响。轴向预裂。结果表明，随着裂纹前倾角的增大，断裂起始压力逐渐增大，且裂纹扩展长度先增大后减小。随着裂纹前长度的增加，断裂起始压力下降，裂纹在更短的距离内扩展。随着压裂液粘度的增加，压裂起始压力增加。当粘度达到40mPa·s时，裂纹扩展长度显着增加。此外，较大的断裂韧性导致较高的断裂起始压力和较短的断裂延伸长度。我们的研究结果可根据煤层顶板控制的围岩特性，帮助优化预裂角和裂隙长度参数以及压裂液的粘度。