SIMULATING THE FATE OF HYDRAULIC FRACTURING FLUIDS IN SHALE FORMATIONS: COUPLING GEOMECHANICS AND TWO-PHASE FLOW IN FRACTURED MEDIA

Hydraulic fracturing (HF) is widely used for oil/gas recovery from unconventional shale formations. The fluids used for HF have raised environmental concerns due to the potential toxicity of their chemical additives and the low recovery ratio (around 20% in most cases). Production of oil/gas from shale formations involve three operational stages: (1) hydraulic fracturing, (2) well shut in, and (3) hydrocarbon production. During these stages, the dynamics of fluid flow are expected to be strongly regulated by geomechanics given the complex hydraulic and natural fracture networks. Here, we present a coupled discrete fracture model with geomechanics to simulate the interactions between fracture opening/closing and two-phase flow in fractures and the shale matrix. High-resolution real-time water and gas production datasets in Horn River shale formations are history-matched to determine the model parameters. Our simulation results illustrate the significant impact of geomechanics on the water flow dynamics in fractured shale formations. The mass transfer between the fracture network and the shale matrix is dominated by the opening and closing of the unpropped fractures. During the hydraulic fracturing stage, fractures connected to the main hydraulic fractures (either with or without proppants) are all kept open due to the large fluid pressure from fracking, which allows water to enter an extensive fracture network and subsequently to imbibe into the shale matrix. While during the shut in and production stages, pore pressure in the fractures decreases rapidly, and as a result, the majority of the unpropped fractures are closed. A large portion of water that was imbibed via these fractures during the hydraulic fracturing stage is then “locked” in the shale matrix, and hence leading to a significantly reduced volume of recovered water. Overall, our physics-based simulations demonstrate that fracture opening and closing due to geomechanics are likely major mechanisms responsible for the low water recovery ratio in field observations.