Molecular Physics Vol. 110, Nos. 11–12, 10–20 June 2012, 1097–1106 INVITED ARTICLE Molecular dynamics study of calcite, hydrate and the temperature effect on CO 2 transport and adsorption stability in geological formations Phan Van Cuong, Bjørn Kvamme, Tatiana Kuznetsova * and Bjørnar Jensen Department of Physics and Technology, University of Bergen, Alle´gaten 55, 5007 Bergen, Norway (Received 14 October 2011; final version received 17 March 2012) Molecular dynamics (MD) simulations at several different temperatures were run to investigate the transport, adsorption, and stability of carbon dioxide (CO 2 ) and water phases in contact with a ð10  14Þ calcite surface. All simulated systems showed evidence of CO 2 transport and interface stability heavily affected by the presence of calcite and the simulation temperature. The number of CO 2 molecules that successfully traversed the water layer and adsorbed on the calcite surface increased with temperature, while the adsorption stability (indicated by the adsorption energy) decreased. It was found that the short-range potential has a significant impact on the preferred CO 2 orientation and adsorption selectivity. Carbon dioxide tended to fill partial hydrate cavities at the water–hydrate interface, potentially promoting the formation of new hydrate. These findings indicate the need to consider the implications that CO 2 injection will have for reservoirs with pre-existing clathrate hydrates. Keywords: calcite; carbon dioxide; water; molecular simulation 1. Introduction Carbon dioxide can be captured, transported, and permanently stored in geological formations including spent petroleum reservoirs [1]. This promising tech- nique of carbon sequestration can contribute both to greenhouse effect reduction and enhanced oil recovery (EOR). However, all processes occurring during injec- tion, post-injection, and storage occur in either porous rock or inside rust-covered pipes, making interactions and reactions between CO 2 , water, and minerals of the utmost importance. Calcite is one of the most abun- dant minerals in the Earth’s crust, with the ð10  14Þ plane being the most stable [2] and by far the most dominant observed morphology of calcite in situ [3]. Atomistic-scale interactions between the ð10  14Þ calcite surface and various substances such as pure water, aqueous solutions, peptides, etc., have been the subject of several numerical studies [4, 5], and continue to attract interest because of the decisive role they play in determining both the macroscopic properties and the kinetics of processes. In this work, we used molecular dynamics simula- tions to study several aqueous interfacial systems involving CO 2 and calcite. Our main focus was on the impact of calcite and temperature variations on the transport, adsorption, and stability of CO 2 molecules and water as affected by the presence of the ð10  14Þ calcite surface. There were two main reasons that led us to use the conventional approach shared by a number of other researchers [4–9] by disregarding the reactions between water and carbon dioxide. The first has to do with the size limitations of even reasonably large numeric simulations. According to our simple calculations, including even a few unbalanced hydronium ions to mimic the effect of the pH would require the presence of at least half a million water molecules. Such a simulation is currently infeasible. A good example of the size necessary to reflect a realistic concentration can be provided by considering the conditions at the Utsira formation, which is the longest-running injection site of carbon dioxide storage into aquifers [1,10]. The pH at Utsira ranges around 3 close to injection, with the temperature in the supercritical range for CO 2 around 310 K at the injection point. While the relevant pressures (depths) have not yet been disclosed in the open literature for confidentiality reasons, they are not likely to exceed 120 bar. Estimates using the geochemical package from the ATHENA reservoir simulator [8, 10] have shown that CO 2 dissolved in water under those conditions will be dominated by molecular CO 2 . As indicated by the ratio between molecular CO 2 and bicarbonate shown in Figure 1, one would need to introduce approxi- mately 88 CO 2 molecules per bicarbonate ion at 277 K, which, for our system, would mean increasing the *Corresponding author. Email: nfyta@ift.uib.no ISSN 0026–8976 print/ISSN 1362–3028 online ß 2012 Taylor & Francis http://dx.doi.org/10.1080/00268976.2012.679629 http://www.tandfonline.com Downloaded by [Universitetsbiblioteket i Bergen] at 03:34 10 September 2014