On the challenges of measuring interfacial characteristics of three-phase fluid flow with x-ray microtomography

Synchrotron-based x-ray computed microtomography contributes high-resolution, three-dimensional observations to investigations of multiphase fluid transport in porous media. Pore-scale observations are valuable to the development and validation of new theory, as well as numerical models. Computed microtomography has been used previously to measure fluid content and interfacial areas in systems containing two fluids (air-water, oil-water) and to a limited extent to measure fluid content and entrapped fluid morphology in systems containing three fluids (air-oil-water). This study addresses challenges that arise when imaging three-phase flow in spreading systems. The first challenge is related to wettability alteration. Observations reported herein suggest that the wettability of solid surfaces changed over the course of a three-fluid phase flow experiment, a phenomenon that has not been observed in similar, previously conducted two-fluid phase experiments. Follow-up experiments showed that wettability alteration is significant when oil-solid contact is combined with x-ray exposure, and is not reversed with a conventional cleaning procedure. The second challenge arises in segmenting three-phase images, and thereby obtaining data from which various measures can be quantified with sufficient accuracy. Partial volume effects and blur often cause the grey-scale values of different fluids to overlap and appropriate steps must be taken to avoid ambiguity at phase boundaries. A comparison of images collected at standard resolution (10.6 microns voxel(-1) ) to those collected at a higher resolution (5.3 microns voxel(-1) ) showed that saturation measurements are within 5% of each other, but interfacial areas for three-phase systems may be underestimated at standard resolution by as much as 25%.