Systematic investigation of stress-induced fracture closure and permeability evolution in Lac du Bonnet granite

Abstract

Fracture permeability is influenced by the mechanical properties of geomaterials, surface geometry, aperture distribution, and stress conditions. However, accurately characterizing fracture surfaces and aperture distributions, along with their effects on permeability under stress remains challenging. This study systematically investigates the evolution of fracture permeability and closure behavior in Lac du Bonnet granite through integrated steady-state flow experiments, high-resolution laser scanning, and 3D numerical modelling. A 3D digital twin model of the fractured sample was developed to quantify key geometric characteristics of fracture surfaces, including surface gradient, height deviation, mean curvature, Gaussian curvature, and spatial distribution of peaks and valleys. These metrics were used to evaluate their influence on fracture openness. Fracture closure measurements were incorporated as boundary conditions in the numerical model to assess the evolution of the aperture field under varying stress levels. Statistical analysis of the computed aperture fields and corresponding fluid activity was conducted to provide insights into heterogeneous fracture closure behavior. The results reveal that fracture permeability follows a quadratic exponential decline under confining stress of 5–40 MPa, leading to an overall reduction of 82.1 %, while fracture aperture decreases exponentially by 60.4 %. Permeability hysteresis was observed after stress relief, indicating significant impact of stress exposure history on fluid flow behavior. Throughout the experiments, hydraulic aperture variations closely aligned with mechanical aperture measurements, validating the applicability of the cubic law. Based on these experiment results, empirical models describing permeability-stress and mechanical-hydraulic aperture relationships were established, and a modification to the cubic law was proposed. The proposed statistical aperture distribution analysis provides a novel approach for quantifying heterogeneous fracture closure, while the modified cubic law, based on the mechanical-hydraulic aperture relationship, enhances rapid predictions of fracture permeability under varying stress levels.