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Using high-resolution micro-CT imaging at 2.98 μm/voxel, we compared the percolation of hydrogen in gas injection with gas expansion for a hydrogen-brine system in Bentheimer sandstone at 1 MPa and 20 °C, representing hydrogen storage in an aquifer. We introduced dimensionless numbers to quantify the contribution of advection and expansion to displacement. We analysed the 3D spatial distribution of gas and its displacement in both cases and demonstrated that in gas injection, hydrogen can only advance from a connected cluster in an invasion-percolation type process, while in gas expansion, hydrogen can access more of the pore space even from disconnected clusters. The average gas saturation in the sample increased from 30% to 50% by gas expansion, and we estimated that 10% of the expanded volume is attributed to hydrogen exsolution from the brine. This work emphasises the importance of studying the combined effects of pressure decline and gas withdrawal in hydrogen storage to assess the influence of gas expansion on remobilising trapped gases. [Display omitted] • High-resolution X-ray imaging studied hydrogen injection and expansion in a sandstone. • Disconnected gas clusters can expand and reconnect during pressure decline. • Gas expansion increased the average gas saturation from 30% to 50% as pressure declined. • Hydrogen exsolution from brine increased the expanded gas volume by 10%. • Hydrogen accessed more of the larger pores by expansion than by injection. [ABSTRACT FROM AUTHOR]

The flow characteristics of methane and water in sedimentary layers are important factors that affect the beneficial exploitation of marine hydrates. To study the influencing factors of methane drive-off water processes in porous media, we constructed nonhomogeneous geometric models using MATLAB 2020a random distribution functions. We developed a mathematical model of gas–water two-phase flow based on the Navier–Stokes equation. The gas-driven water processes in porous media were described using the level-set method and solved through the finite element method. We investigated the effects of the nonhomogeneous structure of pore media, wettability, and repulsion rate on gas-driven water channeling. The nonhomogeneity of the pore medium is the most critical factor influencing the flow. The size of the throat within the hydrophilic environment determines the level of difficulty of gas-driven water flow. In regions with a high concentration of narrow passages, the formation of extensive air-locked areas is more likely, leading to a decrease in the efficiency of the flow channel. In the gas–water drive process, water saturation changes over time according to a negative exponential function relationship. The more hydrophilic the pore medium, the more difficult the gas-phase drive becomes, and this correlation is particularly noticeable at higher drive rates. The significant pressure differentials caused by the high drive-off velocities lead to quicker methane breakthroughs. Instantaneous flow rates at narrow throats can be up to two orders of magnitude higher than average. Additionally, there is a susceptibility to vortex flow in the area where the throat connects to the orifice. The results of this study can enhance our understanding of gas–water two-phase flow in porous media and help commercialize the exploitation of clean energy in the deep ocean. [ABSTRACT FROM AUTHOR]

This study extensively investigates the influence of different pyrolysis temperatures and organic matter contents on the fluid flow and heat transfer properties in oil shale samples. Utilizing CT images to generate three-dimensional digital rock, coupled simulations of CO 2 flow and heat transfer were conducted, analyzing parameters such as velocity fields, permeability, temperature fields, average temperatures, and heat transfer coefficients. The results reveal that, for relatively homogeneous oil shale samples, the permeability exhibits a monotonous increase with rising pyrolysis temperature. While the effect of pyrolysis temperature on the distribution characteristics of velocity and temperature fields is minimal, it significantly impacts the heat transfer coefficients. Specifically, the heat transfer coefficients increase significantly in the direction perpendicular to the bedding plane, while they decrease or remain unchanged parallel to it. Additionally, the organic matter content significantly influences the fluid flow and heat transfer properties of shale samples. After undergoing heat treatment, the heterogeneity of pore structures in shale samples varies significantly, affecting the characteristics of fluid flow and heat transfer. The influence of organic matter content and pyrolysis temperature on fluid flow and heat transfer in shale primarily stems from the effect of organic matter pyrolysis on the original pore structure. The development and connectivity of pore networks are closely related to the distribution characteristics of the original organic matter and are not directly correlated with the organic matter content. These findings provide essential theoretical guidance and technical support for the development and utilization of oil shale resources, while also offering valuable references and insights for future research. [ABSTRACT FROM AUTHOR]

Micro‐continuum models are versatile and powerful approaches for simulating coupled processes in two‐scale porous systems. Initially oriented for modeling static single‐phase flow in microtomography images with sub‐voxel porosity, the concept has been extended over the years to multi‐phase flow, reactive transport, and poromechanics. This paper introduces an integrated micro‐continuum framework to model coupled processes in porous media. It reviews state‐of‐the‐art models and discusses applications in geosciences including Digital Rock Physics with sub‐voxel porosity, moving fluid‐solid interface at the pore‐scale due to geochemical reactions, fracture‐matrix interactions, and solid deformation. Finally, the paper discusses future developments in micro‐continuum models. Key Points: Micro‐continuum models are hybrid‐scale approaches for solving flow and transport in porous mediaState of the art micro‐continuum models handle full Temperature‐Hydrodynamics‐Mechanics‐Chemistry coupling in unsaturated environmentsApplications include Digital Rock Physics with sub‐voxel porosity, pore‐scale reactive transport, and poromechanics [ABSTRACT FROM AUTHOR]

The core pore structure of the target shale oil block was finely characterized using scanning electron microscopy， and a microscopic numerical model of the pore-scale core was constructed to elucidate the intrinsic mechanisms and the main controlling factors of spontaneous imbibition of fracturing fluid within the complex pore space of shale. On this basis， research was conducted on spontaneous imbibition within the fracture-matrix system. The Navier-Stokes （N-S） equations were coupled with the level-set method to track changes in the oil and water interface， thus clarifying the oil and water two-phase flow characteristics and evaluating the efficiency of spontaneous imbibition under the influence of the wettability， viscosity ratio of oil and water， and interfacial tension. The research results revealed that large and small intercommunicating pores in shale form intricate microscopic displacement units under capillary action. When the rock wettability presents a water-wet state， water preferentially infiltrates from the fracture into the smaller pores， expelling oil from larger pores into the fracture， and the produced range of crude oil within the matrix pores is wider， resulting in higher spontaneous imbibition efficiency. When the rock wettability turns oil-wet， crude oil within the small pores cannot be produced during spontaneous imbibition， while the water invades from larger pores， causing crude oil to be displaced from other large pores. This results in a lower spontaneous imbibition efficiency and a small range of crude oil produced in the matrix pores. The spontaneous imbibition in the initial phase is faster but gradually slows over time. Stronger water wettability of shale indicates a lower viscosity ratio of oil and water and higher interfacial tension， resulting in a wider range of crude oil produced within the matrix pores and a cumulative efficiency of spontaneous imbibition of more than 15%.

The flow characteristics of methane and water in sedimentary layers are important factors that affect the beneficial exploitation of marine hydrates. To study the influencing factors of methane drive-off water processes in porous media, we constructed nonhomogeneous geometric models using MATLAB 2020a random distribution functions. We developed a mathematical model of gas–water two-phase flow based on the Navier–Stokes equation. The gas-driven water processes in porous media were described using the level-set method and solved through the finite element method. We investigated the effects of the nonhomogeneous structure of pore media, wettability, and repulsion rate on gas-driven water channeling. The nonhomogeneity of the pore medium is the most critical factor influencing the flow. The size of the throat within the hydrophilic environment determines the level of difficulty of gas-driven water flow. In regions with a high concentration of narrow passages, the formation of extensive air-locked areas is more likely, leading to a decrease in the efficiency of the flow channel. In the gas–water drive process, water saturation changes over time according to a negative exponential function relationship. The more hydrophilic the pore medium, the more difficult the gas-phase drive becomes, and this correlation is particularly noticeable at higher drive rates. The significant pressure differentials caused by the high drive-off velocities lead to quicker methane breakthroughs. Instantaneous flow rates at narrow throats can be up to two orders of magnitude higher than average. Additionally, there is a susceptibility to vortex flow in the area where the throat connects to the orifice. The results of this study can enhance our understanding of gas–water two-phase flow in porous media and help commercialize the exploitation of clean energy in the deep ocean.

• Novel simulation method explores diffusion in bi-dispersed pores. • Study critiques Maxwell and Nield models' predictivity limits. • Modifies random pore model, boosts accuracy in low macro-porosity. The classical diffusion models fail to predict the diffusivity of bi-dispersed porous materials, particularly at low macro-porosity. In this work, a pore-scale simulation is carried out to investigate the diffusion process inside ordered and random bi-dispersed pore structures under different growth phases and shapes. The applicability of existing models is evaluated. The results reveal that Maxwell and Nield models are inadequate to predict the diffusivity of the ordered macropore structure under different growth phase types and shapes. Meanwhile, the ordered distribution of microporous spherical particles is superior to that of macropores. For random pore structure, the macropore-macropore diffusion path contributes to the error of the random pore model, which depends on the ratio of the micropore-macropore diffusivity. [ABSTRACT FROM AUTHOR]

• A 3D model of multiphase reactive flow with solid dissolution is proposed. • The diagram of multiphase reactive flow including several patterns is established. • A new correlation of reactive surface area-porosity-saturation is formulated. • Different pore size distributions and bubble characteristics are quantified. • The mass transfer coefficient and effective reaction rate are evaluated. There are still many unclear mechanisms in the multiphase reactive flow with solid dissolution processes. In this study, the reactive transport processes coupled with solid dissolution and self-induced multiphase flow in three-dimensional (3D) structures with increasing complexity is studied by developing a 3D computational microfluidic method, which considers multiphase flow, interfacial mass transport, heterogeneous chemical reactions, and solid structure evolution. Solid dissolution diagram in a simple channel in the framework of multiphase flow is proposed, with six coupled multiphase flow and solid dissolution patterns identified and the transition between different patterns discussed. Then, multiphase reactive flow in a porous chip is further studied, and the interesting 3D phenomena are discovered, including enhanced solid dissolution in the middle and enriched bubble generation at the corner along the thickness direction. Considering the importance of reactive surface area, correlations of reactive surface area-porosity-saturation with different dissolution patterns are proposed based on the pore-scale results. Finally, the computational microfluidic model is extended to investigate the multiphase reactive flow in a 3D digital core. Different dissolution patterns are recognized using the local porosity evolution character, and the corresponding pore size distribution and bubble characteristics are deciphered. These findings advance understanding of multiphase reactive transport processes and contribute to improve continuum-scale reactive transport modeling. Computational microfluidic study of multiphase reactive flow with solid dissolution and bubble generation in a simple channel, a microfluidic porous chip, and a digital core. [Display omitted] [ABSTRACT FROM AUTHOR]

The phase equilibrium conditions of methane hydrate, particularly the phase equilibrium pressure, are influenced by capillary pressure. In this study, the existing model for phase equilibrium pressure is enhanced, and a novel methane hydrate dissociation model is formulated by integrating it with the established hydrate dissociation model. A numerical simulation is conducted on experimental research, and the reliability of the model is validated through comparison with experimental data, yielding consistent results. The significance and necessity of considering capillary pressure between hydrate and water in the exploration of methane hydrate dissociation in micropores are elucidated. The proposed model is employed to further elucidate the evolution of various factors such as methane generation rate, phase distribution, temperature distribution, and phase equilibrium pressure distribution. The findings underscore the critical role of capillary pressure between hydrates and water, emphasizing its substantial impact on the accuracy of hydrate dissociation models. Specifically, the cumulative gas production and methane generation rate values obtained are relatively diminished when neglecting capillary pressure between hydrate and water. Capillary pressure, hydrate dissociation effects, and the coupled effect of interphase heat transfer collectively influence the distribution of phase equilibrium pressure, methane generation rate, phase distribution, temperature distribution, and phase equilibrium pressure distribution. Capillary pressure elevates the phase equilibrium pressure of hydrate, augments the driving force for hydrate dissociation, and accelerates the dissociation process. • Hydrate dissociation model is proposed based on an improved phase equilibrium model. • Significance and necessity of considering capillary pressure are elucidated. • Coupling effects of capillary pressure and hydrate dissociation are studied. • Capillary pressure raises hydrate phase equilibrium pressure and dissociation rate. [ABSTRACT FROM AUTHOR]

• Colloidal particle clogging is caused by both deposition and aggregation. • Rough concave surfaces play a dominant role in particle deposition. • Surface clogging is the ultimate type of clogging if recharge time is unlimited. Groundwater recharge engineering practices are usually accompanied by the migration and deposition of polydisperse particles. Solid particles may be retained in the pore space leading to clogging of the porous media and reducing groundwater recharge efficiency. However, the mechanism behind this clogging phenomenon remains poorly understood. Specifically, the relation of particle clogging at pore-scale and macroscale is unclear. Here, the influence of polydisperse particle size on clogging development patterns was studied utilizing a series of laboratory-scale column experiments in conjunction with pore-scale scanning electron microscopy (SEM) experiments. Particles with median particle sizes (d p50) of 0.66, 4.05, 11.83 μm were injected separately into the sand column to simulate particle clogging that occurs during groundwater recharge. SEM was used to directly observe particles deposition on porous media grain surface after the column experiments. By counting the size distribution of the particles on the media grain surface, the response of the particle deposition to the surface morphology of the media grains was analysed. Compared to d p50 of 4.05 and 11.83 μm, the spatial distribution of particles deposited in the sand column was more uniform at d p50 of 0.66 μm. The smallest particles (d p50 = 0.66 μm) used in the experiments ultimately formed mixed clogging, which was the result of the combined effects of particle deposition and aggregation. The particle deposition with d p50 = 4.05 and 11.83 μm ultimately exhibited hyper-exponential distribution and formed mixed and superficial clogging, respectively. At the same time, the concave region of the media grain surface played a dominant role in particle deposition. For d p50 = 4.05 μm, 69.41 % of the particles were deposited in the concave region, and for d p50 = 11.83 μm, the proportion was 90.1 %. The results help to better manage particle clogging in groundwater recharge and other engineering applications (e.g., groundwater source heat pump, and filtration project). In addition, these findings provide a theoretical reference for the development of numerical models to help recognize the probability of particles of different sizes to be captured in porous media. [ABSTRACT FROM AUTHOR]

• An Eulerian-Lagrangian solver is developed using the OpenFOAM library. • The effects of Brownian, Van der Waals, and electrostatic forces are considered. • The deposition ratio of nanoparticles is investigated by the pore-scale approach. • Open-cell foams with various porosities and Berea sandstone are studied. Nanoparticle deposition in a subsurface environment decreases the amount of material in the bulk fluid, increases surface roughness, and affects nanofluid flow. Nanoparticle transport in porous media is a complex, multiscale phenomenon. Its macroscopic behavior is determined by phenomena at the pore-scale. Hence, it is necessary and of interest to study the transport and deposition of nanoparticles in porous media using a pore-scale approach. A computational solver is developed using an Eulerian-Lagrangian approach within the OpenFOAM library. A Berea sandstone and three open-cell metal foams with porosities of 0.8, 0.85, and 0.9, constructed by Laguerre-Voronoi tessellation, were analyzed to study nanoparticle behavior in water. Nanoparticle deposition on solid surfaces is influenced by porosity, double layer thickness (N DL), surface potential magnitude (N E1), and nanoparticle diameter. Brownian motion that has a significant impact on nanoparticle transport and deposition is also examined. For nanoparticles with 20 nm diameter, the fraction of particles deposited in foam with 0.8 porosity is about 17 % more than the foam with 0.9 porosity. This difference decreases with nanoparticle diameter. For the particles with 100 nm diameter and more, the effects of Brownian motion on nanoparticle deposition becomes negligible. [Display omitted] [ABSTRACT FROM AUTHOR]

The corrosion-induced permeability changes of nanoscale zero-valent iron (NZVI) are one of the crucial factors constraining the successful application of NZVI in the remediation of contaminated groundwater. It is of great significance to study the dynamic evolution of corrosion products of NZVI after NZVI is injected into porous media and its influence on pore plugging effect from the pore scale. Micro computed tomography (Micro-CT) imaging technology, mineralogical characterization and theoretical calculations were used to understand the details of NZVI corrosion plugging porous media at the pore scale. This study reveals the factors of NZVI corrosion plugging porous media, namely, gas production (H 2 ) in the early and middle stages of corrosion (before 90 days) and solid phase changes (NZVI volume increase and migration) in the later stages (after 90 days). The permeability loss rate of the porous media was 66.8%, 87.3%, 79.4%, and 53.6% at the corrosion times of 30, 60, 90, and 120 days, respectively. After 90 d of corrosion, the particle size of NZVI increases by 7.9%, and the secondary minerals formed by corrosion are mainly Fe 3 O 4 /γ-Fe 2 O 3 and FeOOH. In addition, this study also found that the migration of NZVI after 90 d was due to its corrosion reducing the magnetic attraction between particles, dissociating into smaller particles or agglomerates under the action of fluid dynamics, resulting in its redistribution in the porous medium and causing blockage. This study clarifies that NZVI corrosion plays a vital influence in affecting the permeability and clogging of porous media, providing valuable guidance for optimizing NZVI-based remediation technologies., Competing Interests: Declaration of competing interest The author declares that the research was conducted without any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

Triply periodic minimal surface (TPMS) can control the structural characteristic parameters through implicit functions, which possesses great significance for the customization of porous matrix topologies used in transpiration cooling. An innovative geometric reconstruction and meshing method is developed to reconstruct different TPMS porous matrix, a pore-scale numerical simulation of transpiration cooling coupled with mainstream is performed, and a hot wind tunnel experiment is developed to validate the numerical method. The main conclusions are as follows: The flow resistance loss of coolant within porous matrix is greatly influenced by the shape and size of pore throat, and due to the interaction between mainstream and coolant, higher flow resistance loss doesn't mean a higher coolant injection pressure. The influence of pore morphology on cooling efficiency is more obvious at higher coolant dosage, and considering the coolant injection pressure and cooling efficiency at the same time, the W(B) structure shows the best comprehensive performance. The generation of counter-rotating vortex pairs (CRVPs) in coolant film might lead to the increase in skin-friction coefficient on the structure surface, and the complex outflow angle of coolant can inhibit the generation of CRVPs, so for the case needs to reduce the skin-friction resistance, W(A) structure is recommended. [ABSTRACT FROM AUTHOR]

• A in-situ visualization protocol for pore-scale distribution of MAOM was developed. • The mobility of clay-sized MAOM was lower in the intra-aggregate zones. • Considerable amounts of MAOM in intra-aggregate zone was retained after leaching. • Colloid facilitated transport dominates the mobility of clay-sized MAOM. The mobility of different types of soil organic matter (SOM), including dissolved organic matter (DOM) and mineral associated organic matter (MAOM), along the soil profile in response to infiltration flow can significantly affect the carbon pools in subsoil. In addition, the soil heterogeneity on the pore-scale, comprising pore spaces within the intra-aggregate matrix and preferential pathways within inter-aggregate zones, can also affect the mobility of SOM. This is especially the case for clay-sized MAOM. However, the quantitative understanding of the clay-sized MAOM mobility in presence of pore-scale heterogeneity remains elusive. This study designed a novel microfluidic device to track the pore-scale distribution and mobility of fluorescence labeled clay-sized MAOM in heterogeneous soil pore structures. Results showed that the MAOM mobility was significantly lower in the intra-aggregate zones compared with in the inter-aggregate zones. Only 4.8 % to 47.9 % of MAOM in the intra-aggregate region was mobilized at high leaching flow rates of 81 to 163 mm/d. Moreover, colloid mobilization induced by shearing rather than solute desorption followed by solute dilution is expected to be the major factor facilitating the mobilization of clay-sized MAOM. This study demonstrated a new experimental protocol allowing the in-situ visualization and quantification of OMs mobility in presence of pore-scale heterogeneity. Moreover, our findings also highlighted the potential importance of heterogeneous mobility of clay-sized MAOMs that facilitated by the colloid transport on the SOM turnover and distribution. [ABSTRACT FROM AUTHOR]

[Display omitted] Carbon dioxide-enhanced oil recovery (CO 2 -EOR) is an important means of improving oil recovery while lowering greenhouse gas emissions. However, it is still challenging to determine the pore-scale flow characteristics and interfacial interaction parameters, such as the wettability, interfacial curvature, and capillary pressure, under near-miscible conditions. In this study, X-ray microtomography (micro-CT) was used to determine the morphology of gas and oil phases and characterize the in-situ wettability under near-miscible conditions. The mean contact angle increased from 66.99° to 72.5° as the injected CO 2 pore volume (PV) increased from 5.5 PV to 33.1 PV. The wettability was locally altered as the local contact angle changed from 44.14° to 107.27° because of preferential adsorption of CO 2 molecules over oil molecules on solid surfaces. The mixed-wet condition was reflected by local contact angles both below and above 90°, and the capillary pressure exhibited a tower-shaped distribution with a peak value around zero, which promoted oil recovery. At the end of the displacement process, the interfaces were pushed toward the corners by a reduction in the capillary pressure, resulting in interface recession and a reduction in CO 2 clusters. Pore occupancy studies showed that under near-miscible conditions, gas usually occupies large pores and oil resides in pores of varying sizes, which can result in optimal recovery. [ABSTRACT FROM AUTHOR]

[Display omitted] • Hydrate phase transition and gas–water multi-phase flow were examined under pore-scale. • A LBM model of clayey silt hydrate-bearing sediment pore structure was constructed. • A quantitative description of the relationship between relative permeability and hydrate saturation was included. Unlike conventional oil and gas sources, natural gas hydrate (NGH) is stored in solid form within the pores of loose submarine sediments. Production by any method poses a complex problem involving phase transition and multi-phase flow of gas and water. The evolution of hydrate dissociation from the solid phase to gas and liquid phases in a hydrate reservoir is a dynamic process incorporating pore structure and multi-phase seepage. The solid–gas-liquid phase dissociation of NGH affects reservoir seepage properties by altering the micropore structure of the hydrate-bearing sediments. This study, based on the lattice Boltzmann method of modeling fluids flow, clearly demonstrates the micropore-scale changes in pressure and heat transfer that take place during hydrate dissociation. Pore-scale heterogeneity is found to influence the complex development of hydrates in a porous clayey silt medium during dissociation. The effect of hydrate, gas and water saturation on the evolution of relative permeability is identified, and the relationship between relative permeability and hydrate saturation during hydrate dissociation is quantified. These findings provide theoretical support for the implementation of marine natural gas hydrate production testing and future commercial production in China. [ABSTRACT FROM AUTHOR]

• Pore-scale modeling on the natural convection in reconstructed porous media. • The critical pore size where natural convection become significant is determined. • Effects of pore structure and heating direction on critical pore size are explored. • A general criterion is proposed to determine the significance of natural convection. Heat conduction in porous media is generally accompanied by the natural convection of the fluid within the pores driven by the temperature gradient. However, the criterion to determine the critical pore size of porous media where the natural convection cannot be neglected remains elusive. In this study, a pore-scale model is established to investigate the contributions of natural convection on the effective thermal conductivity (ETC) of porous media, which takes the reconstructed microstructures as the geometry input. We thus examine the effects of the porosity, thermal conductivity of the fluid and solid phases, and heating direction on the critical pore size where the natural convection starts to work. The results show that the critical pore size decreases with increasing porosity and heating temperature, but increases with the rise in the thermal conductivity of solid phase. Moreover, the critical pore size is significantly lower for the lateral heating, compared with the bottom heating. Finally, we proposed a new formula as an indicator to determine the significance of natural convection to the heat transfer in porous media. [ABSTRACT FROM AUTHOR]

We study pore-scale gas bubble ripening in porous media by combining a mass transfer method based on chemical potential differences with a level-set method for updating the gas/liquid configurations. We examine the impact of pore geometry and initial gas bubble distribution on the evolution, as well as the effect of mesh refinement and numerical parameters controlling the adaptive time steps in the model. Our results show that the pore geometry has a significant impact as it determines the rate of bubble pressure change with volume, while the area for mass transfer influences the time scale of the evolution. Simulations with different spatial patterns of bubbles with high and low pressures yield different ripening behaviors, classified as local and global ripening regimes. Simulations on sandstone with volumetrically similar bubble distributions display different ripening paths depending on the initial bubble locations, emphasizing the importance of capturing displacement history before ripening investigations. [Display omitted] • Pore throat geometry impacts bubble pressure–volume relations and ripening rates. • Bubble population ripening regime depends on the initial spatial distribution. • Different gas-bubble populations ripen at different rates and evolve differently. [ABSTRACT FROM AUTHOR]

Methane vents and gas hydrate systems coexist extensively in many regions of the world's oceans, posing challenges to the predictions of global carbon budgets and climate. However, the understanding of how methane gas migrates through deep-sea hydrate-bearing sediments and ultimately enters the overlying seawater is still incomplete. To address this issue, we used microfluidic technology to study the pore-scale mechanisms of free gas methane migration under hydrate-forming conditions. The microfluidic device is used to simulate the porous medium in submarine reservoir. The nucleation and growth of hydrates in saturated solution under the condition of methane and water flow were observed. Results indicate that during seepage, methane bubble surfaces continuously undergo the formation and detachment of tinny hydrate fragments, acting like an upward-flowing hydrate seeder. These flowing hydrate-bearing bubbles and hydrate fragments are capable of penetrating the entire oceanic hydrate system. Once captured elsewhere, these hydrate seeds, like other kinetic promoters, can significantly accelerate hydrate evolution at certain locations. Meanwhile, flowing methane repeatedly collides with the trapped hydrate fragments, either leading to the continuous flow of free methane or adsorption to the surface of the trapped hydrate fragments and eventually nucleation. Furthermore, when hydrates occupy the majority of pore channels, free methane gas can migrate through the water film between hydrates and the matrix. These three migration mechanisms may all contribute to the sustained emission of methane gas in natural seafloor hydrate systems. • Three migration mechanisms contribute to the sustained emission of methane gas. • Hydrate crystal detaching from the bubble may act as hydrate seeds and penetrate the entire oceanic hydrate system. • The constant collision of flowing methane gas with hydrate fragments may lead to hydrate nucleation. • Water film can provide flow channels for pore water and free methane gas. [ABSTRACT FROM AUTHOR]

• The first study of Fick diffusion of CO 2 in kerogen and calcite nanopores. • The confined Fick diffusion of CO 2 in shale oil is quantitatively characterized. • A modified PNM for CO 2 miscible displacement of shale oil is constructed. • The effect of organic and inorganic matter on diffusion is explored separately. • Realizing the estimation of CO 2 diffusion coefficient in shale porous media. Field practices show that CO 2 huff and puff can effectively promote the exploitation of shale resources, and CO 2 diffusion is noteworthy as a crucial mechanism of enhanced oil recovery (EOR). Understanding the confinement effect on CO 2 diffusion is imperative to accurately characterize the fluid transport in shale nanopores and further upscaling. We integrate molecular simulation and pore network model (PNM) to study CO 2 diffusion behavior at the nanoscale and pore scale. We first explored CO 2 diffusion into heptane in kerogen and calcite pores using molecular dynamics, and analyzed the impacts of pore size, molar fraction, temperature, and pore pressure on the confined Fick diffusion. Based on the molecular simulation results, a model characterizing the ratio of the confined diffusion coefficient to that of the bulk phase is constructed for further upscaling. Based on the PNM containing organic and inorganic pores, we established a model to simulate the CO 2 diffusion at the pore scale by introducing the characterization model at the nanoscale into the convective diffusion equation. Considering the microscale effects and the influence of CO 2 concentration on diffusion is beneficial to improve the simulation accuracy at the pore scale. The estimated CO 2 diffusion coefficients are in good agreement with the experimental results. This work provides better insight into CO 2 diffusion behavior in shale oil, which can afford effective support for industrial exploitation and the breakthrough of the technical bottleneck of shale oil production. [ABSTRACT FROM AUTHOR]

The wettability of porous media strongly affects the trapping mechanism and storage capacity during CO 2 geological storage. Various novel measuring methods have been proposed recently with the development of imaging techniques. This review describes the state-of-the-art wettability characterization at different scales in three aspects: conventional laboratory-based contact angle characterization to analyze the porous external surface properties, pore-scale contact angle analysis to summarize the wettability of the porous inner surface based on micromodels and X-ray microtomography, and overall wettability evaluation through nuclear magnetic resonance and thermodynamic contact angle and topology analysis. The impact of the pore structure, mineral composition, and surface roughness on pore-scale wettability is described, including contact angle hysteresis and mixed wettability. The wettability in various pore events and wettability alternation with various storage conditions and fluid displacement patterns provide a description of reservoir properties. This review also illustrates the feasibility of multiscale wettability descriptions. Future studies should focus on the interrelation among the contact angles at different scales. How the contact angle of a porous surface, especially at the pore scale, changes under reservoir conditions and the mechanism of wettability alternation still need to be further investigated. Additionally, the impact of rock wettability on storage capacity in practical applications is concluded. Future research will focus on methods to control wettability in actual storage sites to enhance economic benefits and ensure safety. [Display omitted] • Comprehensive wettability measurements at different scales are summarized. • Pore events and wetting phenomena at the pore scale are analyzed. • Wettability alternation with various components and fluid displacement patterns is revealed. [ABSTRACT FROM AUTHOR]