During imbibition, initially connected oil is displaced until it is trapped as immobile clusters. While initial and final states have been well described before, here we image the dynamic transient process in a sandstone rock using fast synchrotron‐based X‐ray computed microtomography. Wetting film swelling and subsequent snap off, at unusually high saturation, decreases nonwetting phase connectivity, which leads to nonwetting phase fragmentation into mobile ganglia, i.e., ganglion dynamics regime. We find that in addition to pressure‐driven connected pathway flow, mass transfer in the oil phase also occurs by a sequence of correlated breakup and coalescence processes. For example, meniscus oscillations caused by snap‐off events trigger coalescence of adjacent clusters. The ganglion dynamics occurs at the length scale of oil clusters and thus represents an intermediate flow regime between pore and Darcy scale that is so far dismissed in most upscaling attempts.

The onset of oil mobilization during imbibition has been imaged with pore-scale resolution under dynamic flow conditions viscocapillary balance, by using fast synchrotron-based X-ray computed microtomography. Oil mobilization under unsteady-state displacement has been studied for sintered glass, sandstone and carbonate rock, which demonstrate distinctly different behavior with respect to the cluster-size distributions and respective time evolution.For the sandstone sample, which showed the largest saturation change between the three samples, over 50% of the oil was mobilized during imbibition. The cluster-size distribution computed from the tomography images, but also detailed visualization at the pore scale revealed that during imbibition the largest (initially connected) cluster breaks off into smaller segments. Over time, successively larger segments break off, which increases the frequency of intermediate-size clusters. In most cases, each segment breaks off into even smaller segments, but also in fewer cases clusters merge and increase in size.These findings support the view that at the onset of oil-mobilization clusters prefer to break off instead of moving as a large cluster, which gives further insight into the ganglion dynamics flow regime. It is also shown that imbibition dynamics is more complex than assumed under common percolation models, where, for instance, disconnected nonwetting-phase clusters become immobile and remain static. The experimental data instead clearly show time evolution for disconnected clusters that can lead to reconnection or further break off.

Pore-scale events in multiphase flow in porous rock have been directly imaged in real time by using fast (15 to 60 seconds) synchrotron-based X-ray computed microtomography. In the past, pore-scale fluid displacements in porous media could only be imaged under quasistatic conditions where at scanning times of several minutes to hours, fluid distributions were subject to capillary redistribution. Here, pore-scale displacement events in porous rock were imaged in situ in real time in natural rock under dynamic flow conditions, where the pressure gradient and the viscocapillary balance were maintained during scanning. Two elementary processes, Haines jumps in drainage and snap-off in imbibition, were studied in detail for sintered glass, sandstone, and carbonate rock. We found that most Haines jump events do not displace the wetting phase pore-by-pore, but typically involve 10 to 20 individual pores and that filling events are cooperative. We also found that in sandstone rock 64% of the externally applied work is actually dissipated during these jumps and approximately 36% is converted into interfacial energy.