Qualitative and quantitative experimental study of convective mixing process during storage of CO2 in heterogeneous saline aquifers

Abstract

CO2 dissolution is considered as one of the most promising mechanisms for trapping of free-phase CO2 into brine. It causes an increased density of the brine and initiation of gravitational instability that eventually leads to density-driven natural convection in saline aquifers. Correct estimation of the onset time for convection and the rate of dissolution of CO2 into brine is important because the timescale for dissolution corresponds to the timescale over which free-phase CO2 has a chance to leak out. The gravitational instability of a diffusive boundary layer in porous media has been studied in several papers in recent years, but there are few works about the behavior of density-driven natural convection mechanism in heterogeneous saline aquifers. Barriers such as shales and calcites layers are common types of heterogeneities in geological formations that are important in the fluid flow. Despite the recognized importance of convective dissolution in these heterogeneous geological formations, there is no experimental data available for studying the accelerated mass transfer rate of CO2 into these media.In this paper, we investigated the effect of the regular distribution of barriers on the rate of dissolution of CO2 into water and geometries of convection fingers. A series of experiments were performed using a precise experimental set-up with barrier heterogeneous Hele-Shaw cell geometries and by using CO2 and water. The approach and procedure for performing the experiments give us this opportunity to have both qualitative (images and movies) and quantitative (amount of the dissolved CO2 into water) data at the same time. The behavior of convection pattern after onset time and the effect of system properties on the behavior of convective mixing process will be presented and discussed. Moreover, some speeded-up movies from the experiments that are suitable for improving public awareness of the problem have been uploaded on the internet platform. Lastly, the relationships between dissolution flux after onset time for convection and barrier properties are discussed.

Steady Flux Regime During Convective Mixing in Three-Dimensional Heterogeneous Porous Media

“…Two models of reservoir heterogeneity are considered in this study. Following the use of horizontal flow barriers as a simplified proxy for realistic heterogeneity (such as shale filled reservoirs) in two-dimensional studies of convective mixing [15,[26][27][28]30], we extend this idea to three dimensions using flat elliptical disks of various aspect ratios to represent flow barriers. The major and minor axes of the elliptical disks are drawn from Gaussian distributions N(250, 75) and N(100, 25), respectively, where N(µ, σ) refers to a Gaussian distribution with mean µ and standard deviation σ.…”

“…Green and Ennis-King [26] demonstrated through numerical simulations that the average mass flux in the steady flux regime during solutal convection in a two-dimensional heterogeneous porous medium was well represented by an anisotropic homogeneous porous medium with equivalent permeabilities in the vertical and horizontal directions. Several subsequent studies have also observed that the steady flux rate is dependent on the average properties of the porous medium, suggesting that heterogeneous porous media may be suitably modelled using anisotropic homogeneous porous media [15,[27][28][29][30].…”

Effect of Convective Mixing Process on Storage of CO2 in Saline Aquifers with Layered Permeability

“…A limited number of experimental studies exist on the effectiveness of the convective mixing process in the homogeneous media regarding the storage of CO2 in geological formations [2,6,[23][24][25][26][27][28][29][30][31][32][33][34][35][36]. The occurrence of natural convection and the effectiveness of the convection mechanism (on the dissolution of CO2 into brine) are investigated by recording the change in pressure in a bulk media, using a cylindrical PVT cell [25][26][27][28]33], and in a homogeneous porous media, using cylindrical core [6,30,31], where a fixed volume of CO2 is overlaying a column of distilled water.…”

“…Convective currents and dynamics of convective mixing are visualized at ambient conditions using Schlieren and pH indicator techniques, for bulk modules of gas-liquid systems, like CO2-water [23,24,33], and for homogeneous Hele-Shaw cells without porous media [2, 30-32, 34, 35] and with porous media [29][30][31][32]35] by using analog or CO2-water fluids. The initiation, formation, and growth of small convective fingers and progress of this process as a function of time are discussed extensively [36]. The essential parameters of the convective mixing process, i.e., onset time of convection, critical wavelength, shape and growth of fingers over time, and the amount of dissolved CO2 into water, are measured qualitatively (by snapshots of convective mixing of CO2 and water) and quantitatively (by amounts of dissolved CO2 into water) in different homogeneous Hele-Shaw cell geometries with Rayleigh numbers varying in the range of 700-9500 [36].…”

Density‐Driven Convection in a Fractured Porous Media: Implications for Geological CO2 Storage

“…In the theoretical studies, linear stability analysis or nonlinear simulation was used to determine the properties of the fingering pattern, such as the characteristic onset time, growth rate, and finger length and wavelength, for given physical properties of the reservoir (Ennis‐King et al, ; Jafari Raad & Hassanzadeh, ; Xu et al, ). Laboratory experiments have been extensively used to examine the convective mixing process in homogeneous media (Ching et al, ; Kneafsey & Pruess, ; Neufeld et al, ; Taheri et al, ). In most of the experimental studies, Hele‐Shaw cells were used for visualization, and analog fluids were used to replicate the dynamics of the free‐phase CO 2 relative to water, as well as mimic the convective dissolution of CO 2 in water.…”