Abstract
We study the convection and mixing of CO2 in a brine aquifer, where the spread of dissolved CO2 is enhanced because of geochemical reactions with the host formations (calcite and dolomite), in addition to the extensively studied, buoyancy-driven mixing. The nonlinear convection is investigated under the assumptions of instantaneous chemical equilibrium, and that the dissipation of carbonate rocks solely depends on flow and transport and chemical speciation depends only on the equilibrium thermodynamics of the chemical system. The extent of convection is quantified in term of the CO2 saturation volume of the storage formation. Our results suggest that the density increase of resident species causes significant enhancement in CO2 dissolution, although no significant porosity and permeability alterations are observed. Early saturation of the reservoir can have negative impact on CO2 sequestration.
Highlights
It has been widely recognized that a significant reduction of CO2 emissions is necessary to maintain atmospheric greenhouse gas concentrations at around 450 ppm CO2 equivalent, limiting the effect of anthropogenic climate change[1]
Ghesmat et al.[21] used linear stability theory and direct numerical simulation to show that geochemical reactions stabilize the unstable diffusion boundary layer because of the consumption of dissolved CO2
For large Damkohler number (Da) the rapid reaction rate limits the plume depth and the boundary layer restricts the rate of solute transfer to the bulk volume, whereas for small Da the average solute transfer rate is limited by the domain depth and the convection is correspondingly weaker
Summary
We study the convection and mixing of CO2 in a brine aquifer, where the spread of dissolved CO2 is enhanced because of geochemical reactions with the host formations (calcite and dolomite), in addition to the extensively studied, buoyancy-driven mixing. Ghesmat et al.[21] used linear stability theory and direct numerical simulation to show that geochemical reactions stabilize the unstable diffusion boundary layer because of the consumption of dissolved CO2 Their results implied that more CO2 can be trapped through mineral interactions. They used high-resolution simulations to examine the interplay between the density-driven hydrodynamic instability and the rock dissolution reactions and analyzed the impact on the macroscopic mass exchange rate Their conclusion was the geochemical reactions terminate significantly earlier than the time when convective mixing stops. Dai and co-authors[27] developed an integrated Monte Carlo method for simulating CO2 and brine leakage from carbon sequestration and subsequent geochemical interactions in shallow aquifers Their results showed shallow groundwater resources may degrade locally by reduced pH and increased total dissolved solids. Density increase and effects from porosity and permeability change are almost negligible even when the reservoir reaches near 100% dissolved CO2 saturation
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