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NMR CT device for studies on in situ processes in the pore space under high pressure

To study the formation and decomposition of gas hydrates in porous media a novel, universally useable NMR cell was developed, and adapted to the dimensions of the magnetic field of a low frequency NMR-imaging tool manufactured by Magnetic Resonance Instruments. Using this high pressure NMR-cell, equipped with a precise temperature and pressure control including temperature gradient regulation, the growth behaviour of gas hydrates near to equilibrium condition was accurately investigated. A rather universal set up was chosen as peripheral device to be able for fast adaptation on new experimental necessities. This equipment is universally usable for studies on processes under high pressure.

Because non-conductive pressure bearing material is a prerequisite to perform NMR-measurements, a ceramic high-pressure cell for NMR-studies on the distribution of gas hydrate was developed and fabricated. The novel core holder cell is fabricated from a silicon nitride tube. Tests with this type of tube showed, that the pressure bearing part can withstand 1000 bar. The sediment model, sand packs or consolidated sand stone cores are inserted into shrinking tubes made from FEP. For application of overburden pressure the liquid D2O is used in the annular space, to avoid any additional proton signal during NMR-measurements.

For studies on gas hydrate behaviour accurate thermostating is essential. Since the maximum diameter of the core holder is limited to 40mm, the core holder has got only narrow outer annular spaces. For this reason a thermal insulation by evacuation of the outer annular sleeve is applied. D2O was chosen as thermostating fluid. To avoid condensation of H2O into the coolant a closed circuit is installed. A gear pump with a maximum flow rate of 4 L∙min-1 is used at the inlet of the heat exchanger coil to apply a high velocity in the narrow annular space of the NMR-cell. To induce a temperature gradient the top of the cell can be heated additionally.

Because a quantitative measurement of axial distributions is restricted to 40 mm, due to the magnetic field characteristic, the vertical position of the cell can be changed accurately during the high pressure experiments simply by using a lifting truck bearing the mounting of the cell. By this procedure profiles with a total length of 160 mm can be achieved.

The peripheral equipment allows flooding of the core from the top or bottom with gas, water, or water containing a predetermined amount of dissolved methane. The total peripheral device is mounted on a cart near by to the NMR-apparatus. Experiments can be carried out at low formation / decomposition rates. The changes in the vertical distribution of gas hydrates can be measured during these experiments.

The following conclusions can be drawn from the NMR-studies using this equipment:
- The growth- and decomposition processes in the pore space are governed by transport phenomena,

- At high initial gas saturation growth processes expand rapidly over large areas after nucleation. Largely extended pore arrays are commonly involved in unsteady growth behaviour. Under these condition channels for the transportation of free gas remain open.

- At low initial gas saturation the start of the gas hydrate formation is a local phenomenon. Convective transport of the gas component is not initiated; transport channels for gas are not formed.

- After nucleation at the gas/liquid interface solid gas hydrate are formed hindering or even blocking the transport process for some time. Gas bubbles enclosed by such films collapse only slowly accompanied by a slow formation of gas hydrates accordingly.

- The gas hydrate layers between grains form effective transport barriers.

- NMR-studies with single-phase methane transport show, that the liquid H2O-phase in the zone containing gas hydrates can be almost completely displaced by D2O applying 1.2 PV.

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Clausthal University of Technology
Agricola Str. 10
38678 Clausthal-Zellerfeld
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