We investigated the cluster-size distribution of the residual nonwetting phase in a sintered glass-bead porous
medium at two-phase flow conditions, by means of micro-computed-tomography (μCT) imaging with pore-scale
resolution. Cluster-size distribution functions and cluster volumes were obtained by image analysis for a range of
injected pore volumes under both imbibition and drainage conditions; the field of view was larger than
the porosity-based representative elementary volume (REV). We did not attempt to make a definition for a
two-phase REV but used the nonwetting-phase cluster-size distribution as an indicator. Most of the nonwettingphase
total volume was found to be contained in clusters that were one to two orders of magnitude larger than
the porosity-based REV. The largest observed clusters in fact ranged in volume from 65% to 99% of the entire
nonwetting phase in the field of view. As a consequence, the largest clusters observed were statistically not
represented and were found to be smaller than the estimated maximum cluster length. The results indicate that
the two-phase REV is larger than the field of view attainable by μCT scanning, at a resolution which allows for
the accurate determination of cluster connectivity.