Tuning the fluid production behaviour of hydrate-bearing sediments by multi-stage depressurization.

• Hydrate dissociation induced by multi-stage depressurization (MDP) were conducted. • Fluid production behaviour and heat transfer characteristics were analysed. • MDP improves the energy efficiency and increases the minimum temperature approached. Natural gas hydrates (NGH) are globally abundant, high in energy density, and are a clean energy source with great potential. The efficient and safe production of CH 4 from NGH has attracted widespread attention in the scientific and industrial fields. Depressurization (DP) has been assessed to be a technically feasible method of producing CH 4 from hydrate-bearing sediments (HBS). However, technical challenges (e.g. sand production and flow assurance issues) remain when using direct fast single-stage DP. Multi-stage depressurization (MDP) has been proposed to be an effective solution but less understood. In this study, we designed a series of experiments employing MDP processes (2-stage, 3-stage, 6-stage, and up to 10-stage DP) to dissociate water-saturated HBS with high S H > 56 vol%. We examined the fluid production behaviour, the evolution of temperature, and evaluated the energy efficiency ratio of the MDP processes with a comparison with single-stage DP process. Increasing the number of DP stages with finer steps controlled the rate of hydrate dissociation and significantly reduced the cumulative fluid produced in each stage. However, the overall recovery was not altered and largely depends on the initial and final thermodynamic states. Fluid production was driven by different mechanisms: gas production driven by heat transfer yielded the highest recovery at final P = 3.0 MPa, whereas water production driven by pressure drawdown resulted in the highest recovery when P first dropped below P eq = 4.6 MPa. Moreover, increasing the number of DP stages leads to an increase in the minimum temperature by 2.2 °C and an overall energy efficiency ratio improvement by 36.6%. The findings of this study could be significant in tuning the water–gas production performance and in designing the optimal MDP strategies in future field production tests. [ABSTRACT FROM AUTHOR]