Molecular Dynamics Simulation of Wettability of Quartz/CO2/Brine Systems

Abstract

Geological carbon dioxide sequestration is an efficient approach for reducing atmospheric carbon dioxide emissions and guaranteeing a sustainable future. In this context, the structural and residual trapping capacities of CO2 are highly affected by the rocks' interfacial properties and wettability. Thus, a precise characterization and knowledge of the wetting properties of the rock/CO2/brine systems are essential for the security of these trapping mechanisms. However, several factors, e. g. operating pressure, temperature, brine salinity, CO2 dissolution in the brine, and density of silanol groups, play a significant role in characterizing the wetting of these systems and have not yet been accurately studied under reservoir elevating conditions. Consequently, the primary goal of this study is to explore the parameters that influence the wettability features of quartz/CO2/brine schemes in the absence of experimental uncertainties.

The molecular dynamic (MD) simulation approach was adopted in this work, which is arguably an excellent alternative to complex laboratory-scale observations. The effect of surface silanol groups, brine concentration, temperature, and the choice of CO2 force field parameters on the contact angle of quartz/CO2/brine systems was studied.

The surface silanol group effect is based on the idea that silica's surface hydrophilicity improves as the number of surface hydroxyl groups increases. This improvement is due to the enhancement in surface polarity resulting in hydrogen bonds formed between water molecules and the silica surface silanol (OH) groups. In contrast, it was found that the increase in the brine concentration reduces the surface hydrophilicity, making the silica surface less water-wet due to the offsetting of the surface polarity by the double layer formed between the sodium salts and the surface oxygens in the silica substrate. In addition, it was reported that the temperature has a negligible impact on the wetting behavior of the system studied, as the CO2 studied was in the supercritical state.

In molecular dynamics simulations, the selection of the potential and the force field parameters has a noticeable impact on the system representation and accuracy of the results obtained. In this work, two rigid carbon dioxide force fields have been tested: TraPPE and EPM2. In quartz/CO2/water systems, it was found that both force fields produce the same contact angle values in the same environment. This result explains their popularity, as both force fields are simple and can mimic CO2 thermophysical behavior.

This work provides a framework for molecularly investigating the factors that influence the wetting behavior of quartz in carbon storage conditions. The outcomes of this work will offer a molecular-level understanding of the effect of the silanol group, the brine salinity concentration, the temperature, the salt type, and the selection of the CO2 force field parameters on the wettability of quartz/CO2/brine systems.

Date of Award	Dec 2022
Original language	American English
Supervisor	Muhammad Arif (Supervisor)