Investigation of supercritical CO₂ mass transfer in porous media using X-ray micro-computed tomography

Understanding the mass transfer of CO₂ into formation brine both qualitatively and quantitatively is important for improving the security of geologic carbon sequestration. In this study, quasi-dynamic X-ray micro-computed tomographic (MCT) imaging was used to track the time-evolution of supercritical CO₂ (scCO₂) clusters in a sandstone throughout brine injection. A cluster-matching workflow enabled the identification of depletion, merging, and snap-off of the scCO₂ clusters, and subsequently the mass transfer coefficient of individual scCO₂ clusters was found to range between 3.0×10⁻⁵ and 3.5×10⁻⁴ mm/s. The macroscopic average mass transfer coefficient was estimated as 1.4×10⁻⁴ mm/s. For application to geologic carbon sequestration, these values give an indication of the range of mass transfer coefficients that may be expected for similar state and flow conditions. With the macroscopic average mass transfer coefficient evaluated, we back-calculated the in-situ CO₂ concentration field for brine, which provides quantitative insight of the distribution of dissolved CO₂ in the sample. Despite slow injection rate (Ca = 10⁻⁷), mobilization of small scCO₂ clusters was also observed, and was attributed to the combined effect of incomplete dissolution of snapped-off clusters and the reduction in the fluid–fluid interfacial tension (IFT) due to the high local CO₂ concentration in brine accompanying scCO₂ dissolution. This highlights the coupling of dissolution and mobilization processes and demonstrates the need to understand these interlinked dynamics to improve CO₂ storage in geological formations.