Pore-Space Images

As mentioned in the Introduction, the discipline of transport in porous media has been transformed by our ability to image, in three dimensions, the pore space of materials at different resolutions, from the nanometre to centimetre scales. Arguably the most versatile instruments use X-rays to construct three-dimensional images at micron resolution. The principles and methodology used to construct an image are similar to that used in CT scanning for medical examinations, and indeed adapted medical scanners are also used routinely to scan rock cores that are a few cm in diameter and 1–2 m long (similar in size, or at least length, to the patients the scanner was designed for). However, in these cases the resolution is around 1 mm, which is insufficient to see, directly, the pore spaces of most rocks. The limitation though is not the wavelength of the X-rays themselves: typical X-ray energies are in the range 10–160 keV – with corresponding wavelengths 0.1–0.01 nm. To obtain higher-resolution images, it is necessary to scan a smaller sample which is placed close to the X-ray source.

The first micron-resolution images of porous rock were obtained by Flannery et al. (1987) using both X-rays from a synchrotron (X-rays are emitted from electrons moving at almost the speed of light as they are accelerated around a ring by strong magnets) and a bench-top instrument with its own X-ray source (here electrons are accelerated and impact on a metal target that then emits X-rays). Since then the technology both to acquire and process these images has improved enormously; most major companies and many universities now have good laboratory instruments to produce three-dimensional micron-resolution images, combined, in many cases, with access to central synchrotron facilities. We will make use of this technology to illustrate the concepts in this book. For reference, Fig. 2.1 shows schematics of the types of apparatus used for X-ray tomography. However, a discussion of imaging and image analysis, or how the image is taken and how it is processed, lies outside the scope of this work: readers are referred to the excellent scholarly reviews of this topic by Cnudde and Boone (2013), Wildenschild and Sheppard (2013) and Schlüter et al. (2014); here it will be assumed that we can acquire a representative image of the pore space.