| dc.description.abstract | Understanding the phase behavior of gas hydrates is critical to flow assurance, geological stability, energy harvest from hydrate reservoirs, and industrial applications such as gas storage and desalination. Systems containing CH4, C2H6, and CO2 are particularly of interest because they are ubiquitous in nature and many industrial applications, and also due to their microscopic properties, e.g., the structure change of the CH4-C2H6 hydrates and the impact of CO2 on the variation of hydrate stability.In this work, a high-pressure PVT experimental apparatus is designed to measure the hydrate phase boundary and to observe the growth behaviors of the hydrate film formed in the water-gas interface. The phase boundary of CH4-CO2 hydrates formed in NaCl solutions is measured to extend the phase boundary database of CH4-CO2 hydrates with pressure up to 61.99 MPa and the temperature to 295.09 K. The roughness of the water-gas interface is greatly increased at the initial stage of hydrate formation and becomes smoother with time because of the thickness growth of the hydrate film. The fine pillars formed at the water-gas interface may be due to the non-equilibrium state of gas and water phases or due to the non-uniform lateral growth of hydrates at the interface. Three theoretical models with different frameworks are developed to calculate the phase boundary of gas hydrates. The first model employs the original PR EoS for the gas-rich phases and the Henry’s law combined with the Poynting correction for the gas solubility in the aqueous phase. In the second model, a single modified PR EoS is applied to calculate the aqueous and gas-rich phase equilibria. In these two models, the effect of electrolytes on the hydrate phase boundary is determined by employing the Pitzer model. The first two theoretical models are also termed as separate models because different models are used for non-hydrate phases, i.e., gas-rich phase, aqueous phase, and dissolved electrolytes. In the third model, a unified EoS is proposed to account for the non-hydrate phases, i.e., the mPR EoS is used to describe the interactions between water, gas, and uncharged ions, a simplified explicit MSA term for the long-range Coulombic forces, and Born terms for the discharging-charging processes. The vdW-P model is applied in all these three models for the hydrate phase. These developed models are capable of capturing the general trend that the phase boundary pressure of CH4, C2H6, CO2, and binary CH4-C2H6 and CH4-CO2 hydrates increases with the temperature and the NaCl concentration. Also, the phase boundary pressure of the binary CH4-C2H6 and CH4-CO2 hydrates increase with the concentration of CH4 in the feed gas. It is worth noting that the two theoretical models are capable of detecting the sII structure formed by the binary CH4-C2H6 hydrates, which is influenced by various factors, e.g., gas composition, pressure, and temperature. The reliability of the first model is verified by an overall AARD of 5.0% for the 984 data points of CH4, C2H6, CO2, and binary CH4-C2H6 and CH4-CO2 hydrates formed in the bulk pure H2O phase and NaCl solutions. The data points of the binary CH4-C2H6 and CH4-CO2 hydrates formed in the pure water and NaCl solution are employed to quantify the reliability of the second model, with an overall AARD of 5.2% for the 187 data points of the binary CH4-C2H6 and CH4-CO2 hydrates. 734 phase boundary data points of pure and binary CH4, C2H6, and CO2 hydrates formed in single- and mixed-NaCl, KCl, MgCl2, and CaCl2 solutions as well as in the pure water are used to evaluate the reliability of the third model, with an overall AARD of these 734 data points predicted by this newly developed model is 3.4%, which proves the capability of the newly developed model in predicting the phase boundary of hydrates in multi-component systems. The MD simulation is used to explore the structure stability of CH4-C2H6 and CH4-C2H6-CO2 hydrates. It proves that the sI hydrates formed by CH4-C2H6 mixtures with a CH4 composition of 20 mol% are more stable than those with a CH4 composition of 90 mol%. On the contrary, the hydrates formed by CH4-C2H6 mixtures with 90 mol% CH4 in sII structure are more stable than those with a CH4 composition of 20 mol%. This is consistent with the theoretical calculation and also the experimental observations. The impact of CO2 on the stability of sI structure CH4-C2H6-CO2 hydrates is investigated using two CO2 compositions of 20 and 50 mol%. It is observed that the hydrate stability decreases when the CO2 composition increases. The impact of CO2 on the motion of gas molecules in the hydrate cages changes with gas composition. For instance, the motion of CH4 molecules is restricted with the increase in CO2 concentration when the CH4 concentration is high. However, the CO2 has little impact on CH4 when the CH4 concentration is low. Also, the CO2 molecules can move more freely when the CH4 concentration is low as the concentration of CO2 is increased. The added CO2 enhances the free motion of C2H6 and this effect becomes less significant when the concentration of C2H6 is increased. The fluctuation frequency of the MSD and the rotation angle curves of the water and gas molecules can be explained by the potential energy between water-water and gas-water pairs. As the pressure increases, the stability of hydrate cages is slightly enhanced in scenarios with a CO2 concentration of 20 mol%, but the hydrate cages get less stable in scenarios with a CO2 concentration of 50 mol%. Similarly, the impact of pressure on the motion of gas molecules depends on the gas concentration. At high concentrations, the motion of CH4 molecules is slightly restricted with the increase in pressure. The pressure effect on CH4 motion becomes negligible as the concentration of CH4 decreases. The impact of pressure on the motion of C2H6 molecules is negligible except when the concentration of C2H6 is 40 mol%, where C2H6 moves more freely. Overall, the impact of pressure on the average rotation angle and MSD of gas molecules in the hydrate cages is less significant than the gas composition. However, the effect of pressure on the interactions between gas molecules is reflected by the amplitude of fluctuations on the rotation angle and MSD curves. For instance, the fluctuation amplitude of both the rotation angle and MDS curves of C2H6 molecules get larger as the C2H6 concentration decreases. The amplitude of the fluctuation in the rotation angle curves is smaller when the CH4 concentration is higher, due to the interaction between CH4 and CO2 molecules. | |
