NUMERICAL STUDY OF HYDRAULIC FRACTURING IN HETEROGEN-EOUS ROCKS UNDER DIFFERENT HYDRAULIC LOADING CONDITIONS

Many studies focus on the characteristics and mechanisms of the hydraulic fracturing because there are lots of factors influence the fracturing process and results. In this paper, effects of different initial hydraulic pressures on hydraulic fracturing are numerically investigated with a finite element method. It considers the heterogeneous of rocks through Weibull distribution of mechanical and strengthen parameters in a coupled seep-stress-failure model. The simulations reveal that when the initial hydraulic pressure is less than or equal to the tensile strength of the rock specimens and increases with a low growth rate, similar to the static loading method, hydraulic fractures can be initiated and propagated at a comfortable speed, which eventually forms two groups of umbrella-shaped fracture network. However, when the initial hydraulic pressure is greater than the breakdown pressure of the rock specimens, similar to the dynamic loading method, hydraulic fractures can initiated and propagated rapidly in all direction of the hole, which produces a radial fracture network. The processes and characteristics of hydraulic fractures reflect different water-rock interaction mechanisms under different loading modes. As for the reservoir fracturing improvement, the radial fracture network is the optimum result because it creates the maximum fracture surface area for migration and desorption of the shale gas.