Parallel-Simulator Framework for Multipermeability Modeling With Discrete Fractures for Unconventional and Tight Gas Reservoirs

Abstract

Summary Economic gas rate from ultralow-permeability shale reservoirs requires the creation of a complex fracture network in a large volume known as the stimulated reservoir volume (SRV). The fracture network connects a large surface area of the reservoir to the well. It is created by injecting low-viscosity fracturing fluid (slickwater) at very high rates in multiple stages along the horizontal wellbore. Numerical simulation is used to evaluate the stimulation designs and completion strategy. Microseismic (MS) -survey fracture mapping can provide a measurement of the overall SRV and an estimate of the fracture patterns. Special core analyses provide estimates of shale-matrix permeability. The extent of the fracture network indicates that there is insufficient proppant volume, and many stimulated fractures may be only partially propped or may be unpropped. Thus, fracture conductivity will vary spatially caused by uneven proppant distribution and temporally caused by stress sensitivity upon pressure decline during production. Because of the vast contrast in conductivity between stimulated/hydraulic fractures (darcy-ft) and shale matrix (nd-ft), the transient response in matrix/fracture flow cannot be captured accurately if the stimulated fractures are approximated with large dual-continuum (DC) gridblocks. The gridding requirement to achieve an accurate solution in fractured shale reservoirs is investigated and discussed. In this work, the stimulated and hydraulic fractures are discretized explicitly to form a discrete fracture network (DFN). This paper discusses the mathematical framework and parallel numerical methods for simulating unconventional reservoirs. The simulation methods incorporate known mechanisms and processes for shale, which include gas sorption in organic matter; combined Knudsen diffusion and viscous flow in nanopores; stress-sensitive fracture permeability; and velocity-dependent flow in the high-conductivity hydraulic fractures. The simulation system is based on a general finite-volume method that includes a multiconnected multicontinuum (MC) representation of the pore system with either a compositional or a black-oil fluid description. The MC model is used to represent the storage and intercommunication among the various porosities in shale (organic matter, inorganic matter, fine unstimulated natural fractures). Unconventional simulation involves many more nonlinearities, and the extreme contrast in permeabilities will make the problems harder to solve. We discuss numerical implementation of the methods for modeling the mechanisms and processes in fractured shale. In addition, we discuss the MC formulation, the discretization method, the unstructured parallel domain-decomposition method, and the solution method for the simulation system. Finally, we explain our efforts in numerical validation of the system with fine-grid single-porosity simulation. We show numerical examples to demonstrate the applications of the simulator and to study the transient flow behavior in shale reservoirs. The effects of the various mechanisms for gas production are also evaluated.