Quantification of Density-Driven Natural Convection for Dissolution Mechanism in CO2 Sequestration

Abstract

Dissolution of CO 2 into brine causes the density of the mixture to increase. The density gradient induces natural convection in the liquid phase, which is a favorable process of practical interest for CO 2 storage. Correct estimation of the dissolution rate is important because the time scale for dissolution corresponds to the time scale over which free phase CO 2 has a chance to leak out. However, for this estimation, the challenging simulation on the basis of convection-diffusion equation must be done. In this study, pseudo-diffusion coefficient is introduced which accounts for the rate of mass transferring by both convection and diffusion mechanisms. Experimental tests in fluid continuum and porous media were performed to measure the real rate of dissolution of CO 2 into water during the time. The pseudo diffusion coefficient of CO 2 into water was evaluated by the theory of pressure decay and this coefficient is used as a key parameter to quantify the natural convection and its effect on mass transfer of CO 2 . For each experiment, fraction of ultimate dissolution is calculated from measured pressure data and the results are compared with predicted values from analytical solution. Measured CO 2 mass transfer rate from experiments are in reasonable agreement with values calculated from diffusion equation performed on the basis of pseudo-diffusion coefficient. It is suggested that solving diffusion equation with pseudo diffusion coefficient herein could be used as a simple and rapid tool to calculate the rate of mass transfer of CO 2 in CCS projects.

Experimental study of gravitational mixing of supercritical CO2

“…For example, experiments have 83 measured CO 2 mass transfer to water at 25 °C and 1 -5 MPa (Farajzadeh et al, 2009), to water-84 saturated porous media at 25 °C and 2 -6 MPa (Moghaddam et al, 2012), and to water and 85 brine (up to 20 % salinity) saturated porous media at 38 °C and 3.45 MPa (Mojtaba et al, 2014; 86 Moghaddam et al, 2015). Although these experiments are informative to the general behavior of 87 3D density driven mixing, these CO 2 -water experiments were not conducted at relevant reservoir 88 conditions so the CO 2 phase (gas and liquid vs. supercritical), fluid densities, viscosities and the 89 solubility of CO 2 in water are not analogous.…”

Characterizing the Dissolution Rate of CO2-Brine in Porous Media under Gaseous and Supercritical Conditions

“…When brine imbibes to contact with CO 2 , the dispersion between CO 2 and brine dominates and therefore the dissolution rate of this period is increased. As both dispersion and diffusion are active under a supercritical state, the dissolution rate follows the analogous definition of the pseudo-diffusion coefficient (Dps) which can be used for quantification of the dissolution rate of CO 2 in water [28]. In the case of the dissolution in porous media, it was calculated based on the rate of loss of CO 2 from the CO 2 phase,…”

Diffusion of carbon dioxide in formation water as a result of CO2 enhanced oil recovery and CO2 sequestration

“…King et al (1995), who studied the diffusivity of different gases in pure and sea water, reported that there is an uncertainty associated with estimating the molecular diffusion coefficient of a gas in seawater from the values obtained in pure water, demonstrating the poor understanding of the effect of salinity on diffusion. The rate of CO 2 diffusion in saline water has been reported by only a few researchers at certain pressures and temperatures (Nazari Moghaddam et al 2012;Bahar and Liu 2008;Sell et al 2012). However, as the experimental data are so limited, many aspects of the effect of salinity on the rate of CO 2 diffusion are not well understood; further clarification is required to better understand the role of salinity on the rate of CO 2 diffusion.…”