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 similar to 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.