Comparison of fluid production between excess-gas and excess-water hydrate-bearing sediments under depressurization and its implication on energy recovery

Methane hydrates are considered as the future energy due to its vast resource volume and high energy density. The fluid production and thermal response of two types hydrate-bearing sediments (i.e., excess-gas and excesswater) under controlled depressurization are still unclear and warrant investigation. In this study, we devised two different hydrate-bearing sediments (HBS) synthesis methods and synthesized excess-water (SA = -27.2%) and excess-gas (SG = -26.5%) HBS with SH of 72.0%. The hydrate dissociation kinetics and fluid production behavior were examined under three bottom-hole pressures, i.e., 3.0, 5.0, and 7.0 MPa. Gas production from the excess-gas HBS follows two-stage profile, while continuous gas production was observed after SG reaches 6.0% in the excess-water HBS. Water production from excess-gas HBS was significantly delayed compared with excesswater HBS and only started when SA reached above 22.5%. A logarithmic water production profile was observed in all cases. Rapid temperature drop due to hydrate dissociation is significantly delayed in excess-gas cases. Heat transfer from surroundings is relatively slow due to its low composite thermal conductivity. The findings on the contrast fluid production behavior between excess-gas and excess-water cases shed light on optimizing production strategies for future field production trials from these two different types reservoirs.