Formation and dissociation mechanisms of clathrate hydrates
Chapman, Walter G.
Doctor of Philosophy
To better understand hydrate formation and dissociation mechanisms, Nuclear Magnetic Resonance (NMR), Magnetic Resonance Imaging (MRI), and viscosity measurements were employed to examine the hydrate transition processes of tetrahydrofuran (THF) - water (D2O or H2O) solution. Specifically, Spin-Lattice Relaxation Time (T1) and Spin-Spin Relaxation Time (T2) of THF in D2O were measured before hydrate formation, during hydrate formation, during hydrate dissociation, and after hydrate dissociation to probe the local molecular ordering changes around THF molecules. Hydrate formation and dissociation patterns were imaged using MRI. The viscosity of THF/H2O solution was monitored before hydrate formation and after hydrate dissociation using Champion Technologies Hydrate Rocking Cell (CTHRC) to investigate the residual viscosity phenomenon. NMR relaxation time results demonstrated that the presence of hydrate phase strongly influences the fluid structure of the coexisting liquid phase. T2 distribution technique was proven to be an effective tool in measuring the dynamic behavior of THF molecules in the hydrate phase and the liquid phase independently and concurrently. Comparison of T1's of THF in D2O solution during hydrate formation with that during dissociation revealed evidence of residual hydrate structures remaining in the liquid phase. Residual viscosity (as measured by CTHRC) was absent after THF hydrate dissociation. It was suggested that the residual viscosity observed by other groups after natural gas hydrate dissociation was more likely due to higher than equilibrium gas concentration than residual hydrate clathrate structures. To enable direct and accurate measurements of gas hydrate behavior in black oil, liquid-state proton NMR spectroscopy was innovatively applied to monitor the water peak area change in the NMR spectrum of water-in-oil emulsion during hydrate formation and dissociation. Because water in the hydrate phase does not contribute to the water peak area in such a spectrum, as water is being converted into hydrate, the water peak area would decrease. Results validated that it is feasible to directly and accurately monitor hydrate behavior in black oil using this technique.