The Hydro-Mechanical Properties of Fracture Intersections: Pressure-Dependant Permeability and Effective Stress Law

Abstract

Fluid flow through the brittle crust is primarily controlled by the capability of fracture networks to provide pathways for fluid transport. The dominant permeability orientation within fractured rock masses has been consistently correlated with the development of fracture intersections; an observation also made at the meso-regional scale. Despite the importance attributed to fracture intersections in promoting fluid flow, the magnitude of their enhancement of fractured rock permeability has not yet been quantified. Here, we characterize the hydro-mechanical properties of intersections in samples of Seljadalur Basalt by generating two orthogonal, tensile fractures produced by two separate loadings using a Brazilian test apparatus, and measuring their permeability as a function of hydrostatic pressure. We observe that intersecting fractures are significantly more permeable and less compliant than two independent macro-fractures. We formulate a model for fracture intersection permeability as a function of pressure by adding the contributions of two independent fractures plus a tube-like cavity with an effective elastic compressibility determined by its geometry. Permeability measurements during cyclic loading allowed determination of the effective stress coefficient (α in pe = pc − αpp) for fracture and intersection permeability. We observe a trend of lower αintersection values with respect to αfracture, which suggests that the channels controlling fluid flow have a higher aspect ratio (are more tubular) for the intersections relative to independent fractures. Our results suggest that fracture intersections play a critical role in maintaining permeability at depth, which has significant implications for the quantification and upscaling of fracture permeability toward reservoir-scale simulations.