Experimental and Computational Approaches for Prevention and Remediation of Gas Hydrates in the Presence of Novel Inhibitors

Abstract

This dissertation investigated the synergistic inhibition effect of gas hydrate inhibitors, newly designed gas hydrate inhibitors, and gas hydrate remediation method development for the field of production chemistry of flow assurance. In order to perform above research topics, fundamental experimental studies of thermodynamic stability, structural analysis, gas uptakes, cage-filling molecular behaviors in the presence inhibitors were examined with computational methods and tried to synthesize eco-friendly and innovative gas hydrate inhibitors. The sigma (σ) profiles of inhibitor molecules obtained from the Conductor-Like Screening Model for Real Solvents (COSMO-RS) software were used to estimate inhibitor–water interactions for the pre-screening of potential inhibitors. From the σ profile results, candidates for thermodynamic hydrate inhibitor (THI) and kinetic hydrate inhibitor (KHI) were selected. In addition, the stability conditions in the presence of various inhibitors show that thermodynamic inhibition effect is related to molecular interaction between water and inhibitor. As is well known, intrinsic properties and the number of inhibitors are the biggest factors influencing the thermodynamic inhibition effect. Thus, even if two or more inhibitors mixed, it is hard to observe thermodynamic synergistic inhibition effect. Moreover, inhibitors hard to enclathrated in the gas hydrate structure because their sizes are too large (van der Waals radius values), so they reside on the outside of the cages or are partly involved in cages to disrupt the water-water networks. Furthermore, kinetic analyses were conducted to observe kinetic inhibition effect regarding onset temperature, growth rates and conversion of water into gas hydrates. It was confirmed that different results of onset temperature, growth rates and conversion of water into gas hydrates are shown according to types of inhibitor and inhibitor mixtures. Therefore, density functional theory (DFT) calculation, quantum theory of atoms in molecules (QTAIM) calculation, fourier-transform infrared (FT-IR) spectroscopy, and in-situ Raman spectroscopy were used to demonstrate inhibition mechanisms of each inhibitor. The DFT and QTAIM calculations indicated that each inhibitor has different interaction energy with cage, and the smaller negative interaction energy means that inhibitor significantly retards gas hydrate formation. The FT-IR was used to investigate the interaction sites of inhibitors toward water and find the peak shifts to observe the hydrogen bonding sites. In particular, in-situ Raman analysis verified inhibition mechanisms of inhibitors during nucleation and formation process, thus delaying behaviors of cages could be observed directly. The overall experimental and computational results in this dissertation provide invaluable information of THI and KHI, thus can contribute to the opening up a new field for flow assurance in oil and gas industries and CO2 storage process.