Experimental study on the effects of pre-cracks, fracturing fluid, and rock mechanical characteristics on directional hydraulic fracturing with axial pre-cracks

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 center dot 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.

Automated summary

The study shows that the inclination angle of pre-cracks affects the fracture initiation pressure and extension length, while a longer pre-crack length reduces the fracture initiation pressure. Additionally, increasing fracturing fluid viscosity significantly increases the crack extension length.