Quantifying the Physical and Chemical Controls on Permeability Evolution in Sheared Fractures

.Elsworth

0510182

Intellectual Merit: The competition between agents that either destroy or generate porosity controls the evolution of the transport properties of fractured rocks. Changes in permeability resulting from chemo-mechanical effects have been shown to occur under modest stresses 2 MPa) and temperatures (T80C, with H2O as the permeant), to be rapid (c. days), of significant magnitude (permeability reductions of 10-2), and moreover, to surprisingly result in permeability reduction even when dissolution net removes mineral mass.

Despite these observations, a consistent view of the processes and indexing parameters that control the switching between porosity generation and destruction in fractures is still sought. Important controls on rates of precipitation and dissolution are exerted by local shear and normal stresses, the chemical potential field, and the evolving topology of the fracture. In turn, these effects mediate the evolution of the transport (permeability) and mechanical properties (stiffness and shear strength) of fractures in rock. This study will clarify how these transformations progress with paths of deviatoric stress, temperature, fluid flux, and chemical potential, and for different fluid saturations.

These effects will be examined via flow-through tests on fractures continuously sheared within a double direct shear loading apparatus to return continuous measurements of evolving permeabilities, stiffnesses, and shear strengths. Tests will be conducted under controlled temperatures (20-300C), flow rates (0-2 cc/min), ambient stresses (0-50 MPa), and under controlled displacement rates (10-106 nm/s) slow enough to approach rates of mineral redistribution within the fracture. Recorded histories of flow impedance, mineral mass efflux, and normal displacement rate, will provide three independent measurements of evolving fracture aperture or porosity, in vivo. These observations, anchored with pre- and post-test fracture surface profilometry ~O(5 m), will provide uniquely constrained micro-mechanical data to support the development of process-based models. Particulate mechanics models will be developed to represent the essential features of two rough surfaces in contact, and to accommodate the birth and destruction of asperities bridging fractures via mechanical and chemical processes. These models will necessarily incorporate the serial processes of stress-mediated dissolution, diffusive transport, and free-face dissolution and precipitation, which together define the evolution of the mechanical and transport characteristics of the fracture. Transport modeling will be via linked Eulerian-Lagrangian methods that accommodate advection dominated flows, that interface with the evolving topology of the particulate mechanics model, and both constrain experimental observations and enable upscaling of the observations to field scale. Results will define critical processes and constrain the magnitudes of strain rates and fluid and mass fluxes where the generation of porosity out-competes its destruction, for a broad range of ambient stresses, temperatures, and paths of chemical potential.

Broader Impacts: In addition to the broad impacts these transport processes have in the safe entombment of radioactive wastes, the recovery of hydrocarbons, geothermal fluids, and potable water, and in the understanding of fluid cycling within the crust, a number of broader impacts are apparent. These include the cross-disciplinary exchange between the engineering and geophysics communities, the broad training of undergraduate and graduate students, and timely presentation and publication of results in the engineering and scientific literature.