Fracture growth in chemically reactive geologic systems : experimental and field studies

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2023-05

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Fractures in the subsurface play important roles in controlling mechanical and mass transport properties of the brittle crust. Following conventional linear-elastic fracture mechanics principles, natural rock fractures are expected to be elliptical in shape with small aperture-to-length or aspect ratios, in contrast to observed fracture shapes that frequently deviate from an ellipse with large aspect ratios. This research aims to test the hypothesis that deviations from linear-elastic fracture geometric characteristics reflect inelastic deformation mechanisms, especially under, although not limited to, chemically reactive subsurface conditions. The study follows two approaches, laboratory constrained sintering experiments of fracture formation and field-based structural and textural investigations of natural fractures. In constrained sintering experiments, I generated fractures under chemically reactive conditions associated with the reaction of jadeite and quartz to albite. Constrained sintering experiments conducted in a muffle furnace allowed fracture growth at atmospheric pressure and temperatures of up to 1000°C, with partial melt present at temperatures of 850°C and above. Constrained sintering experiments in a hydrothermal furnace were conducted in the presence of aqueous fluid at temperatures between 455°C and 600°C and pressure of 1 kbar. In both types of experiments, solution-precipitation creep is the main deformation mechanism associated with fracture growth. I also observed multiple stages of fracture growth concurring with the degree of sintering of the materials, resulting in the deviation of fracture shapes from an ellipse and large aspect ratios. In the absence of an externally applied tensile stress, fracture growth is driven solely by porosity reduction of the porous reacting mineral aggregate and the formation of a tensile sintering stress. In the second part of the study, I quantified shape characteristics of natural fractures, including fracture aspect ratios and fracture ellipticity or shape, and inferred their deformation mechanisms based on microscale textural and compositional analyses. The fractures are in quartz arenite (Travis Peak Fm. and Boyer Ranch Fm.), quartzite (Campito Fm.), rhyolite (Mono Craters), and sintered oxidized siliceous mudstone (Sisquoc Fm.). The fractures have non-elliptical shapes and large aspect ratios due to different rates of growth in aperture and length occurring in multiple stages due to deformation mechanisms such as solution-precipitation creep. While fractures in quartz arenite, quartzite, and oxidized siliceous mudstone formed under chemically reactive aqueous conditions, rhyolite fractures are associated with viscous flow, demonstrating that large fracture apertures are not necessarily unique to chemically reactive systems. The results demonstrate that inelastic deformation mechanisms, especially those associated with chemically reactive environments, can result in fracture characteristics that differ from those of brittle-elastic fractures. With fracture geometry affecting fracture aperture and connectivity, these results are significant for predicting the flow properties of fractured rock formation in the subsurface under chemically reactive conditions.

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