Co-Evolution of Fracture Permeability and Friction PDF Download

Author: Elif Yildirim
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Languages : en
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Book Description
In the following, we examine fracture permeabilityseismicity relationships in two separate studies. These combined studies separately elucidate (1) the role of preslip frictional healing, and (2) the combined effect of dynamic stressing and fluid-pressure-induced shear deformation on the evolution of fracture permeability and friction throughout the phases of the seismic cycle.Fracture permeability is a dynamic property under conditions of varying stress and responds to fluid overpressures applied during hydraulic stimulation. We use samples from the SIGMA-V site (Sanford Underground Research Facility (SURF), SD) to measure the co-evolution of fracture permeability and friction throughout phases of the seismic cycle. This is accomplished via slide-hold-slide and pore pressure stepping experiments completed in double direct shear. Fracture reactivation results in permeability enhancement only after sufficiently long interseismic repose periods. The magnitude of permeability increase from each reactivation, following the long hold periods, is critically dependent on the degree of fracture healing achieved in each pre-slip hold period. Shear dilation and permeability enhancement only results following a threshold repose period. Permeability enhances continuously with each pressure step with the highest permeability increase rate being with the first reactivation event. Our study establishes a direct linkage between fracture permeability and friction evolution throughout the seismic cycle and hydraulic shear, which applies across different fracture surface roughnesses.Mechanisms controlling fracture permeability enhancement during dynamic stressing remain unresolved. We probe these mechanisms through a series of fluid pressure pulse reactivations on saw-cut fractures in impermeable rock samples confined under in situ stresses. Each spiked pore fluid pressure pulse returns to the background control pore pressure while the evolution of fracture permeability and friction are continuously monitored. Peak magnitudes of the pore pressure pulses are successively incremented to both exclude and then include shear reactivation. Fracture permeability is shown to increase, both in the absence and then presence of reactivation by shear slip. Fracture permeability enhancement is permanent in the short-term despite the transient nature of the pressure pulses. The initiation of injection-induced slip significantly magnifies permeability increase over that due to changes in normal stress alone. The shear-induced permeability increase is apparent with a short delay after the first observed shear slip. Differentiation between the contribution of shear dilation and normal stress-only related processes, including unclogging and asperity damage and reseating, is apparent with a major slope change in permeability increase. Permeability increase scales with pore pressure amplitude and permeability increment scales with the amount of pre-stimulation sealing. This sealing and unsealing behavior is systematic and reversible. Enhanced permeability eventually returning to the pre-stimulated value over the long-term once the effective stress perturbations cease.