An Efficient Discrete-Fracture Model Applicable for General-Purpose Reservoir Simulators

TLDR

The paper presents a simplified discrete‑fracture model for general‑purpose reservoir simulators. It handles 2‑ and 3‑D systems, models fracture‑fracture, matrix‑fracture, and matrix‑matrix connections, employs an unstructured control‑volume finite‑difference method with two‑point flux approximation, is compatible with any connectivity‑list simulator, and uses a star‑delta transformation to eliminate control volumes at fracture intersections. The model delivers results in close agreement with a reference finite‑difference simulator across multiple oil/water flow cases, confirming that removing intersection control volumes enhances numerical stability and timestep size.

Abstract

Summary A simplified discrete-fracture model suitable for use with general-purpose reservoir simulators is presented. The model handles both 2- and 3D systems and includes fracture-fracture, matrix-fracture, and matrix-matrix connections. The formulation applies an unstructured control volume finite-difference technique with a two-point flux approximation. The implementation is generally compatible with any simulator that represents grid connections by a connectivity list. A specialized treatment based on a "star-delta" transformation is introduced to eliminate control volumes at fracture intersections. These control volumes would otherwise act to reduce numerical stability and timestep size. The performance of the method is demonstrated for several oil/water flow cases including a simple 2D system, a more complex 3D fracture network, a localized fractured region with strong capillary pressure effects, and a model of a strike-slip fault zone. The discrete-fracture model is shown to provide results in close agreement with those of a reference finite-difference simulator in cases in which direct comparisons are possible.