Enhancement of hydrocarbon recovery from CH4-C2H6-C3H8 mixed hydrates via gas sweep

Abstract

Mixed hydrates have been found in some ocean and permafrost deposits, and the dissociation of mixed hydrates by depressurization alone has been reported to be difficult and inefficient. However, most studies in the field of mixed hydrate decomposition have mainly focused on dissociation behaviors at the crystal lattice and guest molecular levels, but there has been little discussion about recovery efficiency. The dissociation of CH4-C3H8 or CH4-C2H6-C3H8 mixed hydrates in sandy sediments using depressurization followed by the gas sweep method was investigated to determine the underlying mechanism of difficulty in decomposition and to enhance hydrocarbon recovery. It was found that CH4 and C2H6 were preferentially released over C3H8 from mixed hydrates in the depressurization stage. Particularly, the dissociation of mixed hydrates tended to stop despite substantial hydrates remaining above the water freezing point, which was similar to the self-preservation effect of CH4 hydrates below the ice point. Interestingly, when gas sweep was performed, mixed hydrates continued to decompose, and more C3H8 was recovered at the beginning of the gas sweep. Compositional analysis of the dissociated gas confirmed that the gradual formation of C3H8-rich hydrate shell led to the cessation of hydrate decomposition, which hinders the diffusing hydrocarbons in the internal hydrate to be released. Furthermore, continuous gas sweep resulted in continuous and rapid decomposition of mixed hydrates, but when it was stopped, the decomposition of mixed hydrates gradually slowed down and tended to stop, and the C3H8-rich hydrate shell was re-formed. This suggested that the formation and decomposition of the C3H8-rich hydrate shell may control the decomposition rate of mixed hydrates. These results are of significance for guiding successful and efficient hydrocarbon recovery from mixed hydrates.