Hydraulic fracture propagation of rock-like material with X-type flaws

Unconventional oil and gas resources such as shale gas have gradually become a hot area for resource ex- ploitation. Low permeability of unconventional oil and gas reservoirs is one of prominent problem in oil / gas exploita- tion. Hydraulic fracturing technology is one of main technical means to improve the permeability of oil / gas reservoirs. In order to explore the influence of X-type flaws on hydraulic crack propagation in natural rock mass,the hydraulic fracturing experiments on rock-like material with pre-existing X-type flaws are carried out with the true triaxial hydrau- lic loading system. The influences of X-type flaws position on hydraulic fracture propagation are studied under different horizontal stresses. Some experimental conditions are numerically simulated using the extended finite element method (XFEM) method with the calculation software Abaqus. The testing results show that the X-type flaw tip position may influence hydraulic fracture propagation. The lower the horizontal stress difference,the more easily the X-type flaw tips may induce the hydraulic fracture propagates towards flaw tip position. With the increase of horizontal stress differ- ence,the inducing role of X-type flaw tips on hydraulic fracture propagation may decrease. The hydraulic fracture grad- ually propagates towards the maximum horizontal stress direction. When the main and secondary flaw tips are close,the strain energy density at the location of the two flaw tips is larger than that of other places. In this situation the flaw tip has induction strengthening effect on hydraulic fracture propagation,and the hydraulic fracture is more likely to propa- gate to the flaw tip. When the main and secondary flaw tips are approximately symmetrical about the horizontal maxi- mum principal stress direction,the strain energy density of the main and secondary flaw tips are almost same. In this situation the induction effect of the main and secondary flaw tips on hydraulic fracture is weakened,and the hydraulic fracture is more likely to propagate toward the maximum horizontal stress direction.