NMR-Compatible Sample Cell for Gas Hydrate Studies in Porous Media

The production of methane (CH4) from natural gas hydrate deposits via molecular replacement by injected, thermodynamically more favorable, carbon dioxide (CO2) is a promising method of energy production and carbon sequestration. However, the viability of this technique is constrained by mass-transfer limitations, which are in turn associated with diffusion of the injected CO2 into the hydrate-bearing sediment layers. Generally, the coupled heat- and mass-transfer phenomena associated with this replacement process in complex heterogeneous porous media are poorly understood. To facilitate the noninvasive pore-scale study of this replacement process (ultimately in sediment cores) using a range of nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) techniques, a novel NMR-compatible sediment holder has been designed and constructed. Via the provision of centralized sample cooling, sample temperature control is achieved at high pressures while keeping the NMR magnet system at the required temperature. Hydrate formation and dissociation processes in model porous media were successfully investigated using a CH4/C2H6 mixture and CO2. Novel one-dimensional (1D) MRI images of the residual liquid water and hydrocarbon gas were acquired during the hydrate formation and dissociation processes using a single-point ramped imaging with T-1 enhancement (SPRITE) MRI pulse sequence. Interleaved NMR T-2 relaxation measurements were also obtained to interrogate the pores sizes occupied by the residual water. From these results, the water distribution and subsequent hydrate-formation behavior have been spatially and temporally resolved throughout the porous media.