The study of fracture propagation in pre-acidized shale reservoir during the hydraulic fracturing

Due to the high stress difference, high breakdown pressure and undeveloped natural fracture, the pre-acidizing treatment is an effective method for deep shale reservoir hydraulic fracturing. However, the mechanism of complex fracture propagation in the acidized shale is unrevealed. The linear elastic mechanical theory cannot model the elastoplastic deformation of acidized shale and the changes of mechanical properties after acidizing. In this paper, the acidizing damage effect of the shale formation is studied by presenting a numerical model based on the cohesive zone finite element method which is capable for simulating complex fracture propagation. A constitutive model of acidized shale and an equivalent model for simulating the weak planes and natural fractures are embedded into the hydraulic fracturing model. The simulation results of a two-dimensional fracture intersection model considering acid damage effect show that it is more conducive for generation of complex fracture network. A three-dimensional multi-fracture propagation model considering weak planes and acid damage effect is established, and the effects of treatment parameters are analyzed. The simulation results show that the acid-damaged reservoirs are more likely to form complex fractures, which is consistent with the microseismic monitoring results. According to the modeling, the acid treatment could result in a more complex fracture geometry as well as a larger stimulated reservoir volume.