Flow Simulation of Complex Fracture Systems With Unstructured Grids Using the Fast Marching Method
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2017, Unconventional Resources Technology Conference (URTeC). With the current industry practice of reduced cluster spacing and increased fracturing proppant/fluid volume, the hydraulic fracture treatment tends to generate more complex fracture systems, where an unstructured computational grid, instead of a Cartesian or corner point grid, is preferred to accurately model the fracture geometry. With unstructured grids, the reservoir performance is generally simulated with finite volume simulation, for which one major issue is the potentially heavy computational cost. A novel approach has recently been introduced to provide a rapid simulation of unconventional reservoirs, which first captures the drainage volume during the transient pressure propagation process using the Fast Marching Method (FMM) and then rapidly solves fluid flow equation in an equivalent 1D domain. However, this application is currently limited to calculating the reservoir response with Cartesian or corner-point grids. In the study, we propose an effective workflow to model and simulate the complex fracture system. The fracture propagation process is first modeled, based on the pre-existing natural fracture information and fracturing treatment data, to generate complex fractures, which can be calibrated against micro-seismic data. A 2.5D perpendicular bisector PEBI grid based on a Voronoi tessellation is then constructed with high resolution near the fractures and with larger cells far from the fractures. Next, the Eikonal equation, which governs the transient pressure propagation process, is solved on the basis of subdivided tetrahedrons using the Fast Marching Method. Solving this pressure propagation equation on the unstructured grid, with high resolution near the fractures, provides more accurate calculation of the travel time (i.e. diffusive time of flight, DToF) and the transient drainage volume. Finally, the fluid flow equation is effectively solved in the transformed 1D domain, where DToF acts as the 1D spatial coordinate. Complex fracture systems may be developed from fracture propagation models and accurately represented using unstructured grids. The FMM algorithm is studied on unstructured grids with two types of discretization, which are based on Fermat's principle and Eulerian discretization. The accuracy and convergence characteristics are investigated. The DToF-based fluid flow simulation is validated against finite volume reservoir simulation and can be integrated with industry fracturing modeling software to provide a rapid calculation of reservoir response. Through numerical examples, our proposed workflow is demonstrated to be an efficient approach to model and simulate the complex fracture system in unconventional reservoirs. Unstructured grids allow better characterization of the transient drainage volume for complex fracture systems while the DToF-based fluid flow simulation provides rapid simulation of the reservoir performance based on the drainage volume. Combining the advantages of unstructured grids and the DToF-based fluid flow simulation, we have developed an effective workflow to model and simulate the complex fracture system. This proposed workflow provides orders of magnitude reduction in computational cost, which is attractive for high-resolution models. The approach is also efficient for calibrating the reservoir and fractures parameters and optimizing the well and hydraulic fracturing design.
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Proceedings of the 5th Unconventional Resources Technology Conference
