Complex flow dynamics have been observed, at the pore-scale, during multiphase through porous rocks. These dynamics are not captured in large scale models exploring the migration and trapping of subsurface fluids e.g., CO2 or hydrogen. Due to limitations in imaging capabilities, these dynamics cannot be observed directly at the larger, Darcy scale. Instead, by using pressure data from pore-scale (mm-scale) and core-scale (cm-scale) experiments, we show that fluctuations in pressure measured at the core-scale reflect specific fluid displacement events taking place at the pore-scale. The spectral characteristics of the pressure data depends on the flow dynamics, size of the rock sample, and heterogeneity of pore space. While high resolution imaging of large samples would be useful in assessing flow dynamics across many of the scales of interest, such an approach is currently infeasible. We suggest an alternative, pragmatic, approach examining pressure data in the time-frequency domain using wavelet transformation.

The injection of CO2 into subsurface reservoirs provides a long term solution for anthropogenic emissions. A variable injection rate (such as ramping the flow rate up or down) provides flexibility to injection sites, and could influence the amount of residual trapping. Observations made in cm-scale samples showed that starting at a low flow rate established a flow pathway across the core, leading to a long term reduction in pore space utilization, as increases in flux were accommodated with little change in saturation. In this work we assess the scalability of these observations by performing experiments with variable injection rates in larger samples: 5 cm diameter and 12 cm length, compared to 2.5 cm diameter and 4.5 cm length in our previous work (Spurin et al., 2024). We observed that starting at a low flow rate did not lead to a long term reduction in pore space utilization. Instead saturation increased significantly with increased flux, leading to a higher pore space utilization than experiments where injection started with the higher flow rate. The difference in observations depending on sample size and the role of heterogeneity highlights potential uncertainties in upscaling experimental observations to field-scale applications.

Recent work suggests that upscaling multiphase flow requires the averaging of capillary fluctuations in both space and time. This raises the questions: what are the representative time and length scales of multiphase flow and how are they interrelated? Understanding these intrinsic length and time scales is necessary to determine whether systems are at equilibrium. We propose that the intrinsic time scales of multiphase flow can be probed by examining how a system returns to equilibrium after being perturbed. While multiphase relaxation is known to span hours/days and can alter fluid topology, what governs its precise length and time scales is still unclear, particularly in context of pore-to-cm scale heterogeneity. To better constrain these scales, we investigate μm-scale relaxation dynamics following both drainage and imbibition experiments with nitrogen-brine in continuum-scale Bentheimer cores of varying heterogeneity. Unlike previous work, we examine how relaxation depends on both the pore-scale and the macroscopic capillary numbers. We observe effects due to capillary-viscous force re-equilibration after flow is stopped, even for cases where the pore-scale capillary number is exceedingly low. This manifests as an initial breaking-up of connected gas clusters and as radial fluid redistribution in homogeneous samples. Furthermore, our experiments show the impact of mm-scale heterogeneity on the capillary-viscous balance and the resulting relaxation of the fluid distributions. These insights have important implications to upscaling and experimental design. Additionally, a good understanding of multiphase flow equilibration is important for coupling multiphysics models.

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.