Numerical simulation on gas production from hydrate-bearing sediments by depressurization considering time-varying reservoir compressibility

Depressurization is a promising Natural Gas Hydrate (NGH) recovery method. However, the pressure drop can cause significant pore compression, particularly in clayey-silty reservoirs. Additionally, the decomposition of NGH will further weaken the reservoir strength, leading to the time-varying characteristics of reservoir compressibility. This study analyzed depressurization-based gas production under different NGH reservoir models and accounted for time-varying reservoir compressibility. The results showed that the increased reservoir compressibility caused by hydrate decomposition significantly affected the pressure propagation and permeability evolution, further diminishing gas production. The cumulative gas production from sandy reservoirs with lower reservoir compressibility could reach 60 times that from clayey-silty reservoirs with higher reservoir compressibility. The optimal bottom-hole pressure (BHP) gradually increased with the reservoir compressibility. For the reservoir with the higher compressibility, the cumulative gas production at a BHP of 3 MPa was 58% lower than at 9 MPa. Therefore, considering time-varying reservoir compressibility during hydrate decomposition is essential for accurately forecasting the gas production capacity of depressurization. Furthermore, BHP should be cautiously determined in reservoirs with higher reservoir compressibility.