Understanding why injected water flowback rates are so low is of primary interest from operating and environmental standpoints. In this work, matrix and fracture flow mechanisms are explored using a new fully-coupled fluid flow and geomechanics simulator. Specifically, we look at the injected water during the hydraulic fracturing, how it invades the formation, how it interacts with in-situ fluids, and how it flows (or doesn't flow) back.

In this study, we develop a fully coupled geomechanics and fluid flow simulator based on a new discrete fracture network (DFN) methodology. This novel DFN implementation allows us to grow hydraulic fractures independent of matrix meshing, i.e. fracture orientation and length are fully honored regardless of the matrix model grid. This method is used to fully couple fracture geomechanics with fluid flow from the time of fracture initiation to eventual production. Using this new DFN implementation, we overcome some limitations of current flowback modeling that include (1) problems of free fracture growth in any direction (2) geomechanics and fluid flow not fully coupled from the time of fracture initiation to production.

As matrix permeability increases from nanodarcy to micro darcy scale, leak-off and invasion of hydraulic water increase substantially. The application of geomechanics in opening and closing natural fractures with pressure has a significant effect on water distribution inside reservoirs and the production of fluids. The natural fractures are opened near the injection well due to high pressure but they remain closed near the production well due to low-pressure drawdown. Therefore, geomechanical effects on opening fractures are not effectively helpful in creating high conductive channels through fracture networks. Open fracture networks store a significant amount of injected water. Capillary pressures and critical saturations play a crucial role in water flow back in ultra-tight reservoirs.

This model can be applied to quantify the amount of injected water that is produced back in relation to reservoir water for different scenarios. The results may have significant implications in designing stimulation jobs and production operations. The novelty of this work includes the independence of fracture growth from matrix gridding. In other words, this independence grants fractures to grow freely in any direction. This work serves as a stepping stone into understanding the mechanisms for water flowback using a fully coupled geomechanics and fluid flow framework.

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