Permeability and microcrack geometry: Dynamic loading induced evolution

Abstract

We propose a crack-geometry-based model for permeability evolution in rocks under dynamic loading. Accurate representation of permeability is important for fluid extraction and containment in geological formations, but remains challenging because low-porosity brittle rocks are highly sensitive to changes in pore geometry and connectivity, while dynamic permeability measurements are limited. Rock void space is represented as an evolving network of penny-shaped cracks with prescribed orientations. Permeability changes are described through crack microvariables including aperture, length, and crack distance. The model accounts for crack opening and closure, fracture-energy-driven propagation, plastic-strain-driven nucleation, coalescence after full connectivity is reached, and strain-rate-dependent fracture toughening. The evolving crack network directly modifies connectivity, fluid conductance, and preferential flow paths. The formulation is implemented in the GeoDyn hydrocode and evaluated under tension, shear, compression, and symmetric impact loading. Results show a strong relation between permeability and crack length, with crack propagation playing a major role in permeability enhancement. Permeability growth can nevertheless be limited by poor connectivity and crack closure. The model captures changes of several orders of magnitude depending on stress state, strain rate, crack orientation, and connectivity, consistent with observed behavior of brittle rocks under high strain-rate loading.