Permeability evolution during dynamic stressing of dual permeability media

Changes in permeability due to dynamic loading from earthquakes are observed commonly but the underlying mechanisms are poorly understood. This study reports fluid flow-through experiments on fractured rock that reproduce, at laboratory scale, transient changes in permeability that decay to background over extended periods of time. We explore this response as a particular form of poroelastic loading in dual-porosity and dual-permeability media subject to zero net strain but with incremented fracture fluid pressures. Initial augmentation of pore fluid pressure dilates the fracture and compacts the surrounding, low permeability matrix, resulting in a step-like (order of seconds), transient increase in the effective permeability of the rock mass. With time, fluid pressure diffusion into the low permeability matrix then resets the effective permeability to the background magnitude, with the rate controlled by a diffusive timescale. We show that for an increase in fracture pore fluid pressure, the magnitude of the transient increase in fracture permeability scales with the ratios of the pore pressure increase to the intact modulus and the fracture spacing to the initial fracture aperture, for a broad suite of experiments. The duration of the permeability transient, measured via the time to recover background permeability, scales inversely with matrix permeability and modulus of the intact matrix and directly with the square of the spacing between fractures.