Modeling CO₂-Water-Mineral Wettability and Mineralization for Carbon Geosequestration

Carbon dioxide (CO₂) capture and storage (CCS) is an important climate change mitigation option along with improved energy efficiency, renewable energy, and nuclear energy. CO₂ geosequestration, that is, to store CO₂ under the subsurface of Earth, is feasible because the world's sedimentary basins have high capacity and are often located in the same region of the world as emission sources. How CO₂ interacts with the connate water and minerals is the focus of this Account. There are four trapping mechanisms that keep CO₂ in the pores of subsurface rocks: (1) structural trapping, (2) residual trapping, (3) dissolution trapping, and (4) mineral trapping. The first two are dominated by capillary action, where wettability controls CO₂ and water two-phase flow in porous media. We review state-of-the-art studies on CO₂/water/mineral wettability, which was found to depend on pressure and temperature conditions, salt concentration in aqueous solutions, mineral surface chemistry, and geometry. We then review some recent advances in mineral trapping. First, we show that it is possible to reproduce the CO₂/water/mineral wettability at a wide range of pressures using molecular dynamics (MD) simulations. As the pressure increases, CO₂ gas transforms into a supercritical fluid or liquid at ∼7.4 MPa depending on the environmental temperature. This transition leads to a substantial decrease of the interfacial tension between CO₂ and reservoir brine (or pure water). However, the wettability of CO₂/water/rock systems depends on the type of rock surface. Recently, we investigated the contact angle of CO₂/water/silica systems with two different silica surfaces using MD simulations. We found that contact angle increased with pressure for the hydrophobic (siloxane) surface while it was almost constant for the hydrophilic (silanol) surface, in excellent agreement with experimental observations. Furthermore, we found that the CO₂ thin films at the CO₂-hydrophilic silica and CO₂-H₂O interfaces displayed a linear correlation, which can in turn explain the constant contact angle on the hydrophilic silica surface. In view of the literature and our study results, a few recommendations seem necessary to construct a molecular system suitable to study wettability with MD simulations. Future work should be conducted to determine the influence of brine salinity on the wettability of minerals with high cation exchange capacity. Mineral trapping is believed to be an extremely slow process, likely taking thousands of years. However, a recent pilot study demonstrated that CO₂ mineralization occurs within 2 years in highly reactive basalt reservoirs. A first-principles MD study has also shown that carbonation reactions occur rapidly at the surface oxygen sites of a reactive mineral. We observed carbonate ions on both a newly cleaved quartz surface (without hydrolysis), and a basalt andesine surface after hydrolysis in a CO₂-rich environment. Future work should consider the influence of water, gas impurities, and mineral cation type on carbonation.