The enhancement effect of water-gas two-phase flow on depressurization process: Important for gas hydrate production

Depressurization is one of the most efficient methods in the production testing of natural gas hydrates. However, problems such as hydrate reformation, ice generation and insufficient dissociation driving force in the later period of depressurization adversely affect the gas production. It has been confirmed that controlling the flowrate ratio of a water-gas two-phase flow can help enhance the hydrate dissociation. However, the effect of the water-gas flowrate ratio on the hydrate dissociation behaviors during depressurization is unclear. In this study, three dissociation modes involving a combination of water-gas flow and depressurization were examined: mode 1 (concurrent start of depressurization and water-gas flow), mode 2 (depressurization is first used to dissociate the hydrate, and the water-gas flow is initiated after 15 min), and mode 3 (depressurization is first used, and the water-gas flow is initiated after 30 min). The feasibility of water-gas flow to accelerate hydrate dissociation and mitigate ice generation was confirmed during the depressurization process. In all the modes, the higher water-gas flowrate ratio and lower dissociation pressure significantly increased the energy recovery rate and decreased the energy input. Additionally, the water-gas flow, especially that with a higher flowrate ratio, effectively accelerated the elimination of the dark-zone (mixture of ice and hydrate) by providing continuous heat transfer. Mode 1 corresponded to the highest energy recovery rate, lowest energy input and most rapid disappearance of the dark-zone under the same experimental conditions. Therefore, mode 1 was regarded as the most efficient mode to dissociate hydrate in an actual hydrate production.