A Robust and Practical Modeling and Simulation Workflow with a Non-intrusive EDFM Method for Naturally Fractured Reservoirs

Abstract

AbstractIt is very difficult to characterize, model and simulate the naturally fractured reservoirs (NFRs) due to the high static and dynamic heterogeneity, especially in the low matrix porosity and permeability reservoirs. The common used DFN modeling method is simple in upscale, with low accuracy. Therefore, this method has been challenged increasingly by reservoir engineers. In this study, we used an innovative EDFM (Embedded Discrete Fracture Modeling) method, which can combine the results of geological modeling with reservoir simulation, preserve the complexity of the DFN, and save the run time or steps compared with the conventional fluid flow simulation workflow. With EDFM method, any complex fracture planes can be modelled explicitly without discretization of the matrix domain into unstructured elements. EDFM method is high in computational efficiency, and it is feasible to conduct quick modelling of the fracture reservoirs with high-resolution with EDFM method.With EDFM method, NFRs modeling, calibration and simulation can be conducted. The workflow is as follows: Firstly, characterize the fracture parameters (including aperture, length, connectivity, and intensity) on the basis of the results of fine seismic interpretation, outcrop survey data, core data and borehole image, and build DFN models for individual well. The fractures are divided into two groups according to their size grades and their hydraulic influence on the fluid flow in the reservoir, namely the large-scale fractures and the small-scale diffuse fractures. The large-scale fractures can represent highly conductive corridors, while the small-scale diffuse fractures are lower in overall conductivity and connectivity; Secondly, calibrate DFN models by embedding the individual well reservoir model with EDFM, run well testing forward modeling to match with well testing data(K*h), and conduct reservoir simulation to optimize fracture parameters with the true production data; Thirdly, put the DFN properties of individual well into DFN model of the whole block. Finally, conduct numerical simulation of NFR. During this process, the EDFM method is used to simulate the large-scale fractures, and DPDK (Dual Porosity-Dual Permeability) method is used to simulate the influence of small-scale fractures on fluid flow, in order that both computational efficiency and accurate flow behavior can be improved.This workflow has been used successfully in a granite fractured oil reservoir. A whole block reservoir simulation has been conducted for history matching of 15 wells with EDFM method and DPDK method to test the workflow and understand the fracture system. More than 260 thousand small-scale fractures have been upscaled into DPDK model, and more than 13 thousand large-scale fractures have been embedded into the model. The result shows that the simulation result of oil production rate from 15 wells is matched very well with the actual data. The study result indicated that the new workflow with EDFM method can provide a practical solution for complex NFRs modeling and simulation.KeywordsNaturally fractured reservoirs (NFRs)Embedded discrete fracture modeling (EDFM)Reservoir simulation and calibrationDual porosity-dual permeability (DPDK)