• Energy Research

AbstractThe fluid flow characteristics in shale fractures are of great significance for shale gas reservoir evaluation and exploitation. In this study, artificial tension fractures in shale were used to simulate the hydraulic fractures formed by fracturing, and a gas flow test under different pressure gradients was conducted. The nonlinear gas flow and stress‐dependent permeability characteristics were analyzed. The experimental results show the following: (a) CO2 flow in shale fractures exhibits strong nonlinearity. Forchheimer's law, which considers gas compressibility, satisfactorily describes the nonlinear relationship between the flow rate and the pressure gradients in shale fractures. (b) The permeability sensitivity of shale fractures under stress is very strong, and the exponential relationship better describes the pressure dependency of the permeability for the tested shale samples. The permeability of the shale fractures is similar when measured parallel or perpendicular to bedding. Furthermore, the pressure dependence of fractures in shale obeys the Walsh permeability model. (c) As the effective stress increases, the nonlinear flow behavior appears earlier. Based on the Reynolds number and the nonlinear coefficient, a friction factor model is proposed. (d) The normalized transmissivity exhibits a strong correlation with the Reynolds number. CO2 flow through shale fractures is generally dominated by transitional flow. The critical Reynolds number ranges from 1.8 to 102.88 and decreases with increasing effective stress.

AbstractThe overwhelming majority of experimental and numerical tests show the dependence of mechanical response on discontinuities (such as joints, faults, and bedding plane). In this study, fracture process is numerically investigated using finite‐difference method. A quasi‐continuum model (assuming that the real rock mass represents the sum of intact rock and fracture) has been developed to describe the fracture propagation in a fractured rock. First, this model has been validated against the experimental results and was shown to simulate satisfactorily the fracture propagation in the tensile, shear as well as the mixed (tensile and shear) modes. This model then has been used to investigate the fracture processes during the 2D long‐wall mining and 3D pillar failure. According to the 2D and 3D simulations, it was found that: (a) this model can provide a new reasonable approach to simulating fracture processes in either 2D or 3D cases; (b) roof failure is mainly caused by shear fractures rather than tensile fractures during the 2D long‐wall mining.