Experimental investigation into gas recovery from CH4-C2H6-C3H8 hydrates by CO2 replacement

Abstract

The replacement of CH4-C2H6-C3H8 ternary hydrates in sandy sediments by gaseous and liquid CO2 was studied to explore the gas recovery from structure II (sII) gas hydrate reservoirs. It was found that the swapping pattern in sII mixed hydrates is significantly different from that in the pure methane hydrate, which is a structure I (sI) hydrate. CH4, C2H6 and C3H8 can be recovered heterogeneously from the mixed hydrates by CO2 replacement. The results showed CH4 was the easiest to replace in all of the experimental runs. In addition, there was a preferential release of C3H8 over C2H6 with higher mole ratio of CO2 to initial hydrate. The guest-to-cavity size ratio was used to qualitatively study the gas exchange kinetics in the mixed hydrates. It was proposed that the isostructural conversion and structural transition coexisted in the mixed hydrates replacement. The spatial distribution of multiple components in the replaced hydrate particles was obtained by coupling gas production measurements with compositional analysis of the hydrate dissociation. It was found that further hydrate structure-transition from sII to sI occurred in deeper layers of the hydrate particles in replacement with higher mole ratio of CO2 to initial hydrate. A power function relation was found to exist between the replacement efficiency and mole ratio of CO2 to hydrate. These results are of significance for guiding successful sII hydrate recovery and CO2 sequestration.