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The dissolution of carbon‐dioxide (CO2) in deep saline aquifers is an important trapping mechanism in carbon storage. This process is triggered by unstable high‐density CO2 front, which later promotes density‐driven mixing, hydrodynamic dispersion of CO2, and favors the long‐term sequestration. In many former studies, effects of hydrodynamic dispersion and multispecies geochemical reactions have been ignored. This work elaborates the impacts of these simplifications on the dynamics of convective mixing by numerical simulations. Geochemical effects were studied by the implementation of rock‐fluid and fluid‐fluid interactions for a typical carbonate aquifer. Results show that accounting for the hydrodynamic dispersion decreases the convection onset time and increases the CO2 dissolution flux, which is more significant in larger dispersivities and Rayleigh numbers. Results indicate that carbonate geochemical reactions intense the long‐term overall efficiency of the process, while decrease the total amount of sequestered carbon in the diffusion‐dominated period. Results also reinforce the importance of realistic geochemical representation and importance of spatial and temporal dependence of the reactions pathway, subsequent to the finger development for more detailed simulation of the CO2 storage process. Plain Language Summary: Geological carbon storage is one of the technologies envisaged to play a critical role in decarbonization and achieving the net‐zero carbon emission. It is estimated that 17% of the whole decarbonization by 2050 will be achieved by carbon storage. Based on a modeling study, we demonstrate the role of natural geochemistry of the subsurface is important in the dissolution of carbon in brine and its long‐term performance. Key Points: Effect of hydrodynamic dispersion is studied on the dynamics of CO2 convectionChemical reactions have nonmonotonic effects on carbon storage before and after convection onsetThe importance of realistic geochemical modeling and different reaction pathways subsequent to plume propagation is shown [ABSTRACT FROM AUTHOR]

Density-driven mixing resulting from CO2 injection into aquifers leads to the CO2 entrapment mechanism of solubility trapping. Crucially, the coupled flow-geochemistry and effects of geochemistry on density-driven mixing process for "sandstone rocks" have not been adequately addressed. Often, there are conflicting remarks in the literature as to whether geochemistry promotes or undermines dissolution-driven convection in sandstone aquifers. Against this backdrop, we simulate density-driven mixing in sandstone aquifers by developing a 2-D modified stream function formulation for multicomponent reactive convective-diffusive CO2 mixing. Two different cases corresponding to laboratory and field scales are studied to investigate the effect of rock-fluid interaction on density-driven mixing and the role of mineralization in carbon storage over the project life time. A complex sandstone mineralogical assemblage is considered, and solid-phase reactions are assumed to be kinetic to study the length- and time-scale dependency of the geochemistry effects. The study revealed nonuniform impact of rock-fluid and fluid-fluid interaction in early- and late-time stages of the process. The results show that for moderate Rayleigh (Ra) numbers, rock-fluid interactions adversely affect solubility trapping while improving the total carbon captured through mineral trapping. Simulation results in the range of 1,500 < Ra < 55,000 in the field-scale model showed more pronounced impact of geochemistry for higher Ra numbers, as geochemistry stimulates the convective instabilities and improves the total sequestered carbon. This study gives new insights into the effect of rock-fluid interactions on density-driven mixing and solubility trapping in sandstone aquifers to improve estimation of the carbon storage capacity in deep saline aquifers. [ABSTRACT FROM AUTHOR]

In this paper, the underlying problem with the color-gradient (CG) method in handling density-contrast fluids is explored. It is shown that the CG method is not fluid invariant. Based on nondimensionalizing the CG method, a phase-field interface-capturing model is proposed which tackles the difficulty of handling density-contrast fluids. The proposed formulation is developed for incompressible, immiscible two-fluid flows without phase-change phenomena, and a solver based on the lattice Boltzmann method is proposed. Coupled with an available robust hydrodynamic solver, a binary fluid flow package that handles fluid flows with high density and viscosity contrasts is presented. The macroscopic and lattice Boltzmann equivalents of the formulation, which make the physical interpretation of it easier, are presented. In contrast to existing color-gradient models where the interface-capturing equations are coupled with the hydrodynamic ones and include the surface tension forces, the proposed formulation is in the same spirit as the other phase-field models such as the Cahn-Hilliard and the Allen-Cahn equations and is solely employed to capture the interface advected due to a flow velocity. As such, similarly to other phase-field models, a so-called mobility parameter comes into play. In contrast, the mobility is not related to the density field but a constant coefficient. This leads to a formulation that avoids individual speed of sound for the different fluids. On the lattice Boltzmann solver side, two separate distribution functions are adopted to solve the formulation, and another one is employed to solve the Navier-Stokes equations, yielding a total of three equations. Two series of numerical tests are conducted to validate the accuracy and stability of the model, where we compare simulated results with available analytical and numerical solutions, and good agreement is observed. In the first set the interfacial evolution equations are assessed, while in the second set the hydrodynamic effects are taken into account.