Pengfei Lv

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

Enhanced oil recovery(CO₂-EOR) is an essential technique to improve oil recovery while reducing greenhouse gas emissions. However, the pore-scale flow dynamics and morphological characteristics during the EOR process remain unclear. This study employs X-ray micro-tomography (micro-CT) to perform a displacement experiment through CO₂ flooding. The pore characteristics, in situ wettability, morphological parameters, and interfacial area are analyzed. The spatial distribution of pores and throats is discovered to be even, with both radii distributions exhibiting narrow peaks. The coordination number is predominantly located between 5 and 10, increasing with pore size. As the displacement process proceeds, the percentage of contact angle distribution between 105° and 140° increases, indicating the ability of CO₂ to remove the oil phase from the solid surface. As CO₂ is injected, the oil-solid interfacial area decreases while the CO₂-solid interfacial area increases, demonstrating the reduction in oil-wettability of the sample. The predominant form of residual gas at the end of displacement is residual film gas, characterized by positive Euler number values, indicating its existence as a disconnected phase. The isolated oil phase, characterized by a small volume and simple structure, dominates the remaining oil phase during CO₂ injection. The surface area and volume of the oil phase exhibit a power-law relationship with similar coefficients and exponents at various pore volumes (PVs). Some larger oil phases deviate from the fitted curve, possibly due to dissolution.

With the continuous growth of global energy demand and the gradual depletion of traditional fossil energy reserves, natural gas hydrates have attracted widespread attention as a potential clean energy source due to their vast reserves and wide distribution. Although various extraction methods, including depressurization, thermal stimulation, chemical inhibitors, and displacement methods, have been proposed, there are still challenges, such as low extraction efficiency, poor sustainability, and high costs, making it difficult to achieve large-scale engineering applications. Among these, the use of gases such as CO₂ for displacement extraction of natural gas hydrates can both develop hydrate resources and sequester CO₂, achieving a win–win situation for resource development and greenhouse gas reduction. This paper provides a detailed review of the multi-gas displacement extraction technology for natural gas hydrates, systematically summarizes the latest progress in thermodynamic and kinetic studies, analyzes the technical advantages and feasibility of combining displacement methods with traditional techniques, and explores the effects of multi-gas mixtures, such as N₂, CO₂, and H₂, and their ratios on hydrate extraction efficiency. Finally, this paper summarizes the technical challenges faced by displacement extraction methods for hydrates and offers future research directions to promote the development of multi-gas displacement technology for natural gas hydrates.

For the closure of the subgrid-scale (SGS) stress tensor, an artificial neural network (ANN)-based SGS model that takes account of the inverse energy cascade in isotropic turbulence is developed. The data required for training this ANN-based SGS model are provided by direct numerical simulation of isotropic turbulence with an inverse energy cascade. Two input features, the root mean square of the rate-of-strain tensor and the product of the eigenvalues of the rate-of-strain tensor, are employed to characterize the inverse energy cascade. An a priori test reveals that the ANN-based model adequately predicts the SGS stress tensor in the backward energy transfer process, and the predictive capability of the gradient model is found to be slightly poorer than that of the ANN-based model, while that of the Smagorinsky model is not satisfactory. In comparison with the gradient model, the ANN-based model even predicts a few backward energy transfer events in the stage of excessive energy dissipation. In addition, the off-diagonal component of the SGS stress tensor, rather than the diagonal component, may be intimately associated with the inverse energy cascade. The ANN-based SGS model presented here is expected to provide inspiration for future investigations of the construction of SGS models that take account of the inverse energy cascade.