Prerequisites for density-driven instabilities and convective mixing under broad geological CO 2 storage conditions, Advances in Water Resources (2015),
Highlights We develop an instability onset, onset time and steady convective flux-analysis tool. A systematic analysis of prerequisites for enhanced solubility trapping is presented. Indirect thermal effects on enhanced solubility trapping are non-negligible. Shallow storage depths favor density-driven instability onset and short onset times. Salinity determines the storage depth at which the largest convective fluxes occur.
AbstractDirect atmospheric greenhouse gas emissions can be greatly reduced by CO 2 sequestration in deep saline aquifers. One of the most secure and important mechanisms of CO 2 trapping over large time scales is solubility trapping. In addition, the CO 2 dissolution rate is greatly enhanced if density-driven convective mixing occurs. We present a systematic analysis of the prerequisites for density-driven instability and convective mixing over the broad temperature, pressure, salinity and permeability conditions that are found in geological CO 2 storage. The onset of instability (Rayleigh-Darcy number, ), the onset time of instability and the steady convective flux are comprehensively calculated using a newly developed analysis tool that accounts for the thermodynamic and salinity dependence on solutally and thermally induced density change, viscosity, molecular and thermal diffusivity. Additionally, the relative influences of field characteristics are analysed through local and global sensitivity analyses.The results help to elucidate the trends of the , onset time of instability and steady convective flux under field conditions. The impacts of storage depth and basin type (geothermal gradient) are also explored and the conditions that favour or hinder enhanced solubility trapping are identified. Contrary to previous studies, we conclude that the geothermal gradient has a non-negligible effect on density-driven instability and convective mixing when considering both direct and indirect thermal effects because cold basin conditions, for instance, render higher compared to warm basin conditions. We also show that the largest is obtained for conditions that correspond to relatively shallow depths, measuring approximately 800 m, indicating that CO 2 storage at such depths favours the onset of density-driven instability and reduces onset times. However, shallow depths do not necessarily provide conditions that generate the largest steady convective fluxes; the salinity determines the storage depth at which the largest steady convective fluxes occur.