CO2 Mass Transfer in Porous Media: Implications for Long-Term Carbon Sequestration in Saline Aquifers

Abstract

Abstract Carbon sequestration in saline aquifers is considered a suitable alternative for reducing anthropogenic carbon dioxide (CO2) emissions into the atmosphere. As supercritical CO2 is injected into the subsurface, its density difference with the residing water causes a CO2 buoyant plume to overlay a water-saturated porous layer. The study of diffusive and convective mechanisms during CO2 sequestration in aquifers is paramount for understanding the effects of solubility trapping to determine the long-term fate of the injected CO2. We studied the CO2 dissolution in aqueous solutions in porous media by coupling two experimental techniques: (1) constant pressure method and (2) visualization tests. Each test was performed in a closed visualization cell for at least 36 hours. Capillary tubes filled with glass beads of specific grain sizes are saturated with pH-sensitive solutions. CO2 is continuously supplied into the visualization cell to keep the pressure of the system constant by compensating the gas mass transfer into the aqueous phase. The injected gas is monitored in real- time. Furthermore, as the CO2 mass transfer into the aqueous phase takes place, an acidic solution is created, altering the color of the pH-sensitive solution. The change in color is continuously monitored. Our experimental approach allowed us to study the mass transfer dynamics in two different conditions: (1) diffusive mass transfer with the aid of natural convection; (2) diffusive mass transfer with the isolation of density-driven convection. The diffusive mass transfer was described by measuring the velocity of the interface separating the gas and aqueous phases. Natural convection was characterized by the onset time of convection and flow dynamics. We studied the effects of salinity (NaCl) and the impact of grain size in porous media systems. Results show that as average grain size diameter decreases, natural convection has no enhancing effects on the dissolution process when compared to a diffusive mass transfer process alone. We observed that convection onset time increases as grain size decreases, which in turn, diminishes the mass flux of convection. Salinity has been observed to play an essential role in the convection onset time as it affects parameters such as density, effective molecular diffusivity (affected by pore size), and viscosity. This work introduces a novel experimental approach based on coupling pressure-driven and light transmission-driven techniques to evaluate the mass transfer of CO2 in aqueous solutions in porous media. Results obtained in this study are essential to assess solubility entrapment and its long-term storage integrity.