Rock fracture kinetics of the facture mesh system in shale gas reservoirs

The formation mechanism of fracture mesh and the mechanism of activating natural closed fractures during shale fracturing are studied based on rock fracture kinetics. It is found that steering conditions should be satisfied at both left and right ends of a natural fracture to form intensive fracture mesh after the hydraulic fracture reaches the natural fracture; there exists a minimum critical pump rate (critical pump rate) to form intensive fracture mesh. The critical pump rate increases as the inclination of natural fracture increases and reaches a maximum value (constant) when the inclination of natural fracture is 90°. When the inclination of natural fracture is less than 90°, the critical pump rate first decreases and then increases as the angle between horizontal wellbore and natural fracture increases; the critical pump rate reaches a minimum value for the natural fractures perpendicular to the wellbore. The critical pump rate increases as natural fracture length increases and rock elastic modulus decreases. It is also found that natural fracture surface is rough and not fitting, so when hydraulic fractures reopen existing natural fractures, the release of the original shear stress in the fracture surface would lead to the fracture surface slippage, which would greatly enhance the flow conductivity of the natural fracture. Very small or very large inclination of natural fracture is not conducive to fracture activation, best activating result can be reached at 30°–60° inclination. Rise in elastic modulus inhibits fracture surface slippage and Poissonʹs ratio has little effect on fracture surface slippage.