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Injecting anthropogenic CO2 into the subsurface is suggested for climate change mitigation. However, leakage of CO2 from its target storage formation is a concern. In the event of leakage, permeability in leakage pathways such as faults may get altered due to mineral reactions induced by CO2‐enriched water, thus influencing the migration and fate of the CO2. An example of such fault permeability alteration is found in the Little Grand Wash Fault zone (LGWF), where the fault outcrops show fractures filled with calcium carbonate. To test the nature, extent, and time‐frame of such fault ‘self‐sealing’, we developed reactive flow simulations based on hydrogeological conditions of the LGWF. We measured LGWF water chemistry and conducted an x‐ray powder diffraction (XRD) analysis of the fault‐rock to constrain the model geochemistry. We hypothesized that the choice of parameters for relative permeability, capillary pressure, and reaction kinetics will have a huge impact on the fault sealing predictions. Simulation results showed that precipitation of calcite in the top portion of the fault led to a decrease in porosity of the damage zone from a value of 40% to ∼2%. Over a simulation time of 1000 years, porosity in the damage zone showed self‐enhancing behavior in the bottom portion and self‐sealing behavior in the top portion of the fault. We found that the results were most sensitive to the relative permeability parameters and the fault architecture. A major conclusion from this analysis is that, under similar conditions, some faults are likely to seal over time. © 2017 Society of Chemical Industry and John Wiley & Sons, Ltd.

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Injection of anthropogenic CO 2 into a subsurface reservoir will significantly impact the geochemistry, porosity, and permeability of the reservoir. If a fault or fracture penetrates the reservoir, CO 2‐enriched brine may migrate into that fault, eventually sealing it via precipitation or opening it up via dissolution. The goal of this study was to identify and quantify such conditions of fault self‐sealing or self‐enhancing. We found the Damköhler number ( Da) provides a meaningful framework for characterizing the propensity of (fault) systems to seal or open up. We tailored a spatiotemporally varying Da framework and applied it to simplified fault models with eight conditions derived from four geologic compositions and two reservoir conditions. The four geologic compositions were chosen such that three of them were representative of distinct geologic end‐members (sandstone, mudstone, and dolomitic limestone) and one was a mixed composition based on an average of three end‐member compositions. The two sets of P‐ T conditions chosen included one for CO 2 in a gaseous phase (“shallow conditions”) and the other for supercritical phase CO 2 (“deep conditions”). Simulation results suggest that fault sealing via carbonate precipitation was a possibility for shallow conditions within limestone and mixed composition settings. The concentration of cations in the water was found to be an important control on the carbonate precipitation. A key conclusion suggested by the results of this study is that carbonate precipitation in the near‐surface (top 50–100 m) depths of a fault is the most likely mechanism of “self‐sealing” for most geological settings.

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