The geometric structure variation of the nonwetting phase (NWP) in porous media plays a critical role in numerous energy-related processes. This work conducted a series of gas/liquid displacements in three rock cores using high resolution x-ray microcomputed tomography (micro-CT), including two sandstones and a Berea core with different pore architectures. The morphological and topological characteristics of NWP were investigated at the pore scale in a full cycle of primary drainage (PD), main imbibition (MI), and main drainage (MD). It is found that roof and distal snap-off occurred during drainage, resulting in nonwetting (NW) singlets and ganglia. These NW bubbles occupied 10%–20% NWP volume. Both throat snap-off and pore snap-off were observed in MI, which generally occur at a single-pore scale. The effect of viscous force is significant even at a capillary dominant displacement. The trapped NWP during PD can be partially or totally displaced in MI through further snap-off events or viscous remobilization. Due to the temporal and spatial scale diversion in pore scale events, the amplitude of pressure drop fluctuations among PD, MI, and MD is different. The significant difference in NWP characteristics between PD and MD is connectivity, thus leading to hysteresis.

The mixed wettability characteristics of porous media significantly influence CO2-brine two-phase transport processes. This multi-scale wettability heterogeneity manifests not only across core-to-field scales but also occurs at sub-grain to grain scales due to variations in mineral composition, surface roughness, and pore structure of rock surfaces. This study uses finite element numerical simulations to investigate the mechanisms governing mixed wettability effects on fluid migration at both sub-grain and grain scales. Through two-dimensional pore-scale porous media modeling, we simulate brine displacement of CO2 under four distinct wettability distribution patterns and conduct parametric studies across varying capillary numbers. The research findings demonstrate that at the grain scale, brine flow pathways preferentially propagate through regions with stronger wettability clustering. Under sub-grain-scale mixed wettability conditions, quadruple-directional interfacial propagation occurs, achieving brine saturations of 78.2% and 87.6% under capillary numbers of 10−5 and 10−4, respectively. These results nearly exceed all grain-scale wettability scenarios. At lower capillary numbers, pore filling is primarily characterized by burst and overlap events, while at higher capillary numbers, touch and overlap events dominate. Burst events exhibit a nearly 15-fold acceleration in velocity accompanied by significant pressure gradients spanning over seven pores. At the grain scale, CO2 is trapped in three distinct forms: singlet, ganglia, and clusters, with the spatial distribution of wettability governing the trapped area. Under random-order conditions, CO2 clusters exceeding 4 mm2 are captured. In contrast, at the sub-grain scale, adhesion-dominated trapping mechanisms prevail, resulting in a captured area that remains below 0.3 mm2.

CO₂ sequestration into saline aquifers can significantly reduce atmospheric greenhouse gas concentrations, making it a key geological carbon storage technology for mitigating climate change and achieving carbon neutrality targets. However, current research predominantly focuses on reservoir saturation states, with limited understanding of dynamic mechanisms at the gas-liquid interface. In this study, microfluidic experiments were conducted at ambient temperature to investigate CO₂ drainage and imbibition under varying capillary numbers, incorporating the remobilization process driven by gas-water interphase mass transfer. Collectively, these three processes determine the temporal distribution of CO₂ and water phase saturations within the porous medium, thereby influencing the efficiency and long-term stability of CO₂ sequestration. With the increase of the capillary number, the sweep efficiency of CO₂ during drainage showed an upward trend, increasing to 54.71%. Moreover, this study provides an in-depth analysis of the distribution and morphological evolution of CO₂ under conditions where the aqueous phase is unsaturated. Results indicate that the asynchronous contraction of cluster interfaces results in a heterogeneous and dynamic dissolution process; the gas-water interface evolution of double-pore ganglia resembles the brine snap-off process; and singlet structures undergo shrinkage and deformation during the dissolution process. These findings elucidate the complex interactions between CO₂-water in porous media and underscore the critical roles of capillary forces and interfacial dynamics in geological carbon sequestration. Document Type: Original article Cited as: Xue, R., Chang, Y., Wang, S., Wang, P., Sun, H., Liu, H., Lv, P. Pore-scale microfluidic investigation of unsaturated CO₂ bubble morphology and interface evolution during drainage-imbibition cycles. Capillarity, 2025, 15(3): 74-86. https://doi.org/10.46690/capi.2025.06.03

An investigation of topological features of homogeneous isotropic turbulence with an inverse energy cascade is performed by using a direct numerical simulation (DNS). The inverse energy cascade is induced by reversing the velocity field (i.e., under the transformation from u_i to -u_i) of a freely decaying isotropic turbulence. In the backward energy transfer process, the joint probability density function (PDF) of Q and R exhibits a novel shape which seems to be symmetric with the well-known teardrop shape about the Q-axis (Q and R are the second and third invariants of the velocity gradients tensor, respectively). The unexpected shape of the joint PDF of Q and R indicates that the turbulence with an inverse energy cascade is dominated by vortex compression and tubelike structures. The PDF of the intermediate eigenvalue of the strain-rate tensor in the inverse energy cascade is negatively skewed, which is opposite to the universal feature of the forward energy cascade. Nevertheless, the preferential alignment of vorticity with the intermediate eigenvector is rather robust, no matter the direction of the energy transfer is forward or backward. In addition, it is universal that the vorticity is mostly perpendicular to the eigenvector which corresponds to a strain-rate eigenvalue with the maximum absolute value among the three eigenvalues. Since the velocity gradient invariants are closely related to the local flow topology, the numerical results reported in this work are expected to shed light on the intrinsic dynamics and mechanisms of inverse energy cascade.