Evolution of Strength and Permeability in Stressed Fractures with Fluid-Rock Interactions

We determine the evolution of frictional strength, strain weakening behavior and permeability in fractures subject to dissolution and precipitation. We establish these relations through slide-hold-slide experiments, with hold times from 10 to 3000 s, on split limestone core, under hydraulically open and closed conditions. Fracture friction and permeability are measured continuously throughout the experiments. The limestone displays velocity-strengthening behavior (stable slip) under incremented velocity steps of 1-6 mu m/s. Frictional healing is observed to be time- and stress-dependent, showing higher gains in strength at both longer hold times and under lower effective stresses. Activation of healing is greater in wet samples than in dry samples. Flow-through experiments for flow rates in the range of 1-10 ml/min are conducted to further investigate the role of flow and mineral redistribution in contributing to healing. These experiments show strength gains are lower at higher flow rates where advective mineral dissolution and redistribution is enhanced and cementation concomitantly limited. Concurrently measured permeability decreases throughout the slide-hold-slide sequences indicating that mean fracture aperture reduces during sliding. We combine models representing pressure solution and stress corrosion as models for the growth in fracture contact area and represent the observed time-dependent behavior of strength gain and permeability evolution. The simulated results represent the observed strength gain at long hold times (similar to 1000 s), but underestimate strengthening at short hold times. We conclude that the evolution of strength and permeability are significantly controlled by mechanisms of fluid-rock interactions and that the strengths and nature of feedbacks on these linkages are critical in understanding the mechanical and hydraulic behavior of faults.