Solid/CO2 and solid/water interfacial tensions as a function of pressure, temperature, salinity and mineral type: Implications for CO2-wettability and CO2 geo-storage

Abstract

Wettability of CO2/brine/mineral systems plays a significant role in the underground geological storage of CO2 as it governs the fluid flow and distribution mechanism within the porous medium. Technically, wettability is influenced by CO2 pressure, the temperature of the storage formation, formation water salinity and the type of mineral under investigation. Although a growing number of studies report wettability data for CO2/water/mineral systems, yet the factors responsible for wettability variation with pressure and temperature remain unclear. In this work, we used the concept of surface energy to explain dependency of wettability on pressure, temperature and salinity. Neumann's equation of state approach was used to compute solid/CO2 and solid/water interfacial energies using reliable contact angle and CO2/brine interfacial tension data from the literature at a wide range of operating conditions for quartz, water-wet mica, oil-wet mica and high, medium and low-rank coals. Moreover, the all-important question that why different minerals offer different wettability to CO2/water systems at the same pressure and temperature of investigation is addressed by comparing the interfacial energies of the minerals. We found that for all minerals solid/CO2 interfacial energy decreased with pressure and increased with temperature, and solid/water interfacial energy decreased with temperature except for quartz for which solid/water interfacial energy increased with temperature. Furthermore, the solid/CO2 interfacial energy was lowest for the oil-wet mica surface and highest for quartz which is due to higher hydrophobicity of oil-wet mica surface. The results of the study lead to a better understanding of the wetting phenomenon at the CO2/brine/mineral interface and thus contribute towards the better evaluation of geological CO2-storage processes.