Microscope insights into gas hydrate formation and dissociation in sediments by using microfluidics

Natural gas hydrates (NGHs) have tremendous potential and abundant reserves worldwide. Both the hydrate distribution and the potential inverse formation are related to the efficient exploitation of NGHs; however, studies of the micro-mechanism of hydrate morphology and the evolution of phase transition processes are still lacking. In this study, hydrate formation and dissociation were investigated at the microscale using a microfluidics device. Methane hydrate (MH) was formed at a system pressure of 5 MPa and temperature of 274.15 K and then was dissociated by using the depressurization method. Based on different gas-water contact areas, two kinds of stable crystal structures and two kinds of unstable crystal structures of the micromorphology in the formation stage were identified. During the dissociation process, the direct proof of the induced local reformation of the hydrate by microbubble aggregation was given. First, the distribution of the CH4 bubbles (from 5 mu m to 140 mu m) inside the pores and throats was quantified. Normal bubble distributions were found, and the largest percentage of bubble diameters ranged from 10 mu m to 30 mu m. The average diameter of the bubbles increased with time, and the total number of bubbles decreased with time. The large density distribution of microbubbles with smaller diameters in liquids impeded the pressure propagation and heat transfer, which are keys to restraining the rate of hydrate dissociation. These findings are beneficial for understanding the microscale mechanisms of the hydrate phase transition and MH dissociation efficiency, which may be helpful for the selection and design of NGH exploitation schemes.