Pore-Scale Imaging of Multiphase Flow Fluctuations in Continuum-Scale Samples

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 similar to 0.5 cm(3). 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 cm(3)), 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 representative elementary volume 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 usefulness of large field-of-view, high-resolution imaging to bridge the gap between pore- and continuum-scale multiphase flow studies.

Automated summary

Representative elementary volumes (REVs) are important for studying subsurface multiphase flow at the continuum scale. Previous research on fluctuations in multiphase flow was limited to small samples. In this study, we used high-resolution imaging to observe steady-state co-injection experiments on a larger scale, revealing different behaviors at the cm-scale compared to mm-scale samples. Our findings suggest the importance of considering viscous forces and the potential for non-Darcy like upscaled behavior.