Pore-Level Interpretation of Methane Hydrate Growth and Dissociation with Deionized and Saline Water
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Natural gas hydrates are solid crystalline compounds composed of water and gas molecules, located in vast amounts around the world, in subsurface permafrost and oceanic environments. With an increasing demand of energy worldwide, methane production from natural gas hydrates may play an important role to ensure future energy sustainability. Research on methane hydrate on pore-level may contribute to a greater understanding of the fundamentals and characteristics of hydrate formation and production schemes. This thesis presents a series of experiments conducted in a two-dimensional synthetic micromodel. The main objective of the experiments was to determine and interpret methane hydrate characteristics on pore-level using microscopy, during hydrate formation and dissociation with different water solutions. In the experiments, deionized water and saline water of 2.0, 3.5 and 5.0 wt% sodium chloride (NaCl) were used. The saturation changes for water, gas and hydrate were estimated to determine water and gas behavior in each experiment. There were conducted 16 successful hydrate formations, where ten were primary formations and six were secondary formations. During primary hydrate formation, the temperature and pressure values were fixed, and hydrate growth was induced by forcing agitation on water and gas in the micromodel. The temperature was in the range 1.0-4.1°C, and the pressure was in the range 80.0-110.0 bar. During secondary hydrate formation, the temperature was approximately 4.0°C, and the pressure was increased to above the hydrate stability line. Primary hydrate formation was faster and more homogeneous than secondary hydrate formation, independent of water salinity. Hydrate growth occurred mainly within the gas, but was one time observed to occur in the water phase. Initial hydrate growth occurred on the water-gas interface at the pore walls and continued to grow in the gas towards the pore center. Salt was the limiting factor when hydrate was being formed with saline water of 3.5 wt% NaCl and higher, and in these experiments hydrate required greater driving forces to form. The hydrate formation rate was observed to decrease with increasing salinity. There were performed 13 controlled hydrate dissociations by either pressure reduction or thermal stimulation. The patterns of hydrate dissociation by depressurization were the opposite of the patterns of hydrate dissociation by thermal stimulation. When hydrate was dissociated by depressurization, hydrate generally dissociated from the center of the pores towards the water-gas interface at the pore walls. Further dissociation was favored when the hydrate was in direct contact with free gas bubbles. When hydrate was dissociated by thermal stimulation, hydrate generally dissociated from the water-gas interface at the pore walls towards the center of the pores. The experiments showed that with high connectivity between the hydrates, hydrate dissociated more uniformly. The saturation profiles gained from images showed that dissociation by thermal stimulation was more uniform than dissociation by pressure reduction. Hydrate was dissociated by reducing the pressure in increments of 0.7 bar or by increasing the temperature in increments of 0.1°C. The stepwise pressure and temperature technique showed that hydrate dissociated over a range of pressure and temperature steps. The required number of pressure and temperature steps from initial hydrate dissociation to complete hydrate dissociation, increased with increased salinity. This is believed to be due to a continuous decrease in salinity in the local water phases when pure water was liberated from the dissociating hydrate structure. The hydrate phase became more stable towards the low-saline water phases, and as a consequence of pore water freshening, the hydrate phase required lower pressures and higher temperatures to dissociate. Local hydrate saturations within the gas and isolated hydrate saturations in the pore space were observed to remain, while the surrounding hydrate dissociated. These local hydrate saturations seemed to become stable toward local low-saline water, and are believed to be the main reason why dissociation was prolonged for saline water.
PublisherThe University of Bergen
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