Orientation and Controlling Role of Induced Stress Field on Formation of Hydro-fractures in Multi-hole Hydraulic Fracturing Process

It is necessary but difficult to avoid the occurrence of fracturing blind zone and to prevent the excessive concentration of local stress at the same time in hydraulic fracturing in coal mine. The difficulty lies in that it needs to make much more fractures while limiting the fracturing energy. Based on the mechanism of induced stress, the orientation and controlling role of the induced stress field on the hydraulic fractures in coal seam under the condition of multi-hole synchronous fracturing was analyzed by RFPA^2D-Flow simulation. A new technology, combined with the water injection for directional pressure relief and the multi-hole hydraulic fracturing for controlling the fractures propagation, was presented and applied in the field. The results show that: the hydro-fractures of all the simultaneously fracturing drilling-holes will be induced by each other; under the function of the induced stress field, the direction of the minimum principal stress in the vicinity of the later main hydro-fracture deflects about 47.5°, basically perpendicular to the connection line of the main hydro-fracturing deflection points of two drilling-holes; the water jet drilling expansion could induces the hydro-fractures near expanding to itself; the technology could, in a certain extent, eliminate fracturing blind zone, while avoiding excessive stress concentration, and it would significantly improve the gas drainage content.