Numerical simulation of pressure evolution and migration of hydraulic fracturing fluids in the shale gas reservoirs of Sichuan Basin, China

One of the main environmental issues associated with hydraulic fracturing (HF) is the upward migration of the HF fluids from a shale reservoir to reach the shallow drinking water aquifers through preferential flow paths, such as fractures or faults. Oversimplified model structures or arbitrary boundary conditions were used in the previous studies, resulting in inaccurate results. To better describe the flow and transport in fractures/faults and matrix pores, a dual-medium model including fracture (fault) and matrix was established in this study to understand the evolution that occurs in reservoir pressure during HF and the long-term migration of HF fluids in deep geological formations, considering Sichuan Basin, China, as a case study. Both HF fractures and pre-existing natural faults were characterized as highly permeable preferential flow channels. Simulation results show that the spatial distribution of pressure in shallow aquifers is mainly controlled by the topography while it is controlled by overpressure in the deep strata. HF fluids were not found to contaminate the shallow groundwater throughout the simulation period, even in the worst scenario. A sensitivity analysis is carried out to quantify and understand the influence of a broad range of possibilities of parameters, such as HF fracture, reservoir properties and HF operation. The results suggest that the reservoir properties and the duration over which the injection of HF fluid take place are the most important factors influencing the spatiotemporal distribution of HF. Moreover, HF fluids are removed from the formation by the production of gas, meaning that these fluids are unlikely to contaminate the shallow aquifers of the Sichuan Basin.