Dissolution-driven density instability (DI) occurs when a species A dissolves into a host fluid and introduces a buoyantly unstable stratification. Such an instability has positive effects in related applications and may be affected if species A reacts with solute B in the host fluid. In this paper, the lattice Boltzmann (LB) method is employed to simulate the dynamics of such an instability coupled with reaction A + B → C in porous media at the pore scale. Numerical simulations in homogeneous media have demonstrated that six types of dissolution-driven DI can be classified based on the Rayleigh numbers of three chemical species Ra r (ratio of buoyancy to viscous forces), and reaction can accelerate, delay or even trigger the development of DI. Then, a parametric study has indicated that, increasing Ra CB (Ra C − Ra B) can intensify density instability and reaction, promote the diffusion of species A, and also introduce either stabilizing or destabilizing effects of reaction. Besides, the increase of initial reactant concentration or/and Damköhler number Da (ratio of flow time to chemical time) can enhance the influence of chemistry. Finally, simulations are carried out in three types of heterogeneous media HE1-HE3, and six groups of fingering scenarios can also be observed in each medium. However, compared with the homogeneous case, heterogeneous media HE1 with randomly distributed solid grains can introduce deeper advancing position and rougher density fingering, and media HE2 and HE3 with vertical variations of pore spaces can affect the developing speed of fingering obviously. In terms of the storage of species A in the host fluid, medium HE2 with large pore size in the top layer is favorable. The present study is of significant importance for applications such as carbon capture and storage.

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Based on the lattice Boltzmann (LB) method, pore-scale simulations are performed to investigate the differential diffusion effects on the density-driven instability (DI) with chemical reaction A + B → C in porous media. A partially miscible stratification is considered, and thus only solutes from the top fluid can diffuse down into the host fluid in pore spaces. Tests with different values of the Rayleigh number Ra r and the diffusion coefficient D r of species r (r = A, B, C) are considered. The results demonstrate eight distinct scenarios of DI, and four of them are not observed in equal diffusivity simulations. Two differential diffusion effects, namely, the double-diffusive (DD) and the diffusive-layer convection (DLC) mechanisms, can act upon the gravity field and give rise to new fingering phenomena. The DD mechanism comes into play and results in a local minimum density layer if Ra B /Ra C is small and D B > D C ; and DLC becomes significant and brings in a local maximum density layer if Ra B /Ra C is large and D B < D C . On one hand, when fluid density increases with dissolved A, the DD-induced minimum can act as an inhibiting barrier to suppress fingering propagation, although it can be eventually penetrated by fingering tips; and the DLC-induced maximum can introduce the second DI below the first one. On the other hand, when the dissolution of A contributes to decreasing fluid density, both the DD-induced minimum and the DLC-induced maximum can help trigger the development of DI. Finally, quantitative results are provided to indicate that fingering propagates into the host fluid more deeply with larger D B /D C , and the dissolution of A decreases with the increasing difference between D B and D C .

Global climate change is happening but may be mitigated by the technology of geological carbon dioxide (CO 2 ) sequestration. To gain comprehensive insights into this approach, we perform pore-scale simulations of displacement between two miscible fluids in porous media using a new multiple-relaxation-time lattice Boltzmann model. This study marks the first attempt to investigate viscous fingering dynamics in miscible displacement, considering the coexistence of viscosity contrast and dissolution reaction. Simulation results capture different fingering patterns that depend on dissolution (Damk€ ohler number Da), diffusion (Peclet number Pe), and viscosity contrast (viscosity ratio R). From simulations of unstable viscous flows, dissolution is found to delay fingering onset, slow down fingering propagation, and inhibit or reinforce the late-stage fingering intensity. In simulations with stable viscosity contrasts, the displacement features fingering phenomena when dissolution is fast enough. In addition, we conduct a parametric study to assess the impact of Pe, R, and Da. The results suggest that increasing Pe or R destabilizes fingering, but increasing Da first suppresses and gradually intensifies fingering. Finally, for every fixed Da, we determine the phase boundary between stable and unstable regimes in a Pe-R phase plane. A unified scaling law is developed to approximate boundary lines obtained under different Da values. By comparing reactive and nonreactive cases, we classify four distinct regimes: stable, unstable, reactive stable, and reactive unstable. These pore-scale insights are helpful in understanding and predicting the displacement stability during the geological CO 2 sequestration, which is of importance to the pre-evaluation of the storage efficiency and safety.