Influence of Modified Polyvinyl Caprolactam on Methane Hydrate Formation: Structural and Morphological Insights

Abstract

Controlled injection of kinetic hydrate inhibitors (KHIs) is one of the most effective flow-assurance strategies for delaying hydrate nucleation and crystal growth in multiphase natural gas pipelines. To enhance the structure of the kinetic inhibitor polyvinyl caprolactam (PVCap), oxyethyl and ester functional groups were introduced, resulting in the development of a new inhibitor, PVCap-XA1. PVCap-XA1 has been applied to achieve enhanced inhibition performance against methane hydrate formation. The structural and morphological changes in methane hydrates formed in the presence of PVCap-XA1 were investigated using advanced characterization techniques, including PXRD, low-temperature Raman spectroscopy, and cryo-SEM. Under identical experimental conditions, PVCap-XA1 demonstrates higher kinetic inhibition efficiency compared with PVCap. Microscopic analysis indicates that the clathrate structure of methane hydrate remains unchanged; however, PVCap-XA1 induces lattice-plane distortions that result in smaller crystallites. In the presence of PVCap-XA1, the induction time for hydrate formation was extended to approximately 240 min, and the maximum subcooling increased to 10.8 °C, compared with 8.1 °C for PVCap. In addition, PVCap-XA1 alters the cage occupancy ratio (IL/IS), making it more difficult for methane molecules to occupy hydrate cages. PVCap-XA1 also modifies the microscopic morphology of methane hydrate, shifting it from a porous to a dense, compact structure. This densification blocks gas flow through the hydrate layer, further enhancing inhibition efficiency. Overall, the results demonstrate that PVCap-XA1 is a promising kinetic inhibitor capable of addressing industrial challenges associated with methane hydrate formation in pipelines.