Uniform distribution of fracture apertures is widely used in reservoir simulation of production from hydraulically fractured unconventional reservoirs. Laboratory experiments, however, reveal that changes in effective stress create uneven distribution of fracture apertures, and this phenomenon has been neglected in most numerical models. This work investigates the impact of pressure dependent fracture apertures during the production process; fracture aperture is explicitly represented and the effect of closure stress is incorporated in a numerical simulation model.
A Discrete Fracture Model (DFM) is implemented using an unstructured Voronoi mesh that captures complex fracture geometries. The effect of fracture aperture with respect to fracture grid size is examined via a case study that includes a vertical well intersecting a single stage hydraulic fracture. The effect of dynamic behavior of fracture aperture due to changes in stress, resulting from rock stiffness and uneven distribution of proppants within primary and induced fractures, is investigated. Fractures are divided into three types, each with its own dynamic response to changes in stress. The impact of matrix permeability on aperture closure is also studied in a reservoir model with multi-stage hydraulic fractures intersecting with induced fractures.
The developed numerical simulator is validated against commercial software. Sensitivity analysis reveals that representation of fractures with exact apertures in numerical simulation produces more accurate results compared with an effective permeability approach, and the difference becomes more pronounced as the matrix permeability decreases. Therefore, it is critical to modify, in the connection list, the length of the interface between each two grids embedding the fracture in accordance with the actual fracture aperture. The dynamic behavior of fracture apertures with respect to closure stress is processed by the numerical simulator as input information. Simulation results indicate that reduction of fracture aperture translates into a decrease of its conductivity, which results in lower production rates. It is observed that fracture closure caused by uneven proppant distribution also plays a significant role in gas production. In addition, formations with lower matrix permeability experience a smaller reduction of gas production caused by stress-dependent fracture apertures.
This work presents a new approach to representing complex fracture geometries with stress-dependent apertures and simulating the process of aperture closure due to changes in stress dynamically. The results provide a better understanding of stress dependence of fracture apertures and their spatial variations on well performance for hydraulically fractured wells.