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.