NUMERICAL SIMULATION OF HYDRAULIC FRACTURING IN SHALE GAS RESERVOIRS UNDER FAULT INFLUENCE

Fault has significant impact on the hydraulic fracturing operations in shale gas reservoir, and even induces deep seismic events and near-surface environment issues. This article adopts the multi-physics coupling method and considered the coupling progress of seepage and stress. The appearance、extend and evolution of hydraulic damage area in the fault and closed caprock during hydraulic fracturing in shale reservoirs are simulated and analyzed. The mechanism of fluid migration and diffusion along high permeability channels is analyzed at last. The main conclusions can be drawn as follows: (1)Fault changes the shape of reservoir hydraulic damage area and expands the space of the hydraulic fracturing damage area. High water-injection pressure extends reservoir hydraulic damage area to the closed cap rock and bottom plate. The hydraulic damage area is obviously affected by the fault and developed along the fault rapidly. High water-injection pressure leads to a sharp increase in the height of fault hydraulic damage area, resulting in changes in reservoir sealing integrity. (2)Under the conditions of internal high-risk geological structures and high water-injection pressures in shale reservoirs, hydraulic fracturing operations produce micro-seismic events induced by rock rupture and local reactivation of fractures, but it is difficult to cause destructive seismic events. Most of seismic events are possibly induced by regional hydraulic coupling damage during reactivation of fault or large fractures. (3)The penetration of the hydraulic damage zone into the fault induces fluid migration along the fault zone. Considering the higher permeability in fault damage zone, the well-connected pathway of the hydraulic damage zone and the upper high-permeability rock formation can speed up the escape of fluid and increase the risk of fracturing fluid contaminating the upper formation. This leads to a decrease in fracturing efficiency and reduces the economic value of shale gas development.