[1] The evolution of permeability in fractured rock as a function of effective normal stress, shear displacement, and damage remains a complex issue. In this contribution, we report on experiments in which rock surfaces were subject to direct shear under controlled pore pressure and true triaxial stress conditions while permeability was monitored continuously via flow parallel to the shear direction. Shear tests were performed in a pressure vessel under drained conditions on samples of novaculite (Arkansas) and diorite (Coso geothermal field, California). The sample pairs were sheared to 18 mm of total displacement at 5 mm/s at room temperature and at effective normal stresses on the shear plane ranging from 5 to 20 MPa. Permeability evolution was measured throughout shearing via flow of distilled water from an upstream reservoir discharging downstream of the sample at atmospheric pressure. For diorite and novaculite, initial (preshear) fracture permeability is 0.5-1 Â 10 À14 m 2 and largely independent of the applied effective normal stresses. These permeabilities correspond to equivalent hydraulic apertures of 15-20 mm. Because of the progressive formation of gouge during shear, the postshear permeability of the diorite fracture drops to a final steady value of 0.5 Â 10 À17 m 2 . The behavior is similar in novaculite but the final permeability of 0.5 Â 10 À16 m 2 is obtained only at an effective normal stress of 20 MPa.

[1] 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.Citation: Faoro, I., D. Elsworth, and C. Marone (2012), Permeability evolution during dynamic stressing of dual permeability media,

To improve our understanding of the complex coupling between circulating fluids and the development of crack damage, we performed flow‐through tests on samples of Etna basalt and Westerly granite that were cyclically loaded by deviatoric stresses. The basalt was naturally microfractured, while the relatively crack‐free Westerly granite was thermally pretreated to 500°C and 800°C to generate microcrack damage. Samples were repeatedly loaded and then unloaded under deviatoric stress paths and ultimately to failure. Permeability and water volume content were measured throughout the loading history together with the differential stress. Permeability decreases at low differential stresses and increases at intermediate differential stresses up to a steady value at failure. We use water volume content as a proxy for fluid storage and show that both permeability and storage evolve with damage and evolution of crack density. We use crack models to represent the evolution of permeability as a function of loading state and are able to independently link it to the observed evolution of deformability, used as an independent measure of crack density.

Strong feedbacks link temperature (T), hydrologic flow (H), mechanical deformation (M), and chemical alteration (C) in fractured rock. These processes are interconnected as one process affects the initiation and progress of another. Dissolution and precipitation of minerals are affected by temperature and stress, and can result in significant changes in permeability and solute transport characteristics. Understanding these couplings is important for oil, gas, and geothermal reservoir engineering, for CO2 sequestration, and for waste disposal in underground repositories and reservoirs. To experimentally investigate the interactions between THMC processes in a naturally stressed fracture, we report on heated (25°C up to 150°C) flow‐through experiments on fractured core samples of Westerly granite. These experiments examine the influence of thermally and mechanically activated dissolution of minerals on the mechanical (stress/strain) and transport (permeability) responses of fractures. The evolutions of the permeability and relative hydraulic aperture of the fracture are recorded as thermal and stress conditions' change during the experiments. Furthermore, the efflux of dissolved mineral mass is measured periodically and provides a record of the net mass removal, which is correlated with observed changes in relative hydraulic fracture aperture. During the experiments, a significant variation of the effluent fluid chemistry is observed and the fracture shows large changes in permeability to the changing conditions both in stress and in temperature. We argue that at low temperature and high stresses, mechanical crushing of the asperities and the production of gouge explain the permeability decrease although most of the permeability is recoverable as the stress is released. While at high temperature, the permeability changes are governed by mechanical deformation as well as chemical processes, in particular, we infer dissolution of minerals adjacent to the fracture and precipitation of kaolinite.