A nonlinear flow model and criterion for 3D natural fracture intersections: Effects of surface roughness and mechanical aperture

Abstract

Characterization of hydraulic behavior of intersected fractures is of great importance for evaluating flow properties within fractured rock masses. Nevertheless, existing flow models that correlate fracture geometries with nonlinear flow behaviors remain limited in predictive accuracy. To develop a nonlinear flow model and criterion based on fracture geometries that accurately represent real-world conditions, a series of three-dimensional (3D) intersected fracture models were created using surface morphology extracted from split granite blocks. Then, water flow-through simulations were performed on the generated fracture intersections subjected to varying hydraulic gradient conditions. The results demonstrate complex streamline patterns and significant nonlinear flow processes at fracture intersections with complex geometries. With increased roughness coefficient (JRC), decreased mechanical aperture (e_m) and reduced aperture ratio (e^’), the nonlinearity of flow increases, and the intrinsicpermeability (k) decreases. The critical hydraulic gradient (J_c) associated with the onset of nonlinear flow decreases with increasing JRC, e_m, and e^’. Furthermore, a notable coupling effect of surface roughness and mechanical aperture on the nonlinear flow behaviors of fracture intersections has been revealed. Based on the established relationships between hydraulic properties (J_c and inertial permeability k_i) and geometric parameters (JRC and e_m), a nonlinear flow model and a criterion were proposed to quantitatively determine the fracture permeability under nonlinear flow regimes. This study provides insights into the nonlinear flow behaviors within 3D natural fracture intersections, allowing quantitative identifications of flow regimes and evaluations on the nonlinear permeability. These findings contribute to a more accurate characterization of the hydraulic properties in fractured rock masses.