Gas production enhancement effect of underlying gas on methane hydrates in marine sediments by depressurization

Abstract

The marine hydrates with underlying gas show great commercial potential as a new cleaner alternative energy. Yet the effects of underlying gas on the production characteristics of marine hydrate reservoirs are still unclear. In this study, the clayey-silt marine sediments sampled in South China Sea were used to synthesize the excess-water methane hydrate samples with consistent porosity (38%) and three-phase saturation. Depressurization was conducted to produce gas from hydrate reservoirs and underlying gas with different pressures (3–6 MPa). The results indicate that the underlying gas pressure has a positive correlation on hydrate decomposition, gas production, and water production; yet its effect on the gas–water ratio is not significant. As the underlying gas pressure increased from 3 MPa to 6 MPa, the average temperature drop of the entire hydrate reservoir was reduced by more than 0.5 K, and the time required for 90% hydrate decomposition (t90) was shorted by nearly half. The underlying gas flowing into the hydrate reservoir is beneficial to alleviate the drops of pressure and temperature and promote the formation of gas–water flow channels. Therefore, the production of underlying gas can not only improve the gas production potential but also effectively contribute to hydrate decomposition and avoid ice formation. Through comprehensive evaluation of hydrate decomposition time, reservoir temperature drops, and average gas–water ratio, the enhancement effect of underlying gas on hydrate reservoir exploitation is preliminarily demonstrated. The results of this study prove the importance of underlying gas and show the prospect of further research on marine hydrate reservoirs with underlying gas for commercial exploitation.