The Effect of Initial Water Saturation on Hydrate Saturation and the Electrical Resistivity Response of Methane Hydrate-Bearing Sediments

Methane hydrate, as a new potential energy source with abundant reserves and high energy density, is primarily distributed in deep-sea sediments and permafrost regions. Its exploration and development hold significant importance for the future energy structure. However, the occurrence morphology, formation, and decomposition dynamics of hydrates in reservoirs are difficult to observe directly. Resistivity, as a highly sensitive physical parameter to pore fluid type, connectivity, and phase changes, is considered an effective means of identifying hydrate growth and distribution. Yet, its response mechanism is significantly influenced by the initial reservoir conditions, and the underlying mechanisms remain unclear. This study investigated the variations in resistivity during the formation and decomposition of methane hydrate in sediments, with a focus on the influence of initial water saturation on resistivity. The results indicated that initial water saturation significantly affects the final hydrate saturation. When the initial water saturation increased from 30% to 48%, the final hydrate saturation rose from 42% to 53%. Changes in resistivity not only reflected the total amount of hydrate formed but also its spatial distribution, with high-resistivity regions corresponding to methane hydrate-enriched areas. As the initial water saturation increases from 30% to 48%, the resistivity value rose from 17,856 Ω·m to 123,280 Ω·m. Methane hydrate preferentially forms in the upper layer of the reactor, where the resistivity is higher. During hydrate decomposition, the overall resistivity decreased, but an abnormal increase in resistivity was observed, indicating localized secondary formation of ice or hydrate during decomposition. The resistivity was generally higher than the initial value, which is speculated to the reduction in pore water caused by gas carrying away some water during decomposition. This work demonstrated that resistivity can reflect the growth and distribution of methane hydrate and will provide theoretical guidance for the exploration and development of natural gas hydrates.