Multiphase Relaxation Dynamics at the μm-to-cm Scale During Storage of Gases in Rocks: A Micro-CT Study on Homogeneous and Layered Sandstones

Abstract

Recent work suggests that upscaling multiphase flow requires averaging capillary fluctuations in both space and time. This motivates the need to determine the representative time and length scales of multiphase flow and how they are interrelated. Understanding these intrinsic scales is necessary to determine whether systems are at equilibrium. Here, we probe the intrinsic time scales of multiphase relaxation 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 in samples an order of magnitude larger than those typically used in pore-scale investigations. 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, phenomena not previously observed during relaxation. 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 for upscaling and experimental design, and understanding multiphase flow equilibration is critical for coupling multiphysics models.