Laws of multi-fracture coupling initiation during blasting induced hydraulic fracturing

For figuring out the stress interference of multiphase fracture combinations and its effect on the fracture initiation pressure of subsequent hydraulic fracturing, a calculation model for the coupled stress field with multiple induced fractures preexisted was established based on the calculation model for the stress field with single induced fracture preexisted, and the change laws of circumferential stress field around the wellbore under the effect of induced stress were analyzed. Then, the fracture initiation pressure of subsequent hydraulic fracturing was calculated according to the fracturing mechanics criterion. Finally, the effects of the length, phase, horizontal principal stress difference coefficient and quantity of preexisting fractures on its initiation pressure were analyzed. And the following research results were obtained. First, the circumferential stress difference in the area near the fractures behind the preexisting fractures increase greatly and even the horizontal principal stress is reversed. Second, as the length of new preexisting fractures increases, the initiation pressure of initial preexisting fractures rises first and then drops. And when the length of new preexisting fractures is equal to that of the initial preexisting fractures, the initiation pressure of initial preexisting fractures drops quickly first and then slowly. Third, the initiation pressure of high phase fractures is lower than that of low phase fractures. Fourth, with the increase of fracture quantity, the initiation pressure of new preexisting fractures drops gradually, but the decline trend of initiation pressure difference is not obvious. Fifth, the stress interference of multiphase fractures influences the fracture initiation pressure, and high-phase long fractures and low-phase short fractures are both favorable for the simultaneous initiation of multiphase fractures. Sixth, synchronous propagation of multiple fractures can generate more complicated stress interference and excite the evolution of hydraulic fracture network, so as to realize uniform fracturing stimulation. In conclusion, the research results can provide a theoretical guidance for the design of fracturing operation, e.g. perforation phase of blasting induced fracturing, scale of induced fractures, and pump pressure of subsequent hydraulic fracturing.