A New Three-Dimensional Numerical Model Based on the Equivalent Continuum Method to Simulate Hydraulic Fracture Propagation in an Underground Coal Mine

Hydraulic fracturing has been proven to be the most efficient way to improve the permeability of coal seams. In this work, the hydraulic fracture propagation in an underground coal mine was numerically investigated. A new numerical approach was developed based on the equivalent continuum methodology to model the hydro-mechanical behavior of multiple fractures in three dimensions. It was solved using a hybrid combination of the embedded element method (EEM) and the finite volume method (FVM) using an iterative coupling schema. The FVM was employed to calculate the pressure field, while the EEM was used to track the displacement discontinuity caused by fractures. A fracture constitutive model was implemented to describe the aperture variation, shear slippage, and shear dilation for both contact and open fractures, as well as for contact and open criteria. To verify the developed model, two benchmark examples were presented. Then, the developed model was used to numerically investigate hydraulic fracture propagation in Datong underground coal mine in Songzao in Chongqing. According to the numerical study, it was found that (1) a fracture network created by a hydraulic fracturing operation in a coal mine is more complex than the ideal cross-cutting-shape, H-shape, T-shape, and Z-shape patterns; (2) the orientation of the minimum principal stress controlled the main propagation direction; (3) the complexity of the fracture pattern is controlled by the geological structure, the in situ stress and the injection rate.