Theoretical investigation of threshold pressure gradient in hydrate-bearing clayey-silty sediments under combined stress and local thermal stimulation conditions

Abstract Due to the characteristics of smaller grain size and higher clay mineral content, a threshold pressure gradient (TPG) exists in multi-phase flow within hydrate-bearing clayey-silty sediments (HBCSS), which significantly affects the hydrate production. However, the dissociation of hydrates can result in cementation loss, changes in effective stress, and variation in local temperature. As a result, the TPG in HBCSS differs from that of conventional geotechnical materials. Until now, the understanding of TPG in HBCSS with complex pore structures and hydrate distribution is unclear. In this study, we have developed a theoretical TPG model for HBCSS that takes various factors into account, such as effective stress, temperature increase, pore structures, hydrate saturation, and growth patterns. The proposed TPG model for HBCSS has been thoroughly validated using available experimental data. Additionally, we conducted a parameter sensitivity analysis based on this derived model, revealing a positive correlation between TPG and both effective stress and temperature increase. Furthermore, while TPG generally increases with higher hydrate saturation when other parameters are held constant, the relationship between TPG and hydrate saturation is non-monotonic. This observation suggests that TPG is influenced not only by hydrate saturation but also by other factors, including hydrate growth patterns and pore structures. The findings of this study establish a theoretical foundation for characterizing the nonlinear flow behavior during hydrate exploitation.