Effect of Salinity on CO₂ Thermodiffusion in Aqueous Mixtures by Molecular Dynamics Simulations

Abstract

Some geothermal formations may contain a large concentration of CO 2 and a very high vertical temperature gradient. It is likely that there may be significant variation in the CO 2 concentration in these formations due to the Soret effect. In our work, we evaluate, for the first time, the CO 2 thermodiffusion in brine mixtures via molecular dynamics simulations. In the past, cross-diffusion effects on thermodiffusion have been neglected in the evaluation of thermal diffusion in three or more species despite their significance. In this work, we present a methodology to compute the thermal diffusion factor in multicomponent mixtures by combining equilibrium and nonequilibrium molecular dynamics simulations. As in binary CO 2 –H 2 O, the CO 2 –brine Soret effect may have a pronounced dependency on temperature; by increasing temperature, CO 2 migrates from thermophobic to thermophilic conditions. Water structure may have a dominant effect on CO 2 thermal diffusion. The more organized the hydrogen bond network, the more thermophobic the CO 2; an increase in temperature or salinity may make CO 2 more thermophilic. Our results and formulations advance the knowledge of CO 2 distribution in subsurface formations and set the stage for accurate modeling of formations aimed at carbon sequestration and heat extraction.