Essential Maintenance: All Authorea-powered sites will be offline 9am-10am EDT Tuesday 28 May
and 11pm-1am EDT Tuesday 28-Wednesday 29 May. We apologise for any inconvenience.

Shan Wang

and 2 more

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.