Molecular Modeling of Diffusion Coefficient and Ionic Conductivity of CO₂ in Aqueous Ionic Solutions

Abstract

Mass diffusion coefficients of CO(2)/brine mixtures under thermodynamic conditions of deep saline aquifers have been investigated by molecular simulation. The objective of this work is to provide estimates of the diffusion coefficient of CO(2) in salty water to compensate the lack of experimental data on this property. We analyzed the influence of temperature, CO(2) concentration,and salinity on the diffusion coefficient, the rotational diffusion, as well as the electrical conductivity. We observe an increase of the mass diffusion coefficient with the temperature, but no clear dependence is identified with the salinity or with the CO(2) mole fraction, if the system is overall dilute. In this case, we notice an important dispersion on the values of the diffusion coefficient which impairs any conclusive statement about the effect of the gas concentration on the mobility of CO(2) molecules. Rotational relaxation times for water and CO(2) increase by decreasing temperature or increasing the salt concentration. We propose a correlation for the self-diffusion coefficient of CO(2) in terms of the rotational relaxation time which can ultimately be used to estimate the mutual diffusion coefficient of CO(2) in brine. The electrical conductivity of the CO(2)-brine mixtures was also calculated under different thermodynamic conditions. Electrical conductivity tends to increase with the temperature and salt concentration. However, we do not observe any influence of this property with the CO(2) concentration at the studied regimes. Our results give a first evaluation of the variation of the CO(2)-brine mass diffusion coefficient, rotational relaxation times, and electrical conductivity under the thermodynamic conditions typically encountered in deep saline aquifers.

Diffusion Coefficients of Carbon Dioxide in Brines Measured Using 13C Pulsed-Field Gradient Nuclear Magnetic Resonance

“…Under these circumstances, it can be appropriate to consider the tracer diffusion coefficient of the sparingly soluble component in an otherwise spatially uniform brine that is treated as a pseudopure solvent. Such a reduction has been discussed by Garcia-Rateś et al, 4 using arguments based on the Stefan-Maxwell formalism, and also by Cussler. 5 Thus, in the remainder of this work, we focus on the diffusion coefficient D 11 coupling the flux and concentration gradient of component 1, CO 2 , and denote this simply by D. The main assumptions in applying such a treatment are that the concentration of solute 1 is so low that there is negligible diffusional coupling with other solutes and that interactions between component 1 and the other solutes are negligible, in which case D ij ≈ 0 when i ≠ j.…”

Diffusivity of Carbon Dioxide in Aqueous Solutions under Geologic Carbon Sequestration Conditions

“…Moreover, the assumption that infinite-dilution CO 2 diffusivity will accurately predict the diffusivity of CO 2 in GCS reservoirs 3,4 has never been experimentally tested. To clarify the diffusion behavior of CO 2 under realistic reservoir conditions, and to pursue a research program on fluid and mineral reactions relevant to GCS, we have constructed a Raman microscope that is compatible with high-pressure, hightemperature capillaries and microfluidic cells that operates routinely up to 100 bar and 80°C with imaging capabilities at diffraction-limited spatial resolution (i.e., ∼300 nm).…”

Atomistic Molecular Dynamics Simulations of CO2 Diffusivity in H2O for a Wide Range of Temperatures and Pressures

“…27 Additionally, Vlcek et al 27 28 reported a fairly good agreement with the experimental values and showed a dependence of CO 2 diffusivity on the isotopic mass of the carbon atom. Finally, Garcia-Rateś et al 29 reported values for the diffusivity of CO 2 in H 2 O for a narrow range of temperatures and pressures; their study aimed at the determination of D CO 2 in brines.…”