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