Effects of PVCap on Gas Hydrate Dissociation Kinetics and the Thermodynamic Stability of the Hydrates

Kinetic hydrate inhibitors (KHIs) are used in the oil and gas industry to prevent hydrate plug formation. Understanding the influence of KHIs on hydrate dissociation is important in order to evaluate the implication the additives have on gas hydrate remediation. The present study has examined decomposition of structure I, II, and H hydrates formed with 750, 1500, 3000, and 6000 ppm (i.e., 0.075 to 0.6 wt %) of the KHI polyvinyl caprolactam (PVCap) present in the initial solution under isochoric conditions. The hydrates were formed at various pressures ranging from 45 to 175 bara and dissociated by applying heating rates from 0.0125 to 0.2 degrees C/h. Elevated hydrate dissociation temperatures as compared to the uninhibited system were experienced during all experiments. The measured final dissociation temperature increased with increasing heating rate probably due to effects on dissociation kinetics, and the final dissociation temperature was significantly higher than the equilibrium temperature for the corresponding noninhibited system even at very low heating rates. In a previous study at pressure around 45 bara, we left the PVCap system 1.0 degrees C outside the hydrate region over a period of 13.7 days, and dissociation apparently decayed asymptotically toward a limit around 1.5 bar inside the hydrate region. This indicates that there exists a thermodynamic effect in addition to a kinetic effect for dissociation of hydrates formed with PVCap present. During dissociation, the amount of gas released as a function of increasing temperature was less for the PVCap systems as compared to the uninhibited reference system at similar pressure. This pointed in the direction of reduced fill fraction of gas in the PVCap treated system. Raman spectroscopy showed that structure I hydrates formed with PVCap had a lower large-to-small cage occupancy than hydrates formed without PVCap. Structure II hydrates formed with PVCap indicated that there was no methane in the large cages but only in the small cages. Hydrates formed without PVCap did contain methane in both small and large cages. These findings supported the observations from dissociation experiments that PVCap both increase the thermodynamic stability of the hydrates and affects the dissociation kinetics toward slower dissociation rates.