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Neutron radiography imaging, porosity and permeability in porous rocks

Building 1500, NECSA, Pelindaba, PO Box 582, Pretoria, 0001, e-mail: fdebeer{at}necsa.co.za

M.F. Middleton

Geological Survey of Western Australia (GSWA), Australia; e-mail: mfmiddleton{at}iinet.net.au

The visualization of fluid flow and characterization of porous media, by means of Neutron Radiography (NRad), has been tested using the Safari-1 nuclear reactor in South Africa – the only neutron imaging facility available in South Africa for R & D purposes and yet not fully explored by scientists, engineers and researchers. This has been enhanced by the capabilities of neutron imaging equipment and the subsequent improvement of imaging software features that support this type of characterization and analysis.

A low light level CCD Camera that was converted into a neutron detection system was used to study the porosity and permeability features of laboratory scale porous sandstone and concrete samples and to compare the results obtained from conventional detection methods.

The study develops the theory of mensuration of water saturation fluid of porous media of various types, but specifically concrete and sandstone, using Nrad techniques. Examples of these are fully developed.

The study also investigated a number of often poorly understood situations of liquid with full and partial saturations of porous media, including concrete, sandstone aquifers and petroleum reservoir rocks. Never previously studied using NRad is the situation of two-phase fluid flow through oil-bearing rock inside a Hassler high-pressurized apparatus. The Hassler cell is used to reproduce deep reservoir pressure conditions, and is a standard petroleum petrophysical apparatus. For the present experiments, the Hassler cell was constructed of stainless steel (which is not optimum for neutron attenuation), and "heavy water" (D₂O) was used instead of water in order to enhance the neutron attenuation contrast between petroleum and the resident liquid (normally water, but in this case deuterium). The heavy water and petroleum combination permitted sufficient contrast in neutron attenuation to enable resolution of D₂O versus petroleum concentration within the pore volume to about 10% of the total concentration. Results were obtained of significant quality to suggest that NRad may supply valuable information on multi-phase fluid behaviour in such experimental apparatus.

This study shows the capability and ability of neutron radiation imaging to obtain information about porous media and to validate existing data obtained through conventional methods. Although this study concentrate only on 2D neutron imaging and its advantages, exciting new 3D tomography imaging equipment are envisaged at SAFARI-1 that will enhance these R & D capabilities through future studies.