Numerical Simulation on Depressurization-Driven Production of Class I Hydrate Deposits with Transition Layer and Perforation Modes Optimization

Natural gas hydrate widely exists in the South China Sea as clean energy. A three-phase transition layer widely exists in low permeability Class I hydrates in the Shenhu offshore area. Therefore, taking into account the low-permeability characteristics with an average permeability of 5.5 mD and moderate heterogeneity, a 3-D geological model of heterogeneous Class I hydrate reservoirs with three-phase transition layers is established by Kriging interpolation and stochastic modeling method, and a numerical simulation model is used to describe the depressurization production performance of the reservoir. With the development of depressurization, a specific range of complete decomposition zones appear both in the hydrate and transition layers. The entire decomposition zone of the whole reservoir tends to outward and upward diffusion. There is apparent methane escape in the three-phase transition layer. Due to the improvement of local permeability caused by the phase transition of hydrate dissociation, some methane accumulation occurs at the bottom of the hydrate layer, forming a local methane enrichment zone. The methane migration trends in reservoirs are mainly characterized by movement toward production wells and hydrate layers under the influence of gravity. However, due to the permeability limitation of hydrate reservoirs, many fluids have not been effectively produced and remain in the reservoir. Therefore, to improve the effective pressure drop of the reservoir, the perforation method and pressure reduction method were optimized by analyzing the influencing factors based on the gas production rate. The comparative study demonstrates that perforating through the free gas layer combined with one-time depressurization can enhance the effective depressurization and improve production performance. The gas production rate from perforating through the free gas layer can be twice as high as that from perforating through the transition layer. This study can provide theoretical support for the utilization of marine energy.