Representative Volume and Seepage in Gas Hydrate-Bearing Sands: Insights from Improved Pore Network Modeling

Natural gas hydrates are considered as an important clean energy resource due to their vast reserves and widespread distribution. However, commercial extraction faces significant challenges. Among these, gas–water seepage characteristics are critical factors influencing extraction methods, while the representative elementary volume (REV) remains inadequately understood as a fundamental parameter for evaluating the effectiveness of seepage parameters in hydrate-bearing sediments (HBSs). This study systematically analyzed CT images from different gas hydrate formation processes using an improved pore network model. The pore space of HBSs before and after hydrate formation was compared and analyzed using connectivity and morphological parameters. The REV length of HBSs was measured based on porosity, absolute permeability, and shape factor. Our findings reveal that accurate hydrate distribution patterns can be obtained through the analysis of connectivity and morphological parameters. For granular HBSs, the REV remains present regardless of the hydrate distributing mode, and the increase in the REV length after hydrate formation does not exceed 19.5%. Porosity can serve as a reliable parameter for evaluating the REV length of HBSs. Hydrate formation increases the capillary pressure and irreducible water saturation of HBSs and primarily influences the gas phase’s relative permeability.