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ABSTRACT: Fracturing of rock allows ingress of fluids that may be chemically distinct from native pore fluids. These external fluids can induce dissolution and precipitation, which may be enhanced by highly reactive, comminuted material and metastable highly deformed materials. Reaction products have important feedbacks via dissolution of asperities that affect fracture strength and friction properties. In this study, we examine dynamic fracturing of anhydrite/dolomite using triaxial direct-shear experiments with simultaneous x-ray radiography and tomography. The impermeable cores were fractured at a temperature of 50°C and a confining pressure of 3.5 MPa while being exposed to 500 kPa injection pressure of concentrated (18 wt%) BaCl2 solution. Once fractured, the barium solution invaded the primary shear fracture and the surrounding damaged region. X-ray radiographs and tomography revealed precipitation of barium sulfate and carbonate (barite/witherite). Despite the precipitation, the permeability of the fracture system changed little following the initial fracturing event. Precipitation was likely confined to relatively stagnant flow regions, with more open regions sustaining high flow rates. The experimental results demonstrate laboratory-induced fluid-rock reaction and precipitation in fracture systems and reveal the important role of grain-scale mineralogy and flow pathways on the extent of reaction and the impact of precipitation on permeability. 1. INTRODUCTION Fracturing of rock allows the ingress of fluids that may be chemically distinct from the native pore fluids. These external fluids can induce chemical reactions that are extremely challenging to anticipate or monitor, as they are a complex function of both intermixing among external and native fluids and reactions with the mineralogy of the host rock. The primary reactions of interest are dissolution of primary minerals and precipitation of secondary minerals, which can alter both fracture and void space sufficiently to change critical transport and rock mechanical properties (e.g., Viswanathan et al. 2022). The fracturing process itself can produce highly reactive, comminuted material that can accelerate these types of geochemical reactions. In addition, the complex geometry of fracture systems creates large variations in the ratio of reactive surface area to fluid volume (i.e., tensile opening versus closed shear; Menefee et al. 2020). Furthermore, fracturing under intense shear can generate highly reactive, metastable materials (e.g., heavily deformed crystals, amorphous material, glass) that lead to non-equilibrium precipitates. Such reaction products have important implications for the hydraulic and mechanical evolution of geologic systems, but their formation and growth depend on an array of widely variable reservoir conditions (e.g., geochemistry, mineralogy, temperature/pressure, flow rate, permeability) and thus are poorly constrained. |
