Numerical Simulation and Main Controlling Factors for Volume Cracks in Unconventional Reservoirs

The key to achieving industrial capacity lies in the fracture network formed by the unconventional reservoir hydraulic volume. In addition, the combination of mechanical and physical properties, reservoir defects and construction parameters of reservoirs has significantly affected the scale of the formed network fractures. Based on the cohesive force model, the secondary development for sub-program is used to realize the diagonal expansion of cohesive elements. Through embedding the global technology in the matrix network, a numerical simulation method for the hydraulic fracturing fracture network of unconventional reservoirs is formed and obtained. Through the comparison between the strain field from the digital speckle experiment and the numerical results, the horizontal strain error rate is 7.08%, which verifies the method to be correct. Based on this, the numerical simulation of network fracture propagation is carried out. The calculation results show that the main controlling factors for the fracture network are elastic modulus, natural fracture toughness and construction injection volume. The larger the elastic modulus of the matrix is, the smaller the fracture toughness of natural fractures are. The larger the construction injection volume, the easier it is to form a fracture network; There are best values for Geostress difference and the natural fracture approximation angle with the Geostress difference of 3 MPa, and the approximation angle of 300–450, which is conducive to the formation of network fractures; The matrix Poisson’s ratio and the viscosity of the fracturing fluid have little effect on the network fracture. Through the global embedding function of the cohesive force elements, study is conducted on the ability of large-scale heterogeneous geological structure to form fracture network and on the influence of various factors on fracture propagation couplingly so as to decide main controlling factors for the formation of fracture network in unconventional reservoirs. This study is of great significance for scientific and rational formulation of the fracturing design process and improvement of the fracturing effect of unconventional reservoirs.