An Experimental Study of Methane Hydrates in Sandstone Cores
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Natural gas hydrates exist in large quantities around the world, located in the subsurface of permafrost and oceanic environments. Future energy harvest from production of methane gas encapsulated in natural gas hydrates can be made viable through extended research on fundamental characteristics of hydrates and proposed production schemes. Experimental studies of hydrates on core-scale give fast and valuable input to aid in planning of field tests, and the controlled environment in which laboratory tests are conducted enables the possibility to look at individual parameters. In this thesis methane gas hydrates have been formed in sandstone cores with high intrinsic permeability. The initial brine salinity has been kept at 3.50 wt% sodium chloride and initial water saturation has ranged between 0.57-0.70 [fraction of pore volume]. Three cores were subsequently injected with a mixture of 60% N2 + 40% CO2 [mole percent] and pure nitrogen to induce recovery of methane gas, and the potential of fluid flow through the cores were especially examined. A stepwise pressure reduction scheme was performed on cores containing both pure methane hydrates and mixed CH4 + CO2 hydrates. The pressure depletions were conducted from one end of the cores, and differential pressures were monitored along with recognitions of dissociation pressures. One MRI (magnetic resonance imaging) experiment was performed using cyclopentane hydrates at atmospheric pressure for initial testing of a new MRI instrument. The formation of hydrates was conducted with temperatures varying between 0-4 °C, and the final hydrate saturation seemed to increase when formation temperatures were less than approximately 1 °C. Salinities and initial water saturations were kept fairly constant, but the results have been implemented with earlier research conducted by the hydrate research group at the Department of Physics and Technology. The observed trends related to hydrate growth can be summarized as followed: increased hydrate saturation for intermediate initial water saturations in the range of 0.50- 0.70 [frac.], increased final water saturation with increased initial water saturation and increased hydrate saturation with decreased initial brine salinity. Two hydrate formations were complemented with resistivity measurements which indicated different growth patterns. One successful CH4-CO2 exchange was carried through with injection of 60% N2 + 40% CO2 [mole percent]. Initial injection of pure nitrogen gas led to a vanishing differential pressure, and a conservative estimate of methane recovery from hydrates of 0.25 was obtained after injection of 3.5 pore volumes [frac.]. It was attempted to inject pure nitrogen in two other cores with the result of an immediate build-up of differential pressure due to clogging. Injectivity could only be regained by thermal stimulation inducing hydrate dissociation. This serves as evidence of nitrogen's incapability of guaranteeing fluid flow in hydrate-filled cores with excess water. Stepwise pressure reductions showed that hydrate dissociation occurred at slightly elevated pressures compared with theoretical dissociation pressures. The observed dissociation pressures were not distinct and methane gas was liberated through a pressure range below the start of dissociation. The effect of a decreasing salinity is believed to contribute the most to gradual dissociation in core experiments containing a conserved mass of salt.