The fate of oil clusters during fractional flow: trajectories in the saturation-capillary number space

Fractional flow has been studied at the pore scale under dynamic flow conditions by using fast synchrotron-based X-ray computed micro-tomography. The pore-scale flow regimes have been mapped in a “phase diagram” where the regimes of connected pathway flow and ganglion dynamics are characterized by fractional flow and capillary number. The regimes are identified from the respective pore scale dynamics that can be conveniently characterized by using a saturation-(cluster-based) capillary number diagram. Therein connected pathway flow is represented by a fixed point (because all parameters are constant over time) and in ganglion dynamic regime the oil clusters follow trajectories because saturation and cluster length are changing over time. Ganglion dynamics is composed of breakup and coalescence processes. During coalescence processes, both cluster volume and length increases, i.e. clusters move “up” the trajectory. During break-up processes, on the other hand, both properties decrease and clusters move “down” the trajectory.
Ganglion dynamics occurred even though the (cluster-based) capillary number of the average flow field was at least two orders of magnitude smaller than unity, i.e. the average flow field indicates capillary-dominated regime. However viscous mobilization can also be triggered by more complex break-up and coalescence processes that have much higher local flow velocities than the average flow field suggests. Most situations encountered are a combination of connected pathway flow and ganglion dynamics, where a combination of viscous and capillary-driven processes accounts for the net transport of oil. Static simulation approaches are not capable of capturing such regimes, as they
require connected pathway flow.