The storage of carbon dioxide and acid gases in deep geological formations is considered a promising option for mitigation of greenhouse gas emissions. An understanding of the primary mechanisms such as convective mixing and geochemistry that affect the long-term geostorage process in deep saline aquifers is of prime importance. First, a linear stability analysis of an unstable diffusive boundary layer in porous media is presented, where the instability occurs due to a density difference between the carbon dioxide saturated brine and the resident brine. The impact of geochemical reactions on the stability of the boundary layer is examined. The equations are linearised, and the obtained system of eigenvalue problems is solved numerically. The linear stability results have revealed that geochemistry stabilises the boundary layer as reaction consumes the dissolved carbon dioxide and makes the density profile, as the source of instability, more uniform. A detailed physical discussion is also presented with an examination of vorticity and concentration eigenfunctions and streamlines' contours to reveal how the geochemical reaction may affect the hydrodynamics of the process. We also investigate the effects of the Rayleigh number and the diffusion time on the stability of a boundary layer coupled with geochemical reactions. Nonlinear direct numerical simulations are also presented, in which the evolution of density-driven instabilities for different reaction rates is discussed. The development of instability is precisely studied for various scenarios. The results indicate that the boundary layer will be more stable for systems with a higher rate of reaction. However, our quantitative analyses show that more carbon dioxide may be removed from the supercritical free phase as the measured flux at the boundary is always higher for flow systems coupled with stronger geochemical reactions.

in Wiley Online Library (wileyonlinelibrary.com).Carbon dioxide storage in deep saline aquifers is considered a possible option to bring greenhouse gas emissions under control. The understanding of the underlying mechanisms, such as convective mixing and associated mechanisms, affecting this mixing may have an impact on the long-term sequestration process in deep saline aquifers. One of the significant aspects of the flow of miscible species in porous media is velocity dependent dispersion. The effect of dispersion on dissolution of carbon dioxide (CO 2 ) into brine is investigated by full nonlinear numerical simulations. This study reveals that dispersion may dramatically change the trend of CO 2 dissolution into brine. It was found that the dissolution of CO 2 increases as dispersion strength increases. The mixing pattern also shows three different mechanisms: diffusion, convection, and a highly nonlinear interaction mechanism. However, the medium dispersivity ratios were found to slightly affect the mixing, while having an impact on the fingering pattern.

The viscous fingering of miscible flow displacements in a homogeneous porous media is examined to determine the effects of an anisotropic dispersion tensor on the development of the instability. In particular, the role of velocity-dependent transverse and longitudinal dispersions is investigated through linear stability analysis and nonlinear simulations. It is found that an isotropic velocity-dependent dispersion tensor does not affect substantially the development of the instability and effectively has the same effect as molecular diffusion. On the other hand, an anisotropic velocity-dependent dispersion tensor results in different instability characteristics and more intricate finger structures. It is shown that anisotropic dispersion has profound effects on the development of the fingers and on the mechanisms of interactions between neighboring fingers. The development of the new finger structures is explained by examining the velocity field and characterized qualitatively through a spectral analysis of the average concentration and an analysis of the variations of the sweep efficiency and relative contact area.

In this study, we have made an attempt to address how the nanoparticle flows may affect the hydrodynamic instability around a miscible front. In order to explore the role of nanoparticles in such flows, a linear stability analysis was performed to examine the impact of nanoparticle addition for an already unstable miscible displacement. The growth rates of the temporal modes of the instability are determined for different profiles or physical properties of nanoparticles. The results reveal that the diffusion of either the carrier fluids or nanoparticles initially has destabilizing effects, but demonstrates stabilizing effects at longer times, as the cutoff spectrum is initially shifted to larger wavenumbers, but shifted back later. It was found that deposition of nanoparticles into the medium stabilizes the miscible front, such that the maximum growth rates and cutoff wavenumbers increase continuously.

The viscous fingering instability of miscible reactive-dispersive flows in a homogeneous porous media is investigated through nonlinear numerical simulations. In particular, the role of velocity-dependent transverse and longitudinal dispersions as well as the type and rate of auto-catalytic chemical reactions is analyzed. It is found that for a third-order auto-catalytic reaction, the higher the reaction rate, the more complex the finger structures. Furthermore, major differences between the flow development of third-order and second-order autocatalytic reactions are reported. In addition, the anisotropy and velocity dependence of the dispersion tensor are found to have a more profound effect on the fingering instability in the case of reactive flows than in the non-reactive ones. The qualitative characterization of the finger structures is explained by examining the flow velocity field and further quantified through an analysis of the average concentration and relative contact area.