NMR Investigation of Methane Hydrate Formation and Dissociation Behavior Induced by Heat Flow in Sandy Porous Media

Clarifying the heterogeneity of methane hydrate (MH) formation and decomposition induced by heat flow in different hydrate deposits is of critical importance for gas hydrate drilling and exploitation operations. In this paper, two sand packing samples with different sizes of 250–270 and 96–109 μm, respectively, were adapted to prepare hydrate-bearing sediments (HBSs), and a low-field nuclear magnetic resonance (LF-NMR) system was employed and modified by combining a gas/water two-phase flow system to quantitatively and visually study the heterogeneity of MH formation/dissociation in the two sand samples during the heat flow invasion. Results showed that methane hydrate formed heterogeneously in porous media, and MH formation rates were negatively correlated with the particle sizes of the sediments. In the whole process of heat injection, uneven distributions of hydrate saturation with low left and high right were shown in the two samples, suggesting that it was difficult for the heat to transfer over long distances. In this process, the dissociation rate of methane hydrate was related to the fluid flow resistance and heat supplement simultaneously, and hydrate decomposition induced by heat stimulation could be accelerated with the increase in the size of quartz sand, which was composed of HBSs. Additionally, the hydrate dissociation fronts expanded with distinct patterns for the hydrate sediments with different sand sizes. When hot water was injected into the HBS filled with larger grains, MH preferentially dissociated in the bottom and inlet parts of the chamber, and the MH decomposition front expanded with a wedge-shaped pattern. In the sediment with smaller grain size, MH was given priority to dissociate from the middle to the edge sides along the coronal planes (X–Z planes) because injected water could be hindered to transfer along a radial direction by the tight seepage channel.