Neutron Computed Tomography (NCT) Applied to Studies of Porosity and Fluid Content in Porous Media

Nicolas J. Huerta
UC Davis, Davis, CA

X-ray computed tomography (CT) has proven valuable in determining the pore fraction of a sample due to the contrast in X-ray attenuation between grains and void space. Inherently noisy and smeared data from reconstructed tomographs of a two phase system can be analyzed statistically to determine porosity by fitting histograms of attenuation data with a three component parametric equation (Clausnitzer et al., 1999). Two normal probability density functions with mean values representing grains and air are supplemented by an empirical equation to account for mixed fraction voxels, which contain a percentage of both phases. We applied a modified version of the Clausnitzer et al. methods to NCT data to illustrate the broader potential of these porosity analyses as well as the value of NCT to studies of porosity, permeability, and fluid analysis. NCT images were obtained at the McClellan Nuclear Radiation Center, UC Davis. Images of 3 granular materials using X-ray CT and NCT indicate that current NCT resolution is too low to resolve true porosity for 2 mm grains. Porosity of ~60% was obtained from NCT data, in contrast to both theoretically and experimentally determined percentage of ~40% from X-ray CT. Comparisons of imaged versus true grain size show resolution of 15 pixels per grain diameter is needed to realistically resolve porosity with current noise levels. Ongoing system upgrades at MNRC should provide this resolution for 2 mm diameter grains. When fully developed NCT will provide a good laboratory complement to porosity estimates from petrographic and down-hole neutron measurements.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005