Conforming, non-conforming and non-matching discretization couplings in discrete fracture network simulations

TL;DR

This paper reviews various numerical discretization methods for simulating fluid flow in complex discrete fracture networks, providing guidance on balancing accuracy and computational cost.

Contribution

It offers a comprehensive comparison of conforming and non-conforming discretization approaches for fracture network simulations, including practical guidelines.

Findings

Different methods exhibit trade-offs between accuracy and computational efficiency.

Non-matching discretizations can effectively handle complex fracture intersections.

Guidelines help choose appropriate methods based on network complexity and resource constraints.

Abstract

Simulations of fluid flow in naturally fractured rocks have implications for several subsurface applications, including energy storage and extraction, and waste storage. We are interested in flow in discrete fracture networks, which explicitly represent flow in fracture surfaces, but ignore the impact of the surrounding host rock. Fracture networks, generated from observations or stochastic simulations, will contain intersections of arbitrary length, and intersection lines can further cross, forming a highly complex geometry. As the flow exchange between fractures, thus in the network, takes place in these intersections, an adequate representation of the geometry is critical for simulation accuracy. In practice, the intersection dynamics must be handled by a combination of the simulation grid, which may or may not resolve the intersection lines, and the numerical methods applied on the grid. In this work, we review different classes of numerical approaches proposed in recent years, covering both methods that conform to the grid, and non-matching cases. Specific methods considered herein include finite element, mixed and virtual finite elements and control volume methods. We expose our methods to an extensive set of test cases, ranging from artificial geometries designed to test difficult configurations, to a network extruded from a real fracture outcrop. The main outcome is guidances for choice of simulation models and numerical discretization with a trade off on the computational cost and solution accuracy.