Experimental and Numerical Study of Density-Driven Natural Convection Mechanism During Storage of CO2 in Brine Aquifers
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Storage of CO2 into geological formations is a reasonable technical choice for decreasing carbon dioxide emissions to the atmosphere. Brine aquifers are considered as one of the most favorable options for this purpose and leakage of CO2 from these storage sites is one of the main concerns about it. To decrease the risk of leakage, trapping mechanisms of CO2 into brine should be fully understood. More contribution of trapping mechanisms of CO2 reduces the time available for leakage and is therefore crucial to storage security. The dissolution of supercritical CO2 in formation water is one of the main long term trapping mechanisms of CO2 into brine aquifers. Density-driven natural convection mechanism is predicted to occur, which accelerates the dissolution of CO2 in the brine formation water. This unusual phenomenon arises from the increase in the density of brine when saturated with CO2. The timing of the onset of this instability and the dissolution rate across the phase contact are important operational issues when assessing the feasibility of a potential storage site. The main objective of this study is to increase scientific knowledge mainly about the density-driven natural convection mechanism in dissolution of CO2 in isotropic/anisotropic and homogeneous/heterogeneous underground brine aquifers using experimental and numerical tools and explaining this mechanism and its likely impacts to the general public. For this purpose and at the first step, density-driven natural convection mechanism is investigated using a black-oil numerical solver to study the effects of different properties on this process and the amount of dissolved CO2 into brine in homogeneous and heterogeneous brine aquifer models. The onset times for convection triggered by numerical round off errors found in numerical solvers of isotropic homogeneous models are significantly later than predicted by stability analysis theory that is based on real physics of the problem. So for more accurate predictions of convective mixing behaviour in isotropic and anisotropic brine aquifer models, a new approach for initializing the numerical models is developed. In this approach the gravitational instability is triggered by a weak wavy perturbation with dimensionless wavenumber of K that is consistent with the perturbation in stability analysis theory. This numerical solution is used for prediction of the critical time for the onset of convection and the critical unstable wavelength in semi-infinite anisotropic homogeneous brine aquifer models. Moreover after validating this methodology, it is applied in heterogeneous barrier type numerical models for prediction of the related critical times for the onset of convection for different barrier patterns and geometries. In next step we focus on upscaling of properties in simulation models and its effect on performance of convective mixing process in heterogeneous brine aquifers with variation of permeability and barrier type aquifers. Effect of upscaling on onset time for convection and dissolution rate of CO2 in brine is investigated there. After the numerical and theoretical studies of convective mixing process, we start experimental studies of density-driven natural convection mechanism in different Hele-Shaw cell geometries using two sets of fluids; water/brine and water/CO2. At first step we present the results of preliminary experiments about densitydriven natural convection mechanism performed in a homogeneous Hele-Shaw cell using two fluids with different densities; water and brine. With this analysis, the effects of density-driven natural convection on accelerating the rate of dissolution are investigated. Also the growth and development of convection fingers and the changes in their geometries with depth and time are studied. After these initial and preliminary experimental studies and understanding the real concept of density-driven naturally convection mechanism, we focus on performing a series of experiments about density-driven natural convection mechanism in different Hele-Shaw cell geometries using water and CO2. In this part after introducing our precise experimental set-up and the suitable procedure for performing of the experiments in different Hele-Shaw cell models, the results of several experiments are presented. In these experiments the behaviour of density-driven natural convection mechanism in different geometries like homogeneous models with different permeabilities and dips, heterogeneous models with barriers and layered permeability models is investigated. Onset time for convection, critical wavelength of convection fingers and CO2 dissolution rate into water are objective parameters here for study. The important point in the analyses of the experiments is that there are several specific dimensionless numbers that can be related to each experiment and it can be said that the results of the experiments can be scaled to other systems like real brine aquifers with the same dimensionless numbers. Moreover the experimental results are studied by scaling them to dimensionless forms and also compared with numerical simulation results for investigating the effectiveness of numerical simulators for describing convective mixing process. Furthermore the prepared continuous movies from the whole period of these experiments can be helpful in improving the public knowledge about CO2 storage and one of its trapping mechanisms in underground brine aquifers.