NGI's laboratory is equipped with a state of the art industrial X-ray computed tomography (CT) scanner for non-destructive imaging of soil samples, internal rock structure and pore-fluid distribution in sedimentary rock.

The application of X-ray CT imaging in the geosciences was introduced in the petroleum industry in the 1980s. Most of the studies are using medical scanners with typical resolution of mm and X-ray intensities matching the density of the human body. The evolution in technology has resulted in industrial scanners specifically designed to image geological samples and other high density materials.

Specifications
The industrial scanner at NGI is equipped with a 225 kV micro-focus X-ray tube and a detector panel with 4.2 Megapixels. The spot size of the X-ray beam is typically 3 µm. At low energy levels for high resolution scans the spot size is 1-1.5 µm. 3D images are reconstructed on a powerful 64 bit computer using VGStudio MAX by Visual Graphics.

The industrial scanner in the NGI laboratory

The scanner is designed to image samples from mm in size to samples with length up to about 1 meter and diameter up to about 30 cm. The maximum weight of the sample and holder (e.g. pressure cell) is 70 kg.

The spatial resolution of scanned images varies depending on the sample size and the distance between the source and the sample relative to the distance between the source and the detector panel. In addition, the resolution depends on the pixel size of the detector panel and the spot size of the X-ray beam. For small samples of about 5-10 mm in spatial extension the scanner can provide images with resolution 3-5 µm. For conventional cylindrical rock plugs of 38 mm in diameter (corresponding to plugs used for triaxial testing) the resolution is about 10 µm. Images of plugs inside a core holder would give a typical resolution of about 30-40 µm depending on the size of the holder.

Standard visualization services

• Visualisation of internal heterogeneities, geological features, fractures and fluid or bulk density variations in seabed offshore samples, cored rock samples and seal peels in order to properly select specimen for further geomechanical and rock physic testing.
• Pre- and post-experimental imaging of test specimen for visualisation of internal changes in the rock due to geomechanical testing (failure, fracturing, compaction, fluid effects etc.)

Advanced rock mechanic testing and imaging

• Simultaneous imaging of multiphase fluid distributions and rock acoustic - resistivity measurements during core flooding experiment at realistic pressures. The measurements are used to estimate elastics properties and rock resistivity as a function of fluid saturation and distribution. The results are applied in rock physic models to invert 4D seismic data and electromagnetic imaging (CSEM) for fluid and pressure effects.
• In combination with triaxial loading cell the scanner can visualize the evolution of fractures and fluid flow during sharing. Can be combined with acoustic emission measurements. The measurements can be used to study the hydraulic properties of fractures and faults.
• Investigate the effect of rock heterogeneities on flow patterns. Relevant for studying the injection process of water, hydrocarbon gas or CO2 into host formation
• Micro-CT capabilities (resolution 3-5 mm) on small (mm-size) samples:
  - producing 3D images of pore structure
  - study displacement mechanisms on pore scale
  - visualize mineral composition of different grains.

Examples on images and earlier testing
A few example images obtained with the industrial scanner of typical sedimentary rock (sandstone and chalk) are shown in the Figures 2 a) and b) below. Figure 2 c) illustrates micro-CT capabilities of a Red Wildmoor sandstone.

(Left): Reconstructed 3D image of a Vosges sandstone and (centre): a 2D cross-sectional image of a fractured Liege chalk. Both samples are 25 mm in diameter. (Right): Reconstructed 3D image of a Red Wildmoor sandstone sample with typical grain size of 0.2 mm. Sample size is about 7 mm.

Since 2002, NGI has developed a laboratory method allowing simultaneous imaging of fluid distribution by X-ray CT-scanning and measurements of acoustic velocities during core flooding experiment (see figure below). The setup is based on a medical scanner. The investment of an industrial scanner is a natural continuation in order to investigate the effect of rock heterogeneity on the fluid flow and fracture processes.

Medical CT-scanner at Norwegian University of Life Sciences combined with a special core holder and ultrasonic acoustic measurements to study seismic properties versus e.g. gas saturation distribution.

Publications

Alemu, B., Aker, E., Soldal, M., Johnsen, Ø., and Aagaard, P. [2010] Influence of CO2 on rock physics properties in typical reservoir rock: A CO2 flooding experiment of brine saturated sandstone in a CT-skanner. Energy Procedia 00 (2010) 000-000, GHGT10, 19-23 September, Amsterdam.

Johnsen, Ø., Soldal, M.,  F. Cuisiat, and Aker, E. [2010] Micro Focus Computed Tomography in Geotechnical and Geophysical Research. 72nd EAGE Conference & Exhibition incorporating SPE EUROPEC,  Barcelona, Spain, 14-17 June 2010,  P506.  

Aker, E., Soldal, M., Keuhn, D., and Cuisiat, F. [2010] Relating acoustic emission sources to rock failure around a borehole. 72nd EAGE Conference & Exhibition incorporating SPE EUROPEC, Barcelona, Spain, 14-17 June, P568.

Cuisiat, F., Aker, E., Soldal, M., Huynh K.D.V. [2010] Experimental and numerical investigation of acoustic emission and source mechanisms in rock during failure. Rock mechanics in the Nordic countries, Kongsberg, Norway, June 9-12, 2010.  

Monsen, K. and Johnstad S.E. [2005] Improved understanding of velocity-saturation relationships using 4D computer-tomography acoustic measurements. Geophysical Prospecting, 53(2), 173-181.

Aker, E., Monsen, K., Cuisiat, F., and Westerdahl, H. [2005] Studies of water flooded chalk under high pressure by use of X-ray computed Tomography. International Symposium of the Society of Core Analysts, Toronto, Canada, 21-25 August 2005.