Quantitative Assessment of Field-Scale Natural Gas Hydrate Production Potential and Efficiency via Horizontal Well Depressurization

Natural gas hydrates are widely recognized as a promising alternative to conventional fossil fuels. During depressurization, production efficiency gradually declines due to mass and heat transfer limitations within the maximum recoverable zone. This study integrates analytical solutions with numerical simulations to quantitatively evaluate field-scale production efficiency decline and identify high- and low-efficiency dissociation zones. Several indicators are proposed to characterize the effective production duration and the high-efficiency production zone in horizontal well depressurization. A stratified weighted average method is developed to extend the analytical model, enabling consideration of geothermal and pressure gradients as well as horizontal well configurations. By analyzing the mass and heat transfer characteristics of different reservoir types and well configuration strategies, optimal well configurations are recommended. The recoverable zone with enhanced permeability and the critical well length required for commercial production for field-scale in the South China Sea hydrate reservoir are predicted. Results show that placing the horizontal well in the middle gas hydrate-bearing layer (GHBL) promotes long-term production and yields a higher gas water ratio. During hydrate exploitation through horizontal well depressurization, the recovery factor (Rout) varies slightly with permeability (0.18–0.21). However, with sufficient heat supplementation, Rout increases markedly and continues to rise with increasing permeability. Therefore, a combined strategy of permeability enhancement and heat injection is recommended for commercial exploitation. Energy return on investment (EROI) analysis indicates maximum economic efficiency when the GHBL permeability reaches 144 mD; however, considering fracturing costs, targeting permeability below this value is advisible. These findings provide theoretical guidance for optimizing gas hydrate production via depressurization.