Some interfacial properties of water and CO2/H2S at quasireservoir conditions: A molecular dynamics study
Society of Petroleum Engineers
School of Engineering
Interfacial properties are important in the process of geosequestering acid gases in the presence of formation water. However, to a considerable extent, the information from molecular interactions is not obtainable experimentally. Theoretically, this limitation is due to a dearth of data at reservoir conditions (i.e., high pressures and elevated temperatures). Hence, molecular dynamics (MD) is used to study interfacial interactions such as interfacial tension (IFT) as a function of temperature and pressure through the mechanical pressure tensor method, acid gas adsorption onto water and absorption into water, pair correlation functions, and density profiles. Simulations were carried out isothermally at 77°C with pressures ranging from 0.5 to 15.6 MPa. The predicted water densities, p, and acid gas [CO2/H2S, with the NERD (Nath, Escobedo and de Pablo) H2S potential] densities matched the experimental values well. The two force fields used to simulate water-acid gas IFTs, y, both overpredict the experimental values, especially at the higher pressures, but the water-OPLS (optimized potentials for liquid simulations) H2S acid gas combination’s y is closer to the experimental ideal. The overpredictions are primarily due to the supercritical nature of the fluids and the force fields used. Radial distribution functions (RDFs) of the various combinations were also examined, and they were found to demonstrate the supercritical nature of the fluids and the molecular interaction between the constituent components of the acid gas and water. The interfacial thickness, o, revealed further insights into the molecular structure and was found to be typically in the 4.0–7.5 A range and is influenced by mainly the acid gas adsorption onto the water surface and to a lesser extent absorption into the bulk water. It was found that CO2 is more dominant than H2S at the water interfacial layer and that CO2-water interactions contributed more toward the overall interfacial properties. Our findings further suggest that the predomination of interactions by CO2 in the system, coupled with the weak interactivity between CO2 and H2S, means that CO2 geosequestration, at least in the 70 mol%CO2 and 30 mol%H2S used in this work, and by extension for higher CO2 mole percentages, does not face any meaningful impediment from the H2S presence during the process. In the absence of nigh impossible to achieve experiments at these extreme temperature and pressure conditions, the findings of this MD study thus offer a better understanding of some of the geological interactions of fluid-fluid mixtures in the presence of formation water and the application of this information during geosequestration.