An Improved Grid Generation Approach for Discrete Fracture Network Modelling Using Line Fracture Concept for Two-Phase Flow Simulation

Abstract

AbstractIn traditional Discrete Fracture Network (DFN) method for fractured porous media simulation, fractures are modelled in different scales and orientations using single computational grid. However, a large number of small grids near both ends of fracture are generated, which substantially reduce the computation efficiency. In this paper, a new line fracture approach is presented to avoid generation of small grids. Combined with Multi-Point Flux Approximation (MPFA) algorithm, the simulation efficiency is highly improved without losing accuracy.We present a modified mesh generation algorithm in which the volume of large fractures is set zero by neglecting the fracture width. The fracture aperture (very short line) is treated as single node during mesh generation using the paving method. Therefore, the number of small grids around the fracture is significantly decreased. In this paper, a quadrilateral unstructured grid system is generated using the proposed paving method. Then, by taking the median dual of quadrilateral mesh as the control volume, all fracture properties such as the aperture and permeability are accounted for as independent parameters in discrete fractures. The governing equations of DFN model are derived and solved using MPFA algorithm for oil-water two-phase flow.A new discrete fracture network model using line fracture approach is developed. To show the efficiency and accuracy of the new model, the proposed method is applied to a case study of two separated and two crossed fractures, in the computation domain with the scale of 50 by 50 meters. Compared with the standard DFN model including the width of fractures, the computation efficiency of the new model is highly improved (by 2.5 times), due to the large reduction of grid numbers (20%). Good agreement in water and oil saturation distributions resulted from the two models validates the newly developed method.Simulation of naturally fractured reservoirs has been a challenge for decades. Discrete fracture network modelling is a promising approach. However, without an efficient grid generation method, DFN model is not feasible and practical due to the large amount of computational time. The novelty of the present work is the improved model using line fracture approach. The new grid generation algorithm is coupled with multi-point flux approximation approach for flow simulation in fracture network system. High efficiency and accuracy of the developed method is readily applied to fractured reservoir simulation.Sponsored by NSFC (Natural Science Fund of China - "the numerical simulation of natural fractured reservoir based on the unstructured hexahedral grid system and concurrent computation") (No. 51474179)