Unveil the controls on CO2 diffusivity in saline brines for geological carbon storage

Abstract

Accurate estimation of the diffusivity of CO₂ in formation brine, one of the dominant mass transfer mechanisms, is crucial for characterizing CO₂ migration and distribution in Geological Carbon Storage (GCS). However, the effects of pressure, temperature, and the composition and concentration of ions on the CO₂ diffusion coefficient in saline aquifers are complex. This study used in-house-designed equipment to determine the diffusion coefficient of supercritical CO₂ in synthetic brines, such as deionized water and NaCl, CaCl₂, and KCl solutions under different pressure and temperature conditions. First, CO₂ solubilities at initial pressure and temperature were measured to provide the initial CO₂ concentration at the gas-liquid interface. Then, pressure decay curves were collected for the selected brines and the diffusion coefficients of CO₂ molecules were calculated based on Fick's second law. Experimental results show that CO₂ solubility increases with increasing pressure and decreasing temperature and ionic strength. Moreover, CO₂ solubilities at same pressure, temperature and ionic strength conditions vary in different synthetic brines. We also found that CO₂ diffuses faster in brine at higher pressures and temperatures but slows down as ionic strength increases. The type of ions affects the diffusion differently, except at higher pressures, where their impacts balance out due to hydration effects. In addition, a good linear relationship between CO₂ diffusion coefficient and solubility was observed, indicating the CO₂ diffusion coefficient could be estimated by solubility. The findings in this study shed a light on the CO₂ migration during commercial-scale geological storage and enhanced oil recovery processes.