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

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 CH4 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 CH4 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 degrees 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.

Automated summary

Natural gas hydrates are high-energy-density and clean energy source with great potential for producing methane. Multi-stage depressurization processes have been investigated for dissociating water-saturated hydrate-bearing sediments, showing that increasing the number of stages and refinements can control hydrate dissociation rate and significantly reduce cumulative fluid production in each stage. Different mechanisms, driven by heat transfer and pressure drawdown, affect the fluid production behavior, with increasing number of stages leading to improvements in minimum temperature and energy efficiency ratio.