Impact of fluid-rock interaction on strength and hydraulic transmissivity rvolution in shear fractures under hydrothermal conditions

Abstract

Reactivated shear fractures contribute to the creation of pervasive fracture networks in geothermal systems. The creation, reactivation, and sustainability of fracture networks depend on complex coupling among thermal, hydraulic, mechanical, and chemical (THMC) processes. However, most laboratory experiments focus either solely on how fluid transport properties evolve in stationary fractures at elevated temperatures or on how strength evolves in shear faults at room temperature. Studies that examine the combined evolution of these properties, especially under hydrothermal conditions, are limited. Laboratory restrengthening tests, referred to as slide-hold-slide (SHS), measure the difference between the steady sliding strength and the peak shear strength immediately following a hold period at constant stress or displacement. We present the results of five SHS experiments on Westerly granite bare surfaces. These experiments are combined with in-plane flow tests and conducted at an effective confining pressure of 20 MPa and temperatures of 23 and 200˚C. Consistent with previous studies, we observe a greater reduction in fluid transmissivity at 200˚C than at 23˚C and, generally, strength increasing with log(hold duration). However, there are deviations from the expected frictional healing behavior that likely relate to mineralogy and pore fluid chemical equilibrium. For example, under no-flow conditions where fluid is allowed to equilibrate with the rock, restrengthening rate of coefficient of friction is initially 0.02 per decade change in hold time (decade -1 ) for holds ≤ 5x10 4 s. Contrary to expected behavior, time-dependent reduction in frictional healing occurs at a rate of -0.03 decade -1 for holds longer than 5x10 4 s. For flow-through tests in which undersaturated fluid is continuously introduced to the fault surface, this reduction begins earlier, starting at 5x10 3 s, but at the same rate of -0.03 decade -1 . The earlier onset of reduction in frictional healing under the continuous flow conditions suggests that the process underlying the behavior is related to chemical equilibrium. The mechanisms behind the complex healing behavior are not fully understood but it is apparent that fault zone restrengthening under hydrothermal conditions is the result of multiple interacting processes.