Numerical Study of Long-Term Natural Gas Production of Hydrate Composite Reservoirs in the South China Sea by Depressurization Using Horizontal Wells

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The South China Sea (SCS) holds abundant natural gas hydrate resources. Results from the second trial production project have confirmed the high development potential of clayey-silty composite reservoirs, which consist of the hydrate layers, the mixed layers, and the gaseous hydrocarbon layer from top to bottom, and demonstrated the wide applicability of the depressurization method and horizontal well technology. Investigating the evolution of these composite reservoirs under natural gas production using the depressurization method and horizontal wells is critical for advancing the commercial viability of natural gas hydrates in the SCS. In response, this study combines geological data from the second trial production site in the SCS with laboratory mechanical data, employing COMSOL MULTIPHYSICS software to model the production process in natural gas hydrate composite reservoirs using horizontal wells. This model comprehensively simulates gas and water production trends, along with the evolution of physical fields within the reservoir, over a three-year depressurization period. The research findings indicate the following: (1) In the initial production phase, gas production rapidly rises to 5.61 × 10 4 m 3 /day and then declines continuously, stabilizing around 0.65 × 10 4 m 3 /day, while water production consistently increases. (2) Hydrate dissociation is concentrated primarily near the wellbore, with significant changes in hydrate saturation and pore pressure. Due to permeability differences, the mixed layer shows a more pronounced variation, while the hydrate layer remains relatively stable. (3) A reduction in pore pressure initiates stress reorganization within the reservoir, producing significant stress concentration around the wellbore. Here, the Von Mises stress and mean effective stress are markedly higher than in the far-field regions, creating a persistent stress concentration zone that has critical implications for reservoir stability. (4) Long-term hydrate production induces notable deformation in strata surrounding the horizontal well, with the most pronounced deformation observed in the hydrate layer above the well and the overlying layer. Maximum deformation reaches 6.2 m, exhibiting a “U”-shaped downward trend toward the seabed. The results of this study are expected to provide support for understanding the evolution of clayey-silty composite reservoirs during the long-term production of natural gas hydrates.