Gas hydrate reservoirs are receiving increased attention as potential locations for CO2 sequestration, with CO2 replacing the methane that is recovered as an energy source. In this scenario it is very important to correctly characterize the cage occupancies of CO2 to correctly assess the sequestration potential as well as the methane recoverability. In order to predict accurate cage occupancies, the guest-host interaction potential must be represented properly. In previous efforts, the potential parameters used for CO2 hydrate equilibrium calculations were obtained by fitting to experimental hydrate equilibrium data. These fitted parameters do not match with those obtained by second virial coefficient or gas viscosity data. Ab initio quantum mechanical calculations provide an independent means to directly obtain accurate intermolecular potentials.
A potential energy surface (PES) between H2O and CO2 has been computed at the MP2/aug-cc-pVTZ level and corrected for basis set superposition error (BSSE), an error caused due to the lower basis set, by using approximately half of the counterpoise correction. Intermolecular potentials were obtained by fitting the Exponential-6 and Lennard-Jones 6-12 models to the ab initio PES, correcting for many-body interactions. Using this potential, the pure CO2 hydrate equilibrium pressure was predicted with an average absolute deviation of less than 2% from the experimental data. Predictions of the small cage occupancy ranged from 22-38% and the hydration number for the CO2 hydrate was calculated to be above 7.0, whereas the large cage is fully occupied. Through this work we show that intermolecular interactions occurring on the nanometer scale influence phase equilibria and macroscopic properties. This ab initio intermolecular potential for the CO2–H2O interaction has been tested using molecular dynamics simulations against other intermolecular potentials from the literature by calculating gas hydrate reference chemical potentials, the densities of pure CO2 liquid and vapor, and the solubility of CO2 in water.