Influence of Aspect Ratio of Migration Space on Gas Migration and Accumulation Mechanisms of Different Types of Gas Reservoirs

With the continuous development of unconventional natural gas resources, the formation mechanisms of different types of gas reservoirs have become a hot topic of current research. The migration mechanisms of gas in various types of conductive media play a crucial role in studying the formation and distribution of different types of gas reservoirs. In studying natural gas migration, the pressure difference between the source and reservoir and buoyant force are generally considered the main driving forces for gas migration, while the resistance mainly comes from the capillary pressure of the reservoir. In studying capillary pressure, a circular shape is typically used as the basic model for pores or throats. The magnitude of the capillary pressure is inversely proportional to the radius of the pore or throat. However, this study conducted experiments on gas migration in circular pore models, fracture models, sandstone rock models, and pore-fracture dual models. The experimental results showed that the aspect ratio of the migration medium has an important impact on gas migration. In spaces with high aspect ratio, the gas can undergo deformation during migration, significantly reducing the capillary resistance it encounters, and under certain conditions, capillary pressure can also become a driving force for gas migration. In circular spaces, the buoyant rise of gas must satisfy the condition that connected free water can form above and below the gas column, and water can freely flow downward during the gas column's ascent. Otherwise, even if the buoyant force experienced by a continuous gas column of a certain height exceeds the capillary force of the pores, it is difficult for gas to migrate. In pores of reservoir rocks, gas often migrates in the form of bubbles, making it difficult to form a continuous gas phase, and so gas migration under buoyant force is relatively difficult. However, gas migration is easier in fractures and faults with high aspect ratio. Faults are important pathways for gas migration from deep to shallow layers, and they are also crucial for studying the correlation between shallow gas reservoirs and deep enriched gas reservoirs. This paper proposes that the aspect ratio of the migration space positively affects gas migration from the perspective of capillary pressure, improving the existing models of natural gas migration and accumulation. This is significant for understanding the formation mechanisms of different types of gas reservoirs. However, this study primarily focused on quantitative research. Further research is needed to explore the numerical relationship between the aspect ratio of pore spaces and capillary pressure, as well as the specific impacts of factors such as the density and viscosity of two-phase fluids on the experimental results and the evaluation methods of the aspect ratio of reservoir pores.