Research on the Kinetics of Carbon Dioxide-Rich Natural Gas Hydrate Formation and the Applicability of Commercial Kinetic Inhibitors

During the development of deepwater and ultra-deepwater gas fields, the low-temperature and high-pressure conditions are highly conducive to the formation of natural gas hydrates. Particularly in produced gas with high CO2 content, hydrates nucleate and accumulate more readily compared to conventional natural gas. Kinetic inhibitors, as an effective method for preventing hydrate formation, have been employed in conventional oil and gas extraction operations. It is not known whether the kinetic inhibitors used for normal gas can be applied to high-carbon natural gas. To answer above question, it was first conducted that a study on the growth kinetics of high-carbon natural gas hydrates. 50% CO2/CH4 gas mixture was used in experiments. The results showed that it was easier to nucleation in the CO2-rich system. The nucleation subcooling of the 50% CO2/CH4 hydrate was 5. 69 °C, lower than that of the methane hydrate (6.31 °C). Moreover, the initial growth rate of the CO2-rich hydrate was approximately twice that of the methane system, but the growth rate decelerated in the later stage, and the total formation time extended to more than twice that of the methane system. Then, the inhibitory performance of PVCap in different solvents was examined for the CO2-rich system and methane system. Experiments showed that under the same conditions, PVCap had a kinetic inhibitory effect on high-carbon natural gas hydrates, but the effect was weaker than that on methane hydrates. The PVCap in diethylene glycol monobutyl ether increased the subcooling of the CO2-rich system to 7.21 °C (an improvement of 1.52 °C), while that of the methane system was raised to 10.70 °C (an improvement of 4.39 °C). The inhibitory effect of PVCap on the growth rate of the CO2-rich system was also not significant. The above results indicate that the existing commercial inhibitors can be used in high-carbon natural gas pipelines, but their effectiveness is somewhat reduced. New inhibitors that are more suitable for high-carbon natural gas should be developed.