Accelerated Mass Transfer of CO₂ in Reservoir Brine Due to Density-Driven Natural Convection at High Pressures and Elevated Temperatures

Abstract

In this paper, the mass transfer of CO 2 into a reservoir brine sample is studied experimentally at high pressures and elevated temperatures. The equilibrium concentration of CO 2 in the reservoir brine and the density of CO 2 -saturated brine are measured by saturating the brine with CO 2 . The mass-transfer rate of CO 2 into the brine is determined by monitoring the pressure decay inside a closed, visual, high-pressure PVT cell. It is found that the density of the brine with dissolved CO 2 increases linearly with CO 2 concentration. As CO 2 gradually dissolves into the brine by molecular diffusion, a concentration-induced density gradient is generated near the CO 2 -brine interface. Under the influence of gravity, this concentration-induced density gradient causes natural convection, which accelerates the mass-transfer rate of CO 2 into the brine. The modified diffusion equation with an effective diffusivity is applied to model the mass-transfer process. It is found that the determined effective diffusivities of CO 2 in the reservoir brine are almost two orders of magnitude larger than the molecular diffusivities of CO 2 in water or similar reservoir brines. The detailed experimental results show that the density-driven natural convection greatly accelerates the dissolution process of CO 2 in brine. This means that loss of CO 2 in brine can be significant in an enhanced oil recovery operation using CO 2 flooding in an oil reservoir with a bottom water aquifer. More importantly, the accelerated mass transfer due to the density-driven natural convection significantly increases the geological sequestration rate of CO 2 in deep saline formations.

Modeling mass transfer of CO2 in brine at high pressures by chemical potential gradient

“…CO 2 geological storage is a promising means to mitigate CO 2 emission [1][2][3][4][5] and storage in deep saline aquifers appears to hold the largest potential capacity [4,[6][7][8]. The sequestration capacity, long-term CO 2 behavior in receptor formations, and the quantification of possible CO 2 leaks are the main concerns [2,4,9,10], and there remains a need to study the potential mobility of CO 2 dissolved in brines over a wide range of spatial and temporal scales [4,9,11], the CO 2 concentration distribution in saline aquifers, as well as the density distribution in geological media [8].…”

“…The sequestration capacity, long-term CO 2 behavior in receptor formations, and the quantification of possible CO 2 leaks are the main concerns [2,4,9,10], and there remains a need to study the potential mobility of CO 2 dissolved in brines over a wide range of spatial and temporal scales [4,9,11], the CO 2 concentration distribution in saline aquifers, as well as the density distribution in geological media [8]. This highlights the significance of research on CO 2 dissolution and the mass transfer of CO 2 dissolved in brines.…”

“…Numerous investigations of the mass transfer of CO 2 in high-pressure water or brines have been pursued in order to further simulate the geological and ocean CO 2 disposal processes [8,[12][13][14][15][16][17][18][19]. It has been shown that the dissolution of CO 2 in brines increases the density, and then induces density-driven natural convection [8,12,[16][17][18], and the density-driven natural convection then significantly accelerates the diffusion of CO 2 in the brine [8,[16][17][18].…”

“…It has been shown that the dissolution of CO 2 in brines increases the density, and then induces density-driven natural convection [8,12,[16][17][18], and the density-driven natural convection then significantly accelerates the diffusion of CO 2 in the brine [8,[16][17][18]. Yang and Gu [8] investigated experimentally the dissolution of CO 2 in brine at elevated pressures and described the mass transfer of CO 2 in brine using a modified diffusion equation with an effective diffusion coefficient. The effective diffusion coefficients obtained were two orders of magnitude larger than the molecular diffusivity of CO 2 in water [8], which indicates that the density-driven natural convection greatly accelerates the mass transfer of CO 2 in brines.…”

“…Yang and Gu [8] investigated experimentally the dissolution of CO 2 in brine at elevated pressures and described the mass transfer of CO 2 in brine using a modified diffusion equation with an effective diffusion coefficient. The effective diffusion coefficients obtained were two orders of magnitude larger than the molecular diffusivity of CO 2 in water [8], which indicates that the density-driven natural convection greatly accelerates the mass transfer of CO 2 in brines.…”