Molecular Dynamics Study On The Interfacial Characteristics Of CO₂-fluid System In CO₂ Geological Sequestration

CO2 geological storage is one of the most promising methods to manage CO2 emission and accumulation. The optional geological formations include deep saline aquifers, oil and gas reservoirs and deep coal beds. In the injection and storage process, the interfacial tension (IFT) between CO2 and geological fluid governs the capillary-sealing efficiency, the injection energy cost and the maximum storage height. Therefore, the interfacial interactions and properties are of central importance in successful CO2 storage. Currently, experimental research has achieved the IFT variation with salt species, sanity, temperature and pressure in CO2-saline solution system and the influence of CO2 composition on IFT of (CO2+hexane)-brine interface under miscible state. However, the physical mechanisms of IFT dependence on each variable factor have not been investigated, as well as the changes of interfacial micro properties and structures during CO2 injection. Molecular Dynamics Simulation (MD Simulation) is a powerful tool to study the physical and chemical process. It could not only calculate the macroscopic thermodynamic properties, but also observe micro structures and molecular actions. The MD investigations on the interfacial characteristics of CO2-fluid system could provide explanations for the experimental IFT phenomenon and give insight for interfacial properties in molecular level during CO2 geological storage.First, the effect of salt species and sanity on IFT of CO-saline solution was studied. CO2-NaCl、CO2-CaCl2 and CO2-(NaCl-CaCl2) systems were calculated at 343K and 20MPa using MD simulation under different sanity to investigate the intrinsic reasons for IFT variation from atomic parameters and ionic strength aspects and provide guidance for IFT prediction. The results showed that in the effect of salt species, about 80% of IFT variation is caused by cation charge and that Lennard-Jones potential well depth has the second important influence. The IFT growth compared to CO2-water system shows a linear correlation with ionic strength in both CO2-NaCl and CO2-CaCl2, and the slopes of the two systems are close, which proved that the influence of electric field caused by ions on IFT may be identical among different cations. In the simulation of CO2-(NaCl-CaCl2) under various Na+/Ca2+ ratio and sanity, it was found that IFT growthper ionic strength tends to be a constant value in each system, based on which the IFTprediction equation of CO2-electrolyte was summarized.Second, the CO2-NaCl systems were simulated under 300-373K,4-30MPa to study physical explanations for influence of temperature and pressure on IFT. CO2 bulk density, surface excess of CO2 and ions, molecular orientation angle of water and CO2 within interface were calculated to analyze reasons for IFT variation, especially the critical pressure point phenomenon Pplateau.CO2 phase density and solubility in brine both increase with pressure and the growth slows down at Pplateau, which cause changes to the force that water molecules bear at interface, and IFT shows smaller value. The surface adsorption of CO2 and deficiency of ions all become larger at higher pressure, so the influence of ions on interface decreases with pressure. This explains why Pplateau value has no relationship with salt species and sanity at constant temperature condition. The orientation patterns of CO2 molecules at interface and water molecules at water side of interface are consistent with T type CO2-water complex, and the water dlpoles point towards water phase more obviously after Pplateau.The micro structure observed here illustrates the formation of interfacial complex which has the surfactant property, and it caused the Pplateau phenomenon.Finally, hexane was used as a representation of oil to conduct the primary research of CO2-oil-brine system. The interfacial properties of the ternary CO2, n-hexane and 1.5mol/L NaCl solution system under 330K,20MPa were studied with different CO2 compositions by MD simulations. We observed that CO2 mixes well with n-hexane and a clear interface separates the CO2-hexane mixture with the NaCl solution. The interfacial width and roughness increase with the CO2 composition, indicating deeper molecular penetrations and shorter capillary wave lengths, which leads to the reduced interfacial tension. Interestingly, the surface adsorption of CO2 reaches peak when CO2 molar fraction equals to 62.5%, as well as the deficiency of n-hexane. This implies the surfactant feature of CO2 and the hydrophobicity of n-hexane towards the interface investigated here. The orientational preferences of CO2, n-hexane and water molecules at the interface are more random at higher CO2 compositions, as a result of the increased absolute amount of CO2 and the absence of n-hexane at the interface.