Comparative Analysis of Dual Continuum and Discrete Fracture Simulation Approaches to Model Fluid Flow in Naturally Fractured, Low-Permeability Reservoirs

摘要

Originally proposed for conventional reservoirs, the dual-continuum idealization is being applied directly to simulation in ultralow-permeability reservoirs. Recent gridding techniques can mesh the geometries of large discrete natural fracture networks (NFNs) into an unstructured grid to simulate fluid flow. Insufficient literature exists to help identify the tradeoffs in selecting one approach over another. This paper not only analyzes two approaches with respect to their underlying assumptions and applicability but also proposes a hybrid approach. A commercial reservoir simulator supporting both dual-continuum and unstructured simulation grid capability was used to compare these approaches in two separate stages using CPU times and accuracy of results as metrics. First, small-scale reservoirs with simple two-dimensional (2D) fracture patterns were simulated to examine the impact of matrix permeability, fracture spacing, and fracture orientation. Second, drilling-spacing-unit-size reservoirs with networks of stochastically generated three-dimensional (3D) fracture surfaces were simulated to compare the effect of density and clustering of fractures. These models were also simulated using a hybrid approach, modeling one portion of the fracture network as discrete and the remaining as dual continuum.