Permeability of simulated anhydrite fault gouge meter-scale samples in a dynamic flow-through system, combined with static batch experiments: implications for CO2-rock interaction
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Numerous geological reservoirs considered for CO2 storage, both in the Netherlands and elsewhere, are topped by anhydrite caprock. Faults cross-cutting these formations may form potential pathways for CO2 leakage, possibly affecting storage integrity. To confidently assess the impact of such faults, an understanding is required of how CO2-induced chemical reactions, such as the potential transformation of anhydrite to calcite, affect fault permeability. Prior to this study no clear consensus existed on the extent to which acidic CO2-saturated water promotes the transformation of anhydrite into calcite and whether buffer capacity is of importance in this transformation. In this context, the present study addresses the effects of acidic CO2-saturated fluid and alkaline NaHCO3 solution on the mineralogical and transport properties of simulated anhydrite fault rock. To this end, we performed batch reaction experiments (duration 9 to 65 days) and meter-scale reactive flow-through experiments, both conducted at 80°C and 10 MPa fluid pressure, simulating storage reservoir conditions at about 3 km depth. The batch experiments involving CO2-saturated water showed only very minor calcite precipitation, which we attribute to dissolution of dolomite (impurity in the anhydrite sample material) and subsequent re-precipitation as calcite upon experimental depressurization and CO2 degassing. This inference is supported by the fact that calcite precipitation was not detected in a similar batch reaction experiment performed using dolomite-free sample material. By contrast, extensive calcite precipitation was observed in batch experiments employing NaHCO3 solution, indicating that alkalinity is required in order for the anhydrite to calcite transformation to occur. No microstructural or permeability changes were observed in meter-scale flow-through experiments employing CO2-saturated deionized water. We therefore conclude that here is no risk of calcite formation in anhydrite bearing fault zones, due to the presence of CO2-saturated water (pH=3-4), in the absence of alkaline buffering. However, in the flow-through experiment using 0.5 M NaHCO3 solution, we observed a zoned and intensely calcified flow-path that penetrated two-thirds of a meter into the sample, in under 60h of flow-time. We suspect this flow-path to be a self-enhancing pathway, which allows reactive alkaline fluid penetration, coupled with positive feedback involving porosity and permeability increase. Our results imply that acidic CO2-bearing pore fluid will not cause calcite formation and runaway porosity/permeability increase in faulted anhydrite caprock. However, if alkaline fluids penetrate faulted or fractured anhydrite caprock, localized calcification may severely alter transport and geomechanical characteristics of the fault or fracture system. We therefore express the need for caution if alkaline fluids are injected into a reservoir topped by anhydrite, which could the case in relation to carbon capture and storage combined with alkaline waste disposal or alkaline enhanced oil recovery.