Fluid production behavior from water-saturated hydrate-bearing sediments below the quadruple point of CH4 + H2O

Abstract Driven by the large resource volume, extraction of CH4 from hydrate reservoirs has received worldwide attention. Depressurization is a feasible method and has been widely used in past field tests. However, fluid production behavior from water-saturated hydrate-bearing sediments below the quadruple point of CH4 + H2O has not been well understood and warrants investigation. In this study, we designed a series of experiments using ultra-deep depressurization with bottom hole pressure (BHP) of 1.0 MPa, 1.5 MPa and 2.1 MPa in water-saturated hydrate-bearing sediments to investigate the fluid production behavior, particularly in the region with potential ice or secondary hydrate formation. The effect of hydrate saturations and pressure drawdown rates on fluid production below the quadruple point were systematically examined. Our experimental results show that ice formation was likely based on the temperature response, however, the self-preservation effect by ice formation prevents a further drop in temperature to its equilibrium temperature at the corresponding BHP. The gas and water recovery ratios increased by 6.5% and 49.1%, respectively for BHP below the quadruple point. No significant benefit of water trapping due to ice formation was observed. Faster pressure drawdown rate results in a more substantial temperature drop, which could trigger ice formation. Still, interestingly gas production was continuous and not affected in the phase diagram of ice + CH4. Higher hydrate phase saturation improves gas production rate and final gas recovery ratio, yielding a lower water-gas ratio for more economic-viable CH4 recovery. Our results indicate that ultra-deep depressurization could be one technical feasible option for increasing both gas production rate and final recovery ratio. The findings provide valuable information for designing an ultra-deep depressurization scheme based on SH of interest in future production tests.