Effects of H2/N2 on CO2 hydrate film growth: Morphology and microstructure

Abstract

The addition of small-molecules gases such as H2/N2 can significantly improve CO2-CH4 hydrate replacement efficiency. However, the physical/chemical mechanism behind this phenomenon is not completely clear. Since CO2-CH4 hydrate replacement is the sequential process of CH4 hydrate decomposition and CO2 hydrate reformation, the microscopes and Raman spectroscopy were combined to clarify the effects of H2/N2 gas on CO2 hydrate film growth. The experimental results showed that H2 gas significantly affected the initial morphology, thickening and compactness of CO2 hydrate films. With the increase of H2 gas concentration, the size of the initial single hydrate crystal increased, the granulation phenomenon weakened, and hydrate films became rougher after thickening. Raman experiments showed that the presence of H2 made the CO2 hydrate film looser, which was conducive to the sustainability of hydrate growth. However, their growth rates did not change monotonously with the H2 concentration. We consider that H2 played roles in reducing the local CO2 gas concentration, slowing down the CO2 supply rate to the reaction interface, and delaying the time of the gas pores being blocked by further formed CO2 hydrate. N2 gas could also play the similar roles as H2 gas, however, due to its participation in CO2/N2 mixed hydrate formation, the growth of hydrate film changed adaptively. The results obtained are not only important for understanding the mechanism of CH4 hydrate replacement by CO2 and small molecule mixed gas, but also referential for other hydrate technologies involving mixed hydrate formation.