Servizio Comunitario di Informazione in materia di Ricerca e Sviluppo - CORDIS

X-Ray CT device for studies on in situ processes in the pore space under high pressure

A novel, universally useable high pressure CT-cell was developed and adapted to the X-ray CT-tool manufactured by Bio Imaging Research. Using this high pressure CT-cell equipped with a temperature and a pressure control, the growth behaviour of gas hydrates in porous media and the displacement of sediment constituents by the formation of gas hydrate crystals were accurately investigated. Additionally a volumetric measurement in the annular space is applied to follow up possibly occurring expansion of sediment models. Related to the time dependent displacement of sediment constituents, the detection limits of this equipment amount to
X-ray: £Gl/l = 2*10-3, volumetric measurement: 1/V dV/dt = 1*10-10 s-1
This device is universally usable for studies on processes in porous media, especially precipitation of gas hydrates and minerals.The novel high-pressure ceramic cell for high resolution CT-studies is fabricated from an Al2O3- tube (Alsint). The x-ray beam is not affected by any strongly absorbing metallic component assuring a high contrast level for quantitative measurements. The sediment model, sand packs or consolidated sandstone cores are inserted into shrinking tubes made from FEP. A floating end piece serves for compensation of axial expansion.

For studies on gas hydrate behaviour accurate thermostating is essential. To enable long-term measurements (>2000 h) at constant temperature below ambient and constant pressure in the annular space of the cell, an air bath was installed and implemented into the CT-device. The thermostating / cooling system including fans and radiator grille was installed in an insulating polystyrene housing below the free rotating core holder.

A rather universal set up was chosen as peripheral device to respond adequately on changing experimental necessities. The total peripheral device is mounted on a cart close to the CT-apparatus. (Technical drawings are available.)

The device allows flooding of the core with gas, water, or water containing an adjustable amount of dissolved methane. For the studies proppants, England sand 20/40 mesh, were used to construct sandpacks with a diameter of 20 mm and a length of 350 mm.

To achieve settled packing, the installed sediment models were compacted by an overburden pressure of 4 MPa. A method was developed for generating specific large cavities in sandpacks, in which large bubbles of free gas can be entrapped. The method is based on the fact, that large voids can be stable in sandpacks due to the large friction at the contact areas of grains, proven by the stability of proppant faces in fractured reservoirs. By inserting single NaCl-crystals into the sandpack the large voids were preformed. These crystals were dissolved by water after the compaction of the model.

The following conclusions can be drawn from the CT-studies with sandpacks using this equipment:
- The growth of gas hydrates is preferentially initiated at the inner surface of the voids and the first adjacent grain layer. After a short time (less than 200 h) the large pores are surrounded by grain layers cemented by gas hydrates.

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

- Due to the increasing pressure differences between the periphery and the inner space of the void, a compaction of the pore system is resulting.

- In collapsed pores further displacement of grains occur.

- The compaction causes a further reinforcement of the transport barriers.

- Gas included in the large pores with unchanged geometry is not accessible
(time scale 2000 h).

Reported by

Agricolastrasse 10
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