Abstract
Representative elementary volumes (REVs) are an important concept in
studying subsurface multiphase flow at the continuum scale. However,
fluctuations in multiphase flow are currently not represented in
continuum scale models, and their impact at the REV-scale is unknown.
Previous pore-scale imaging studies on these fluctuations were limited
to small samples with mm-scale diameters and volumes on the order of
~ 0.5 cm3. Here, we image steady-state co-injection
experiments on a one-inch diameter core plug sample, with nearly two
orders of magnitude larger volume (21 cm3), while maintaining a
pore-scale resolution with X-ray micro-computed tomography. This was
done for three total flow rates in a series of drainage fractional flow
steps. Our observations differ markedly from those reported for mm-scale
samples in two ways: the macroscopic fluid distribution was less
ramified at low capillary numbers (Ca) of 10-7; and the volume fraction
of intermittency initially increased with increasing Ca (similar to
mm-scale observations), but then decreased at Ca of 10-7. Our results
suggest that viscous forces may play a role in the cm-scale fluid
distribution, even at such low Ca, dampening intermittent pathway flow.
A REV study of the fluid saturation showed that this may be missed in
smaller-scale samples. Pressure drop measurements suggest that the
observed pore-scale fluctuations resulted in non-Darcy like upscaled
behavior. Overall, we show the importance of large field-of-view
high-resolution imaging to bridge the gap between pore- and
continuum-scale multiphase flow studies, in particular of pore-scale
fluctuations.