Elucidation of Hydrate Inhibition Mechanisms via Experimental and Molecular Simulation Approaches for Development of Next-Generation Inhibitors

Abstract

Gas hydrate formation remains a significant operational challenge in the oil and gas industry, as the accumulation of hydrate particles causes flowline plugging, resulting in severe consequences including safety hazards and environmental risks. Among various mitigation strategies, the injection of chemical additives, referred to as hydrate inhibitors, has been regarded as the most effective and technically feasible method to prevent hydrate formation. Even though conventional inhibitors such as methanol (MeOH), mono-ethylene glycol (MEG), and poly(N-vinylcaprolactam) (PVCap) have exhibited excellent performance in the inhibition of hydrate nucleation and growth, their injection is often accompanied by major drawbacks including high cost, toxicity, and operational issues, which limit their long-term sustainability. Consequently, it is strongly required to develop novel hydrate inhibitors with enhanced characteristics from financial, environmental, and safety perspectives. To achieve this objective, we employed a combination of experimental investigations and molecular simulations to assess the inhibition performance of hydrate inhibitors, followed by unveiling their inhibition mechanisms in detail. In topic 1 (Chapter 3), physically and chemically modified microcrystalline cellulose (MCC) was evaluated as a kinetic hydrate inhibitor for CH4 and CO2 hydrates through experimental and computational approaches. To overcome the strong hydrophobicity of MCC, high-pressure homogenized cellulose (HPHC) was prepared by dispersing MCC homogeneously in water, while surface-modified ionic cellulose (SMIC) was obtained by modifying the surface of MCC. The onset temperature and gas uptake of the hydrates were experimentally measured to examine the inhibition performance of HPHC and SMIC, followed by molecular dynamics (MD) simulations to clarify the inhibition mechanisms during hydrate cage formation and hydrate growth. The findings from topic 1 propose the great potential of cellulose as a novel and green KHI. In topic 2 (Chapter 4), eight types of imidazolium ionic liquids (ILs) were investigated as thermodynamic and kinetic hydrate inhibitors for CH4 and CO2 hydrates. This study especially aims at investigating a crucial factor determining the inhibition performance of ILs the hydrates, focusing primarily on the effect of their hydrophobicity through a combination of experiments and MD simulations. In addition to hydrate inhibition, the corrosion inhibition performance of the ILs was assessed by measuring the corrosion rate of mild steel immersed in the CO2-saturated 0.1 M HCl solutions. The capability of ILs acting as multifunctional inhibitors reduces the incompatibility issue between hydrate and corrosion inhibitors in operations, as well as provides economic advantages. The findings from topic 2 contribute to the development of novel hydrate inhibitors and corrosion inhibitors based on ILs. In topic 3 (Chapter 5), the dissociation behavior of CH4 hydrate in the absence and presence of PVCap was closely investigated using a combination of experimental techniques and MD simulations. A slow and two-step dissociation of CH4 hydrate in the presence of PVCap was observed in experiments, followed by a computational investigation examining molecular behavior during the hydrate dissociation to figure out the origin of the slow dissociation. The findings from topic 3 provide valuable insights into the mechanisms of hydrate dissociation in the presence of kinetic hydrate inhibitors.