Experimental study on initiation pressure and mechanism of fracture propagation of hydraulic fracturing in coal and rock mass

To study the hydraulic fracturing initiation pressure and mechanism of fracture propagation in coal and rock mass, the physical simulation experiment program "exerting three-dimensional stresses-injecting fluid from the top" of coalbed hydraulic fracturing was established based on engineering applications by using briquette samples and self-developed hydraulic fracturing testing system. According to the above program combining the existing experimental conditions, the water pressure and hydraulic fracture were analyzed under varied conditions. The experimental results show that initiation pressure increases in the process of increasing flow rate. Hydraulic fractures propagate along the direction perpendicular to σ_h under the condition of σ_H > σ_v > σ_h. With the increase of σ_H, the initiation pressure of coal rock increases at first and then decreases when σ_v and σ_h are constant, in addition, the fracture propagation path becomes more straight. As σ_H is much larger than σ_v and σ_h, the fracture propagation path gets more complex and the angle of bifurcate fractures is larger. The results are of significance and importance for the improvement of hydraulic fracturing theory in coal rock mass.