Abstract:
The evolution of fracture permeability during shearing is crucial in defining the impact of hydraulic stimulation in geothermal and hydrocarbon reservoirs and in describing earthquake mechanisms in induced seismicity. In exploring this phenomenon we link permeability evolution to strength evolution during fracture shearing. In particular, permeability is expected to be incremented by shear-induced dilation for velocity-weakening (i.e., seismic slip) rock fractures and decremented by shear-induced compaction or neutral deformation for velocity-strengthening (i.e., aseismic slip) failure. To confirm our assumptions, a series of experiments are conducted in a triaxial pressure vessel, where confining pressure, pore pressure, and shearing velocity are applied independently, and the evolution of fracture permeability is concurrently monitored. We explore rock rheology through these experiments for both velocity-weakening (e.g., Westerly granite) and velocity-strengthening (e.g., Green River shale) states. The results of comparison study are different from what we expected, but are useful to link between permeability and strength evolution during fracture shearing. This concept will be, furthermore, probed by linking permeability evolution to concepts of dilation and wear recovered from rate-state characterizations of frictional behavior (see Fang et al., 2016 for detail).
Introduction
Fluid injection into geothermal and hydrocarbon reservoirs (i.e., hydraulic stimulation) is recognized as one of the most general processes to improve or maintain the ability of reservoirs. However, at the same moment, fluid injections into underground can cause earthquakes (Ellsworth, 2013; Majer et al., 2007), and the construction of modeling frameworks for such induced-seismicity are desirable to determine the best scenario of fluid injection (Fang et al., 2015b; Norbeck et al., 2015).
To adequately model such induced-seismicity during fluid injection or define the impact of hydraulic stimulation, mechanical property (i.e., strength/friction) and hydraulic property (i.e., permeability) of rock fracture during shearing are necessary to be clarified. Though there are significant number of previous studies for the mechanical property (Fang et al., 2015a; Kohli and Zoback, 2013; Marone, 1998), previous studies which note on the hydraulic property are limited. Although the hydraulic properties of fracture has been explored through laboratory experiments (Esaki et al., 1999), numerical models (Ishibashi et al., 2016) or field experiments (Guglielmi et al., 2015a, 2015b), it is difficult to explain these results uniformly. Specifically, in the former two studies, the effects of shearing velocity on hydraulic properties are ignored. Furthermore, no discussion on the link between strength and permeability during shearing are presented.
