Jun Fan / China University of Mining and Technology
Linming Dou / China University of Mining and Technology
Xuwei Li / China University of Mining and Technology
Directional hydraulic fracturing (DHF) is more and more widely used in coal mines in China for hard roof and rock/coal burst control. The key to success of this technology is the initial pressure of the fracturing fluid that affected by the shape of the artificial notch and the stress state around the fracturing hole. Reasonable and simple formula for fracturing pressure calculation is essential since the fracturing bump used in coal mines is usually limited by the harsh conditions and hardly replaced once determined. Based on superposition principle in elasticity, the simplified 2D model of DHF was established as the elliptical hole with the internal water pressure in the in-site stress and solved by using the complex functions method. The analytical solution of tangential stress on the inner surface was obtained meanwhile the corresponding criterion of fracturing pressure can be set up. Considering the characteristics of DHF in coal mines, we further got a simplified formula that controlled by the ratio of major to minor axis of the ellipse-like notch, the ratio of the minimum to the maximum principal stress, as well as the tensile strength of the rock. The formula also gave a guide to the design of the notch that major diameter should be at least twice the minor diameter, and the optimal solution for the ratio is to 2～4 and recommended 4, which can resist the initiation pressure to a large extent affected by the in-situ stress. Once the pressure of the fracturing fluid is high enough to satisfy the equation in the paper a crack would arise at the tips of the notch along the major axis which belongs to mode Ⅰ crack and would grow unsteadily and rapidly. A PFC simulation model was used to verify the analysis, the results of which are very consistent with the theoretical solutions.