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Most of the implemented marine gas hydrate test exploitation in the world adopt the depressurization method to break down the hydrate in the reservoir into natural gas and then extract it, but because the gas production results are still a certain distance away from the commercial exploitation, and it mainly stays in the stage of theoretical research and trial exploitation. Based on two trial productions in the Shenhu area of the South China Sea, this study established a model for hydrate exploitation and investigated the impact of different well types on the recovery rates of hydrates and free gas in different development layers during depressurization. For the Class 1 hydrate reservoirs, horizontal wells are the optimal solution to extract hydrate and free gas simultaneously when exploiting the hydrate three-phase layer. Meanwhile, the effect of different well spacing in vertical wells on the recovery rate of hydrate and free gas was studied. It is found that the best recovery efficiency is achieved when the spacing between two wells is 80 m. The lower the bottom flow pressure of the well, the higher the production capacity, but its influence is limited. [ABSTRACT FROM AUTHOR]

Failure mode prediction of hydrate-bearing sediments (HBSs) is crucial for accurate evaluation of reservoir mechanical properties and safe exploitation of hydrate resources. In this study, the variation of HBS failure mode with hydrate dissociation degree (D h d ) and effective confining stress ( σ 3 ′) and its micro-mechanism were investigated. The results show that the stress-strain curve of HBS shifts from strain-softening to strain-hardening as D h d and σ 3 ′ increase. The failure strength and residual strength of the sample are negatively correlated with D h d but positively correlated with σ 3 ′. The increase of D h d results in a reduction in the cohesion of HBS from 608.74 to 2.49 kPa and a shift in the internal friction angle from increasing to decreasing. The post-shear sample indicates a minimum porosity of 11.47% under the highest D h d and σ 3 ′ conditions. The increased pore volume available for particle rearrangement resulting from the increase in D h d , and the enhanced confining effect on the sample due to the increase in σ 3 ′ , result in a volume shrinkage characterized by plastic strain-hardening failure mode. A prediction model of HBS failure mode based on the classification of strain-hardening and strain-softening degrees of stress-strain curves was developed and verified using published results. • The failure mode and its microscopic mechanism of HBS were studied. • The strain-hardening and softening degrees of stress-strain curve was classified. • A prediction model of HBS failure mode was established. [ABSTRACT FROM AUTHOR]