Impacts of Mineral Reaction Kinetics and Regional Groundwater Flow on Long-Term CO₂ Fate at Sleipner

Abstract

We conducted coupled reactive mass transport modeling of CO2 storage in a sandy aquifer resembling the uppermost layer in the Utsira Sand, Sleipner, North Sea, in order to investigate the general effects of rate laws and regional groundwater flow on long-term CO2 fate in saline aquifers. The temporal and spatial evolution of CO2 plume and the fate of injected CO2 were simulated with a series of scenarios with different rate law formulations for dissolution and precipitation reactions and different flow regimes. The results indicated the following: (1) Changing the dissolution rate laws of the main soluble silicate minerals can influence the silicate reactions and mineral trapping by impacting the sensitivity of the relevant coupled reaction’s rate to the acidification of brine. The steeper the slope of rate−ΔG r (Gibbs free energy of reaction) relationships, the more sensitive the coupled reaction rate and the mineral trapping are to the acidification of brine. The predicted fraction of CO2 mineral trapping when using the linear rate law for feldspar dissolution is twice as much as when using the nonlinear rate law. (2) Mineral trapping is more significant when regional groundwater flow is taken into consideration. Under the influence of regional groundwater flow, the replenishment of fresh brine from upstream continuously dissolves CO2 at the tail of CO2 plume, generating a larger acidified area where mineral trapping takes place. In a Sleipner-like aquifer, the upstream replenishment of groundwater results in ∼22% mineral trapping at year 10 000, compared to ∼4% when the effects of regional groundwater are ignored. (3) Using linear rate law for silicate dissolution reactions can exaggerate the effect of groundwater flow on the reaction rates and mineral trapping and can overestimate the theoretical mineral trapping capacity, compared to using the nonlinear rate law.

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