Abstract: Physical experiments are limited by experimental conditions and the number of experimental samples available. Thus, it is difficult to conduct large-scale studies on fracture propagation patterns. Hence, a numerical simulation study on fracture initiation and propagation patterns during hydraulic fracturing of shales is conducted based on certain mechanical tests of rocks, rupture tests on hydraulic fracturing of shales, and physical model tests on fracture propagation during hydraulic fracturing of shales. Based on fluid-solid coupling through Biot's consolidation theory and Darcy's seepage law, the maximum tensile strength criterion, and the Mohr-Coulomb criterion as a damage threshold for damage determination of units, a new material distribution algorithm is introduced to construct a finite element calculation model of fracture propagation during hydraulic fracturing. Parameter calibration tests were performed on rock samples, and the influences of key physical parameters on fracture propagation in shales were investigated through the finite element calculation method. The key physical parameters are ground stresses, brittleness indices of shales, the viscosity of fracturing fluids, and bedding characteristics. The general view is that when brittleness indices are small, hydraulic fractures propagate in the shale matrix mostly along the direction of the maximum principal stress. The fractures hardly change direction to form a complex network of fractures. For highly cemented beds, hydraulic actions cannot deviate at a large angle continuously, even in partially open natural beds, and thus form only relatively uniform fractures. Fracture networks become more complex when ground stress ratios and the viscosity of fracturing fluids are lower, and bedding densities are higher.

Key words: hydraulic fracturing, fracture propagation, shale, horizontal well, FEM