New insights into gas production behavior and seepage-wettability evolution during methane hydrate dissociation in sand matrix by NMR investigation

The commercial and sustainable exploitation of NGH reservoir requires a comprehensive understanding of gas production behaviors and seepage-wettability evolutions during hydrate dissociation process in hydrate bearing sediments. In this paper, the in-situ real-time CH4 hydrate dissociation process in sand pack was acquired with the aid of NMR technique. The results showed that the step-wise depressurization method could effectively prevent the ice generation and/or secondary hydrate formation phenomenon during NGH exploitation. When the pore pressure reached the hydrate dissociation surging point, the CH4 hydrate dissociation rate would enter a surge stage. Then, the hydrate dissociation rate of simple step-wise depressurization method kept steady for a relatively long time, while the hydrate dissociation rate of combined method showed a pressure-sensitive feature. And the optimum final CH4 recovery ratio of 92.85% was obtained in Case 7 with the finer step-wise depressurization combined method. The comparison between measured gas effective permeability and prediction value indicated that the accuracy of current widely used permeability prediction models was limited due to lacking consideration of wettability evolutions of hydrate bearing sediments. In this study, a novel NMR-based approach for quantitatively characterizing the overall wettability evolution of hydrate bearing sediments was proposed. It was found out that the hydrophilicity of hydrate bearing mixing system increased with the decrease of hydrate saturation during the dissociation process. And a modified Tokyo model coupled with wettability term was proposed to increase the normalized permeability prediction accuracy by 9.75%, which indicated the flow seepage behavior was governed by the coupling effect of hydrate pore habits and wettability of hydrate bearing sediments.