In this study, we have developed a novel microfluidic high-pressure high-temperature vessel to house geomaterial (natural rock or mineral chips) micromodel specimens. Realistic fracture patterns were laser-scribed on the organic-rich shales of Draupne Formation, the primary caprock for the Smeaheia CO2 storage site (the full-scale CCS project) in Norway. The primary research objective was to examine salt precipitation in fracture networks of shale during CO2 injection under different thermodynamic conditions and for various CO2 phase states to investigate whether authigenic precipitation of salt crystals can partially or entirely block potential CO2 leakage pathways in caprocks. Moreover, the impact of CO2 injection flow rate on the extent of salt accumulations was studied. A conceptual framework was introduced that suggests salt precipitation may be not only a near-well phenomenon but also a sealing mechanism that can impede CO2 leakage. We observed that salt crystals precipitate in two distinct forms: (a) large and semi-large single cubic crystals of halite in the aqueous phase, and (b) dense porous aggregates of micrometer-sized halite crystals that form on the interface of rock and CO2 stream. Experimental observations demonstrated that injection of different CO2 phase states affects the magnitude, distribution and salt precipitation patterns. Analysis of the fracture network after complete drying of shale specimen showed that the higher the injection flow rate, the lower the salt coverage. Three drying regimes (diffusive, capillary and evaporative) governed precipitation of salt crystals through stabilized capillary and evaporative fluxes. The CO2 phase states influence the relationship between injection rate and extent of precipitated salts. A higher impact of the rate on salt coverage was found for supercritical- compared to gaseous CO2. Reducing injection flow rate caused the initial salt nuclei to form closer to the inlet and from there toward different branches of fracture pattern. It is shown that bulk and dense aggregates of micrometer-sized halite crystals, that precipitate on the interface of fracture walls and CO2 stream, has the potential to partly or entirely block the fracture aperture and consequently leakage pathways. The development of salt crystals toward the point where leakage begins, the affinity of salt bodies to become connected, and extent of accumulations suggest that the salt precipitation during injection of CO2 into the geologic formations can be considered as a fracture healing mechanism.
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