Using Unstructured Grids for Modeling Complex Discrete Fracture Network in Unconventional Reservoir Simulation

Abstract

Abstract Unstructured gridding has become an important subject matter in reservoir simulation. Its flexibility of grid generation and enhanced simulation accuracy have gained popularity in reservoir simulation, especially when modeling reservoir with complex wells and faults. In this work, unstructured gridding is being extended to model the complex discrete fracture network in unconventional reservoir simulation. The typical way of gridding in reservoir simulation is using structured grid due to the easiness of cell indexing but the drawback of zig-zag approximation of reservoir geometry is also apparent on the cells near well, fault or fracture. However, such challenge is addressed when using unstructured grid, where the grid cells can be generated conforming to the geometry without large extent of approximation. In this work, 2.5D unstructured PEBI gridding is being used to model the complex discrete fracture network where the fracture geometry can be straight line or irregular. Grid density is controlled by a multi-level scheme based on region of interest in the reservoir and geometry locations of well and fracture. Near-well and near-fracture flow modeling are being enhanced with the use of high resolution grids in the area close to wellbore and fracture pathway. Particularly in the near-fracture area, the unstructured gridding method generates Logarithmically Spaced Locally Refined (LSLR) grid to capture the transient in the ultralow-permeability shale, to satisfy the requirement to model the large fracture pressure gradient which is sensitive to stress changes. This adaption of unstructured gridding method in modeling fracture in unconventional reservoir brings the gridding flexibility to model the complex fracture geometry. By using the fracture oriented grid density control scheme, pressure gradient change in near-fracture area can be modeled with enhanced accuracy. Benefits of using unstructured grid are demonstrated on simulation example by using unstructured grid and being compared with the structured grid. This presented work introduces an enhanced way of complex fracture modeling in unconventional reservoir simulation by using unstructured gridding framework. It improves the grid generation efficiency and simulation accuracy, which is valuable in unconventional reservoir simulation and development.