Macroscopic and microscopic understanding of the effects of additives (salts, surfactants, and polymers) on gas hydrate formation

Abstract

Gas hydrates, or clathrate hydrates, are ice-like solids formed by the encasement of suitably sized nonpolar gas molecules in cages of water molecules. Gas hydrates play a vital role in flow assurance in the oil and gas industry and show promising applications in gas storage and transportation, carbon dioxide sequestration, gas separation, and desalination. In all applications, the controlled formation of gas hydrates is necessary to eliminate the hazard or realise the opportunities. A feasible method to control gas hydrate formation is the addition of chemical additives, which inhibit or promote formation. Surfactants and salts have been intensively investigated as gas hydrate additives. However, the formation of gas hydrates in dilute solutions of salts and surfactants is poorly understood. In addition, a statistical study on the effect of additives on nucleation is limited in the literature due to the stochastic nature of nucleation.  The objectives of this thesis are to determine the influence of salts, surfactants, and polymers on gas hydrate formation, and investigate the relationship between hydrate nucleation and changes in interfacial water structure induced by additives. Since gas hydrate formation typically initiates at the gas-water interface, a study on water structural changes at the interface can provide information for finding the mechanism of gas hydrate formation in the presence of additives. In addition, this thesis aims to fill a knowledge gap and quantify the nucleation probability in gas hydrate formation in the presence of additives. The findings of this thesis provide valuable new and direct insights into water structural changes at the interface in the presence of the various additives. The effect of additives was studied at the molecular level using Sum Frequency Generation (SFG) spectroscopy, which is an ideal tool to study the molecular structure of fluid interfaces as it is surface-specific and capable of detecting molecular vibrations from only the topmost layers of an interface. The effect of additives on the formation of gas hydrates was studied with an extensive set of high-pressure experiments in autoclaves and in the High-Pressure Automated Lag Time Apparatus (HPS-ALTA).  The key findings of this thesis are as follows. First, this work has confirmed the promoting effect of low concentrations (50-70 mM) of sodium halides and alkali metal chlorides. Simulated gas pocket break-up studies and bubble zeta potential measurements showed the gas-water interfacial area and ion-specific effect play critical roles in gas hydrate formation. Secondly, the synergistic effects of NaI and sodium dodecyl sulfate (SDS) at low concentrations was investigated. Adding NaI to dilute SDS solutions reduced the induction time significantly. The analysis of water structure based on SFG results revealed the added NaI diminishes the alignment of interfacial water, thereby favouring the nucleation of the gas hydrate. Thirdly, the results from HPS-ALTA showed the significant effects of cyclodextrins (CDs) on methane hydrate formation. The statistical analysis of large sets of data revealed that α-CD accelerated the hydrate nucleation rate by up to six times compared to water and the effect on accelerating hydrate formation was faster than γ- and β-CDs. Finally, the effect of Polyvinylpyrrolidone (PVP) at ultra-low concentrations on sII gas hydrates was investigated. It was confirmed that PVP could be considered as a universal hydrate inhibitor since it acts as an inhibitor even at ultra-low concentrations. In summary, this thesis provides an understanding of hydrate formation in the presence of different additives. The findings fill several significant knowledge gaps in the current literature and uncover the critical role of water structural changes at the gas-solution interface in gas hydrate formation.