Application of Multi-level and High-Resolution Discrete Fracture Modeling Based on Unstructured Grid in Fractured Reservoir Simulation Study

Abstract

AbstractAn integrated approach of fractured reservoir modeling is presented that uses multi-level and high-resolution discrete fracture model on account of unstructured grid. Results are compared to the conventional dual-porosity/dual-permeability modeling approach that is based on corner point grid.The new approach considers the advantages of discrete fracture model (DFM) and dual-porosity/dual-permeability model respectively. The large-scale fractures, which are mapped out from post-stack seismic data with ant-tracking and curvature analysis, are treated explicitly and individually. The relatively small-scale fractures, which are interpreted using image logs and seismic anisotropy analysis, are up scaled into dual porosity grids by global upscaling procedures under multipoint flux approximation scheme. And then the reservoir is gridded into unstructured tetrahedral or prism meshes to fully resolve these large fractures. The DFM is introduced to carry a reservoir simulation study based on such grid system.We apply the approach for simulation study and history match of a reef limestone oil reservoir which is highly fractured in Eastern South China Sea. Currently, there are 58 production wells in this oil reservoir, among them 20 horizontal wells are in operation. We partition the reservoir block into about 0.4 million unstructured cells, in which 220 large-scale and thousands of small-scale fractures are fully resolved. The simulation study is carried out and history match is successfully achieved. The time consuming and calculation precision for the DFM are compared to conventional dual-porosity/dual-permeability model. The most significant new finding is that the time consuming of DFM based on unstructured grid system is only 11 h on single machine single CPU, which is half the INTERSECT simulator operating on 16 node cluster. Furthermore, 74.5% wells’ main production-index before history match is agree with observation value in DFM, which is far higher than that of dual-porosity/dual-permeability model.This study presents a systematic new way of modeling and simulating fractured reservoirs that effectively captures multi-scale and multi-physics flow behavior caused by complex fracture system. A field-scale case study demonstrates that the proposed workflow certainly improves predictability in development of fractured reservoirs.KeywordsUnstructured gridDiscrete fractureReef limestoneFractured reservoirNumerical simulation