Dynamic neutron imaging of solute transport and fluid flow in sandstone
before and after mineral precipitation
Abstract
Advances in micro-scale imaging techniques, such as X-ray
microtomography, have provided new insights into a broad range of porous
media processes. However, direct imaging of flow and transport processes
remains challenging due to spatial and temporal resolution limitations.
Here, we investigate the use of dynamic three-dimensional neutron
imaging to image flow and transport in Bentheim sandstone core samples
before and after in-situ calcium carbonate precipitation. First, we
demonstrate the applicability of neutron imaging to quantify the solute
dispersion along the interface between heavy water and a cadmium aqueous
solution. Then, we monitor the flow of heavy water within two Bentheim
sandstone core samples before and after a step of in-situ mineral
precipitation. The precipitation of calcium carbonate is induced by
reactive mixing of two solutions containing CaCl2 and Na2CO3, either by
injecting these two fluids one after each other (sequential experiment)
or by injecting them in parallel (co-flow experiment). We use the
contrast in neutron attenuation from time-lapse tomograms to derive
three-dimensional fluid velocity field by using an inversion technique
based on the advection-dispersion equation. Results show mineral
precipitation induces a wider distribution of local flow velocities and
leads to alterations in the main flow pathways. The flow distribution
appears to be independent of the initial distribution in the sequential
experiment, while in the co-flow experiment, we observed that higher
initial local fluid velocities tended to increase slightly following
precipitation. These findings suggest that neutron imaging is a
promising technique to investigate dynamics processes in porous media.