Leon Keim

Abstract

Abstract Convective dissolution of CO2 \$CO\_2\$ is a well-known mechanism in geological storage of CO2 \$CO\_2\$. It is triggered by gravitational instability which leads to the onset of free convection. The phenomenon is well studied in porous media, such as saline aquifers, and the literature provides substantial evidence that onset times and effective flux rates can be estimated based on a characterization of instabilities that uses the Darcy velocity. This work extends the study of convective dissolution to open water-filled fractures, where non-Darcy regimes govern the induced flow processes. Numerical simulations using a Navier-Stokes model with fluid density dependent on dissolved CO2 \$CO\_2\$ concentration were used to compute scenario-specific results for effective CO2 \$CO\_2\$ entry rates into an idealized fracture with varying aperture, temperature, and CO2 \$CO\_2\$ concentration at the gas-water interface. The results were analyzed in terms of dimensionless quantities. They revealed a Rayleigh invariance of the effective CO2 \$CO\_2\$ flux after the complete formation of a quasi-stationary velocity profile, that is, after a certain entry length. Hence, this invariance can be exploited to estimate the effective CO2 \$CO\_2\$ entry rates, which can then be used, in perspective, in upscaled models. We have studied convective CO2 \$CO\_2\$ dissolution for two different fracture settings; the first one relates to karstification scenarios, where CO2 \$CO\_2\$ is the dominant driving force, and were stagnant-water conditions in fractures have not yet received attention to date. The second setting is inspired from geological CO2 \$CO\_2\$ storage, where the literature provides only studies on convective CO2 \$CO\_2\$ dissolution for porous-media flow with Darcy regimes.

Abstract

Dissolved CO2 in karst water is the key driving force of karstification. Replenishment of CO2 concentrations in karst water occurs by meteoric water that percolates through the vadose zone, where CO2 produced from microbial activity is dissolved. CO2 can thus be transported with the percolating water or in the gas phase due to ventilation in karst systems. We measured seasonally fluctuating CO2 concentrations in the air of a karst cave and their influence on aqueous CO2 con- centrations in different depths of a stagnant water column. The observed data were compared to numerical simulations. The data give evidence that density-driven enhanced dissolution of gaseous CO2 at the karst water table is the driving force for a fast increase of aqueous CO2 during periods of high gaseous concentrations in the cave, whereas during periods of lower gaseous concentrations, the decline of aqueous CO2 is limited to shallow water depths in the order of 1 m. This is significant because density-driven CO2 dissolution has not been previously considered relevant for karst hy- drology in the literature. Attempts at reproducing the measured aqueous CO2 concentrations with numerical modeling revealed challenges related to computational demands, discretization, and the high sensitivity of the processes to tiny density gradients.