Evaluations of Heterogeneous Dissociation Front Evolution and Long-Term Production Performance Under Different Depressurization Schemes in the South China Sea

Abstract

As natural gas hydrate production has transitioned from feasibility validation to industrial development, the engineering objective shifted toward achieving efficient, stable, and economically viable production. There are still some challenges in heterogeneous hydrate geological modeling in optimizing the depressurization strategy. Based on the first hydrate production test in the Shenhu area of the South China Sea, this study constructed a novel heterogeneous geological model by discretizing hydrate heterogeneity. Different depressurization schemes were then developed according to real-time geological responses, including pressure, temperature, and hydrate saturation. Modeling and simulation results showed that different depressurization schemes had little effect on long-term production performance. In contrast, gradient depressurization could better balance dissociation efficiency and sediment stability, and was thereby regarded as the optimal depressurization scheme. In heterogeneous hydrate sediments, the depressurization preferentially propagated rapidly along low-saturation pathways, forming discrete dissociation hotspots, while high-saturation regions retained residual hydrate islands, significantly reducing final gas recovery. The free gas layer (FGL) acted as a high-speed pathway for pressure propagation and a gas buffer, rapidly transmitting the depressurization effect over the long-term of production and providing a sustained heat supply for hydrate dissociation. The challenges posed by heterogeneous hydrate distribution could be overcome by employing fine-scale geological modeling to identify preferential pathways and by developing adaptive depressurization control and multifield coupled models. These findings can provide theoretical guidance for optimizing heterogeneous geological modeling and depressurization strategies for hydrate industrial developments.