Experimental Study On Hydrates In Porous Media Using Magnetic Resonance Imaging

Hydrate has attracted worldwide attention because of its close association with energy recovery, flow assurance, transportation, climate change and safety issues. It gives a chance to the development of science, technology and human society, but it also brings about a lot of problems and challenges. In this study, a magnetic resonance imaging (MRI) experimental platform is designed and built up to study hydrate formation and dissociation rules in porous media. Studies on THF hydrate, CO2 hydrate and CH4 hydrate are carried out. The porosities of the porous media and hydrate saturations are calculated. Hydrate growth patterns are determined. And the effects of temperature, pressure and porous media on hydrate formation and dissociation are analyzed. This work has great theoretical and practical significances on the studies of hydrate formation and dissociation mechanism and hydrate exploitation.The structures of the porous media are studied using MRI. The experimental result shows the MRI mean signal intensity method and the bimodal threshold method can be used to determine the porosities of the porous media, but the former is not applicable to small size grains, and the latter will bring subjectivity into the selection of the threshold. The permeability calculated by the parallel capillary model or the Kozeny-Carman model or the improved Kozeny-Carman model is generally higher than the experimental result. A more accurate value is expected using the flow data acquired by MRI.The experimental studies on the effect of porous media on hydrate formation and dissociation suggest, the smaller the grain size is, the shorter hydrate nucleation time is and the faster hydrate growth rate is. As the temperature increases, the grain size has an increasing impact on hydrate nucleation. As the grain size diminishes, the dissociation rate becomes faster. But for THF hydrate, the grain size has little impact on the dissociation rate compared to the dissociation temperature.The experimental studies on the effect of temperature on hydrate formation and dissociation suggest, as the formation temperature decreases, the nucleation time gets shorter, and the growth rate becomes faster. Under the same pressure, the nucleation time has an exponential relationship with the undercooling, but the growth rate has a linear relationship with it. For the dissociation, the higher the dissociation temperature is, the faster hydrate dissociation rate is. The experimental studies on the effect of pressure on hydrate formation and dissociation suggest, as the formation pressure increases, the nucleation time is shorter, and the growth rate becomes faster. The growth rate almost has a linear relationship with the pressure at the same temperature. As the temperature decreases, the pressure has an increasing impact on hydrate growth rate. For the dissociation, the lower the pressure is, the faster the dissociation rate is.In summary, this work is undertaken to study hydrate formation and dissociation processes in porous media, which provides theoretical and experimental supports for the understanding of associated hydrate dynamics.