Pore-scale Simulations Research Articles

A Pore-Scale Simulation of the Effect of Heterogeneity on Underground Hydrogen Storage

Using underground hydrogen storage technology has been recognized as an effective way to store hydrogen on a large scale, yet the physical mechanisms of hydrogen flow in porous media remain complex and challenging. Studying the heterogeneity of pore structures is crucial to enhance the efficiency of hydrogen storage. In order to better understand the pore-scale behavior of hydrogen in underground heterogeneous porous structures, this paper investigates the effects of wettability, pore–throat ratio, and pore structure heterogeneity on the behavior of the two-phase H2–brine flow using pore-scale simulations. The results show that the complex interactions between wettability, heterogeneity, and pore geometry play a crucial role in controlling the repulsion pattern. The flow of H2 is more obstructed in the region of the low pore–throat ratio, and the obstructive effect is more obvious when adjacent to the region of the high pore–throat ratio than that when adjacent to the region of the medium pore–throat ratio. In high-pore–throat ratio structures, the interfacial velocity changes abruptly as it passes through a wide pore and adjacent narrower throat. Interfacial velocities at the local pore scale may increase by several orders of magnitude, leading to non-negligible viscous flow effects. It is observed that an increase in the pore–throat ratio from 6.35 (low pore–throat ratio) to 12.12 (medium pore–throat ratio) promotes H2 flow, while an increase from 12.12 (medium pore–throat ratio) to 23.67 (high pore–throat ratio) negatively affects H2 flow. Insights are provided for understanding the role of the heterogeneity of pore structures in H2–brine two-phase flow during underground hydrogen storage.

A one-field fluid/meso-structure coupling approach for multiscale transport in heterogeneous porous media

Modeling transport phenomena within heterogeneous porous media poses considerable challenges, particularly on account of the complexity of the involved geometries combined with nonlinear transport interactions. In the present study, a novel one-field modeling approach for multiscale fluid–solid interactions is proposed that does not need any a priori information on permeability. This approach implicitly considers the existence of multiscale structures through a penalization function that encompasses merely one single effective parameter. The definition, determination, as well as the response of the effective parameter to influencing factors are elaborated in detail. It is demonstrated that this approach is effective in representing properly the heterogeneity of solids. The method has been successfully applied to both nonlinear porous media flows and Darcian transport problems, exhibiting comparable accuracy but substantial computational savings as opposed to pore-scale simulations. It leads to more accurate interphase mass transfer predictions and lower computational cost in comparison with the Darcy–Brinkmann–Stokes approach. Overall, this method appears to be highly effective in forecasting realistic, industrial-scale porous media transport problems.

Multiscale simulation of water/oil displacement with dissolved CO[formula omitted]: Implications for geological carbon storage and CO[formula omitted]-enhanced oil recovery

This study combines molecular and pore-scale simulations to enhance our understanding of water, carbon dioxide (CO2), and oil flow in a porous system for high-efficiency geological carbon storage and enhanced oil recovery. We use molecular dynamics simulations to study the variation of interfacial tension (IFT), viscosity, and contact angle across varying system CO2 concentrations. The study reveals that the IFT simulation accuracy in the three-component mixture system relates positively to the volume-to-interfacial area ratio, and IFT decreases with CO2 concentration. Oil–CO2 mixture viscosity decreases and shows an intersection with water viscosity as CO2 concentration increases. The contact angle on the hydrophobic surface of kaolinite first increases and then decreases with CO2 concentration, while the hydrophilic surface contact angle is not sensitive to the CO2 concentration. Visualizing CO2 density distribution reveals its accumulation at fluid-fluid and fluid-solid interfaces and the combined effect results in the highest CO2 density at contact-line regions. Meanwhile, the elevated CO2 density at fluid-fluid interfaces reduces IFT, contributing to the initial contact angle increase, while the CO2 density increase at fluids-solid surface explains the subsequent contact angle decrease. These findings are then upscaled into a core-scale system using lattice Boltzmann simulations. The results are summarized into a phase diagram, which reveals the non-monotonic variations of the initial-residual (I-R) curves across different CO2 concentrations, and the residual saturation overall decreases with CO2 concentration for high initial saturations. These findings underscore the critical roles of system CO2 concentration and rock components, particularly kaolinite clay minerals, in subsurface multiphase seepage confronted in geological carbon storage and CO2-enhanced oil recovery, highlighting the importance of digital rock physical chemistry.

Artificial Neural Network Surrogate Model for Geochemical Calculations in Pore-Scale Reactive Transport Simulations

Artificial Neural Network Surrogate Model for Geochemical Calculations in Pore-Scale Reactive Transport Simulations

Fractional Flow Analysis of Foam Displacement in Tight Porous Media with Quasi-Static Pore Network Modeling and Core-Flooding Experiments

Fractional flow analysis is an efficient tool to evaluate the gas-trapping performance of foam in porous media. The pore-scale simulation study and the core-scale experimental work have been bridged via the fractional flow analysis to distinguish the characteristics of foam displacement inside the tight porous media with varying absolute permeability, injection rate, and foam quality. In this work, the combined investigation suggests that conventional foam-enhancing strategies, pursuing higher foam quality and stronger foam regime, are inefficient and restricted in tight reservoirs that the critical Sw corresponding to the limiting capillary pressure has increased around 37~43%, which indicates severely weakened gas-trapping capacity as permeability reduces one order of magnitude. The moderate mobility adjustment and corresponding optimized fluid injectivity exerting from the “weak foam” flow presents a staged decline feature of decreasing water fractional flow, which implies the existence of the delayed gas-trapping phenomenon when water saturation reduces to 0.5~0.6. The finding has supported the engineering ideal of promoting low-tension gas (LTG) drive processes as a potential solution to assist field gas injection applications suffering from gas channeling. Also, the validation with core-flooding experimental results has revealed several defects of the current pore network model of foam displacement in tight porous media, including exaggerated gas trapping and overestimated confining water saturation. This study has innovatively demonstrated the feasibility and potential of optimizing the foam performance of gas trapping and mobility control in tight reservoirs, which provides a clue that may eventually boost the efficiency of the gas injection process in enhanced oil recovery or CO2 sequestration projects.

Pore-Scale Simulation of Deep Shale Gas Flow Considering the Dual-Site Langmuir Adsorption Model Based on the Pore Network Model.

As a key resource in the oil and gas sector, shale gas development profoundly influences the advancement of the global energy industry. Deep shale gas reservoirs, typically found at depths exceeding 3500 m, represent a significant portion of total shale gas reserves. Currently, pore network models primarily simulate middle and shallow shale gas, insufficiently addressing the unique challenges posed by high-temperature and high-pressure conditions in deep shale gas formations. There is an urgent need to explore the microscale flow dynamics of deep shale gas. In this work, we use the pore network model to simulate the flow dynamics of deep shale gas, particularly under nanoconfinement conditions. The simulation integrates adsorption, slip flow, Knudsen diffusion, and bulk flow phenomena. Utilizing the dual-site Langmuir adsorption model tailored for high-temperature and high-pressure conditions enhances the accuracy of gas conductivity calculations for the adsorbed phase, thereby reflecting the specific characteristics of deep shale gas reservoirs. Moreover, this research investigates the flow characteristics of deep shale gas under varying sensitivity parameters, such as water saturation, temperature, and pressure. It explores methane flow dynamics, focusing on effective diffusion coefficients and permeability. The modified approach to calculating adsorption phase conductivity using the dual-site Langmuir adsorption model accurately captures the adsorption behaviors and characteristics of deep shale gas reservoirs.

Pore-scale simulation of two-phase flow in biporous media

Enhancing both permeability and capillary pumping in porous structures has emerged as a key focus for researchers, leading to the development of biporous media. While experimental studies on these structures have been conducted recently, there is a lack of numerical simulations due to difficulties in describing the geometry. To address this gap, the present study explores pore-scale numerical simulation of two-phase capillary flow in biporous media. A new simplified biporous structure is proposed, featuring a staggered arrangement of clusters, with each cluster composed of closely packed solid particles. For comparison, a monoporous media case is contrasted and represented using a conventional staggered arrangement of solid particles. Both passive and active capillary flow modes are considered in the present study. The numerical results align well with previous experimental findings on biporous media, indicating that the proposed biporous geometry effectively models two-phase flow in complex structures at a reasonable computational cost. The results show that capillary effects in biporous media are up to two times more effective than in monoporous structures. Simultaneously, permeability is enhanced by a factor of four in biporous media under similar circumstances, with most of the mass flow rate (more than 95%) passing through the larger pores between clusters. This combined impact of increased capillary action and higher permeability leads to enhanced wicking performance in biporous structures. The outcomes can help to understand two-phase flow physics in the biporous structure and develop reliable models for the simulation of biporous media on a macroscopic scale. Numerical modeling and comprehension of capillary structures play a crucial role in designing optimized geometries to enhance their performance.

Uncovering the impact of morphology of porous media and capillary number on immiscible oil recovery using computational fluid dynamics

Pore-scale simulation and analysis of oil displacement in digital rocks has shown great promise in tracking oil recovery in porous media. This paper examines three digital rocks with varying pore and throat radii but the same porosities, studying their behavior under immiscible oil recovery through waterflooding. Computational fluid dynamics was used to conduct numerical simulations to observe the impact of rock morphology on breakthrough times, residual oil saturations, and oil displacement patterns. The validity of the simulation mechanism is confirmed in both imbibition and drainage conditions through the pore doublet Chatzis' experiment. This study consisted of 33 separate simulations which varied in mobility ratios, interfacial tensions, and injection velocities. The findings suggest that smaller pore and throat radii result in higher oil recovery and water percolation and faster breakthrough times that facilitates the effects of viscous fingering. An increase in capillary number was found to have a significant impact on oil recovery in larger pores and throats.

Multiscale modeling of heat and mass transfer in dry snow: influence of the condensation coefficient and comparison with experiments

Abstract. Temperature gradient metamorphism in dry snow is driven by heat and water vapor transfer through snow, which includes conduction/diffusion processes in both air and ice phases, as well as sublimation and deposition at the ice–air interface. The latter processes are driven by the condensation coefficient α, a poorly constrained parameter in the literature. In the present paper, we use an upscaling method to derive heat and mass transfer models at the snow layer scale for values of α in the range 10−10 to 1. A transition α value arises, of the order of 10−4, for typical snow microstructures (characteristic length ∼ 0.5 mm), such that the vapor transport is limited by sublimation–deposition below that value and by diffusion above it. Accordingly, different macroscopic models with specific domains of validity with respect to α values are derived. A comprehensive evaluation of the models is presented by comparison with three experimental datasets, as well as with pore-scale simulations using a simplified microstructure. The models reproduce the two main features of the experiments: the non-linear temperature profiles, with enhanced values in the center of the snow layer, and the mass transfer, with an abrupt basal mass loss. However, both features are underestimated overall by the models when compared to the experimental data. We investigate possible causes of these discrepancies and suggest potential improvements for the modeling of heat and mass transport in dry snow.

Process-based reconstruction of digital rock based on discrete element method considering thermal-mechanical coupling effect and actual particle shape

The digital rock is a crucial platform for numerical simulations of pore-scale flow in various fields such as geo-energy, carbon (CO2) sequestration, and hydrogen (H2) storage. Under these conditions, rocks deform, and pore structures change due to temperature and stress effects. However, existing methods for reconstructing digital rocks, such as physical experimental, stochastic simulation, and machine learning, can not consider the influence of high temperature and stress. Therefore, a process-based reconstruction method based on the discrete element method (DEM) considering the thermal-mechanical coupling effect is proposed in this paper. Initially, the computed tomography (CT) images are segmented based on the watershed algorithm, and a contour database is established using the spherical harmonic analysis method. A clump template library is then constructed in PFC3D (Particle Flow Code in 3 Dimensions). Subsequently, the DEM model is established using clumps from the template library according to porosity and particle radius distribution, and the model's accuracy is evaluated based on two-point and linear path correlation functions. Micro-mechanical and thermal parameters between particles are calibrated, and different temperature and stress boundary conditions are applied to obtain digital rocks under varying conditions. Finally, digital rocks' geometric and topological structures under different conditions are analyzed, and permeability and relative permeability are calculated. Using Bentheim sandstone as an example, digital rocks are constructed under various temperature and stress conditions. The research findings indicate that high temperatures and stress result in decreased pore and throat radii, elongated throats, deteriorated connectivity, reduced porosity, and permeability, making the digital rocks more water-wet. This study provides theoretical guidance for the accurate pore-scale flow simulation of geo-energy fluids, CO2, and H2.

Pore-Scale Simulation of the Effect of Wettability Alteration on Flow Transport Kinetics in 3D Natural Porous Media

Summary Wettability alteration commonly occurs in subsurface two-phase displacements, such as enhanced hydrocarbon recovery, hydrogen storage, and carbon dioxide sequestration. A comprehensive understanding of two-phase flow transport kinetics during wettability alteration in natural rocks is essential for optimizing these processes. To address this, a wettability alteration model induced by low-salinity waterflooding (LSWF) was implemented based on the volume of fluid (VOF) method and the compressive continuous species transfer (C-CST) method in the OpenFOAM platform, which integrates the pore-scale two-phase fluid flow and the advection-diffusion of species. Following validation against experimental data from existing literature, extensive direct numerical simulations (DNSs) were conducted in an actual 3D sandstone sample obtained by microcomputed tomography (micro-CT) images. The effects of the wettability alteration degree, wettability alteration model, and capillary number on dynamic salt dispersion and fluid redistribution are considered in simulation works. The findings indicate that a higher wettability alteration degree facilitates the release of more oil trapped in smaller pores toward the outlet, while the mobilized oil might become trapped again due to snap-off in larger downstream pores. Moreover, due to the presence of alternative flow pathways in the system, the backflowed oil induced by heterogeneous salinity distribution might not be effectively recovered. A faster wettability alteration rate enhances the performance of LSWF because of the rapid reduction of entry capillary pressure and the delayed negative effect of salt dispersion. In terms of the capillary number, a higher capillary number accelerates the diffusion of species to the three-phase contact line and reduces the occurrence of snap-off retrapping, thereby increasing ultimate oil recovery. This study contributes to a deeper understanding of the microscopic displacement mechanism during the wettability alteration processes, especially for LSWF, in 3D heterogeneous porous media.

Displacement-imbibition coupling mechanisms between matrix and complex fracture during injecting-shut in-production process using pore-scale simulation and NMR experiment

Displacement-imbibition coupling oil production, which involves adjusting the operations of a well group (water injection huff and puff in one well, continuous oil production in another well), is an effective technique for enhancing oil recovery (EOR) for tight oil reservoirs. However, the research on displacement-imbibition coupling mechanisms is still missing, especially the combination of pore-scale numerical simulations and physical experiments. In this paper, a novel self-designed displacement-imbibition online nuclear magnetic resonance (NMR) experiment technique is developed. The effect of changes in fracture morphology on the mechanism of displacement-imbibition (oil saturation field and oil displacement efficiency) has been investigated. Based on the fracture morphology of physical experiments, a pore-scale model is established and solved by the finite element method. The pore-scale oil-water two-phase displacement-imbibition during the injecting-shut in-production process is simulated. The results show that the pressure oscillation makes the counter-current imbibition and co-current imbibition occur simultaneously in the matrix pores, which promotes oil recovery in matrix-fracture systems. The strength of displacement and imbibition effects is affected by the complexity of fractures. When the fracture complexity increases, the imbibition of the dense part of the fracture is stronger, while the displacement effect is weakened.

The role of Voronoi catalytic porous foam in reactive flow for hydrogen production through steam methane reforming (SMR): A pore-scale investigation

Complex pore structures in catalysts can significantly impact flow, heat transfer, and ultimately, reaction efficiency. In steam methane reforming (SMR) for hydrogen production, replacing simple catalyst geometries with intricate, interconnected structures can enhance heat transfer and optimize the SMR process. Reactive flow in Voronoi catalytic foams (VCFs) with conjugate heat transfer is investigated using OpenFOAM for pore-scale simulations. The study examines the impact of porosity, pore density (PPI), PPI distribution (uniform vs. gradient), flow direction, inlet velocity, and temperature on hydrogen production rates within a selected representative elementary volume (REV). The results show that reducing the porosity of the simple foam from 0.8 to 0.6 has led to a 25% increase in the hydrogen production mass flow rate. Additionally, replacing the simple foam with a porosity of 0.6 with a gradient foam with the same porosity can improve the hydrogen production rate by 17%. Also, it is observed that the inlet velocity has a more significant effect on the hydrogen production rather than the inlet temperature, such that increasing the inlet velocity from 0.5 to 2 m/s results in a remarkable 75% increase in the hydrogen production rate.

Pore-Scale Simulation of Tortuosity in the Catalyst Layer of Proton Exchange Membrane Fuel Cells

Pore-Scale Simulation of Tortuosity in the Catalyst Layer of Proton Exchange Membrane Fuel Cells

The impact of diffusiophoresis on hydrodynamic dispersion and filtration in porous media

It is known that the dispersion of colloidal particles in porous media is determined by medium structure, pore-scale flow variability and diffusion. However, much less is known about how diffusiophoresis, that is, the motion of colloidal particles along salt gradients, impacts large-scale particle dispersion in porous media. To shed light on this question, we perform detailed pore-scale simulations of fluid flow, solute transport and diffusiophoretic particle transport in a two-dimensional hyper-uniform porous medium. Particles and solute are initially uniformly distributed throughout the medium. The medium is flushed at constant flow rate, and particle breakthrough curves are recorded at the outlet to assess the macroscopic effects of diffusiophoresis. Particle breakthrough curves show non-Fickian behaviour manifested by strong tailing that is controlled by the diffusiophoretic mobility. Although diffusiophoresis is a short-time, microscopic phenomenon owing to the fast attenuation of salt gradients, it governs macroscopic colloid dispersion through the partitioning of particles into transmitting and dead-end pores. We quantify these behaviours by an upscaled analytical model that describes both the retention and release of colloids in dead-end pores and the observed long-time tailings. Our results suggest that diffusiophoresis is an efficient tool to control particle dispersion and filtration through porous media.

Pore-Scale Numerical Simulation of Acid-Rock Reaction Processes in Carbonate Reservoirs.

In the process of acidizing carbonate reservoirs, dissolution is employed for reservoir modification to enhance recovery rates. This study establishes a numerical model at the pore scale for acid-rock reaction flow based on a microscopic continuum medium model, integrating phase-field theory and component transport models. Subsequently, the results of the Darcy-Brinkman-Stokes model are compared to those of the arbitrary Lagrange-Euler method to validate the accuracy of the model. Finally, the flow behavior of the acid solution at the pore scale and the complex dissolution mechanisms in carbonate reservoirs are analyzed. The research indicates that the microscopic pore-scale dissolution in carbonate reservoirs mainly manifests as five dissolution modes: uniform dissolution, compact dissolution, conical wormholes, dominant wormhole, and ramified wormholes. Different distributions of microfractures will alter the flow state of the acid solution and the rock-acid reaction process within the pores. Once the wormhole breakthrough occurs, there is an increased probability of acid flow through the wormhole to the outlet, leading to a decrease in the effectiveness of the acidizing carbonate reservoirs. A proper understanding of pore-scale acid-rock reaction laws is of great significance for the development of carbonate oil and gas reservoirs.

CO2 transport through swelling organic-rich nanoporous media: Insights on gas permeability from coarse-grained pore-scale simulations

Sorption-induced swelling reduces porosity and permeability in porous media, impacting long-term CO2 storage injectivity. This study employs an innovative coarse-grained model to couple fluid flow with sorption-induced solid deformation at the pore-scale. Our model preserves the advantages of molecular dynamics in simulating complex fluid-solid interaction under nano-confinement while extending the time and length scales. Adjusting the gas-solid interaction energy controls solid swelling and gas slippage at pore walls. Gas permeability curves demonstrate a linear decline with the increasing gas-solid interaction energy in both swelling and non-swelling nanoporous media, with the former experiencing a greater drop. Surprisingly, in the regime of weak gas-solid interactions, swelling is not yet initiated and the porosity and permeability of flexible porous media increase, possibly due to gas flow-induced pore throat opening. Nanoporous media with lower initial porosity experience a greater permeability decline during swelling. The relationship between permeability and porosity changes shows a linear increase characterized by different slopes with varying initial porosities. These findings provide valuable insights into the complex interactions among gas transport, solid deformation, and porosity changes in nanoporous media, with implications for understanding and optimizing gas production and CO2 storage in realistic geological environments.

Pore-scale simulation of water pathway plugging using gel particles in oil reservoirs

Pore-scale simulation of water pathway plugging using gel particles in oil reservoirs

Pore-Scale Simulation for the Fully-Developed Flow Through a Fixed-Bed Reactor Regularly Packed with Mono-Sized Spheres with Extension to Random Packing

Pore-Scale Simulation for the Fully-Developed Flow Through a Fixed-Bed Reactor Regularly Packed with Mono-Sized Spheres with Extension to Random Packing

Melting of PCM-graphite foam composites with contact thermal resistance: Pore-scale simulation

Latent heat energy storage systems have emerged as a solution to intermittency and instability in renewable energy sources, the phase transition of phase change materials (PCMs) within these systems is hindered by their poor thermal conductivity. Graphite foam, renowned for its superior thermal properties, stands as an ideal choice to improve the melting efficiency of PCMs. In the current study, the melting behavior of PCM-graphite foam composites is simulated by the pore-scale numerical simulation (PNS) by employing cubic idealized cells with spherical pores and cylindrical channels. The computed results demonstrate that the porosity and contact thermal resistance of graphite foam are key factors affecting the melting efficiency of PCMs. The full melting time (FMT) of PCMs increases with porosity, achieving a 53.1 % improvement as porosity decreases from 0.90 to 0.75. In addition, the absence of natural convection markedly diminishes the maximum melting rate of PCM, reducing it by 33.5 % to 50.3 % for porosity from 0.75 to 0.90. Notably, it needs to be emphasized that the melting rate of PCM is significantly reduced when the contact thermal resistance surpasses 1.0 × 10–4 m2·K·W−1. Compared to the idealized condition without contact thermal resistance, the dimensionless FMTs for porosities of 0.75, 0.80, 0.85, and 0.90 extend by 27.3 %, 27.1 %, 22.2 % and 23.9 % for a contact thermal resistance of 5.0 × 10–4 m2·K·W−1, respectively.