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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]

• 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]

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]

The phase change problem in porous media has attracted extensive attention due to its complexity of both the heat transfer process and pore structure. This study investigates the mechanisms governing the freezing of water at the pore scale in a porous model, employing a combination of experimental and numerical approaches. Mathematical-physical model is used to simulate the phase change process in porous structures. Experiments are conducted on water freezing in porous resin and steel, with a pore distance of 500 μm. The evolutions of the phase interface and temperature field during the freezing process are analyzed. Results demonstrate good agreement between the simulations and experiments. Moreover, when the thermal conductivity of the solid skeleton is lower than that of the fluid (porous resin), the phase interface in the pore evolves from concave to convex, with a non-linear change in relative curvature. The temperature gradient of the fluid and skeleton within the pore channels first decreases and then increases. However, when solid skeletons have higher thermal conductivities than the fluid (porous steel), the evolution of the phase interface and its relative curvature, as well as the temperature gradient in the pore are different from the other case. • Pore-scale freezing in porous media: experiment and simulation. • Simulation results match well with the experimental observations. • Analyzed phase interface and temperature field evolutions. • Different thermal conductivities of skeletons cause varied freezing interface evolutions. [ABSTRACT FROM AUTHOR]

• We report on the effects of fluid rheology on macroscopic flow and transport properties in porous media. • Fluid viscoelasticity increases the shear rate and changes the local flow to be more elongational. • FENE-P fluid in porous media behaves like a Newtonian, shear-thinning and thickening fluid by increasing the Deborah number. • Above a critical Deborah number, flow does not reach a steady-state and the pressure gradient fluctuates. These fluctuations increase with the Deborah number. • The long-time particle dispersion in a random porous media cannot be modulated by fluid rheology. Flow of viscoelastic fluids in porous media is ubiquitous in biological and industrial processes, and the choice of viscoelastic fluids is of foremost importance for their effect on macroscopic flow and transport properties. In this paper we study the macroscopic properties of flow and transport of viscoelastic fluids through a model porous media by means of Direct Numerical Simulation (DNS). We show that flow of a viscoelastic fluid modeled via a FENE-P constitutive model features three distinct regimes of flow resistance: a plateau at low Deborah numbers, and a shear-thinning phase followed by an abrupt flow thickening above a critical Deborah number, consistent with experimental observations. Congruous to this shear-thickening, we observe the onset of hydrodynamic instabilities resulting in fluctuations in pressure drop and flow resistance. These fluctuations intensify with increasing the fluid elasticity. Finally, we investigate Lagrangian attributes of viscoelastic flow and transport through porous media. We find that although the fluid elasticity broadens the Lagrangian velocity distributions, it does not alter the long-term particle dispersion in disordered porous media. [ABSTRACT FROM AUTHOR]

Abstract Chemical-assisted waterflooding enhances the ultimate recovery of a conventional waterflooding process through several mechanisms such as reducing interfacial tension between water and oil, formation of in-situ emulsions, treatment of adverse mobility ratio, and wettability modification. Application of chemical-assisted waterflooding for recovery of heavy oil is still an active research topic. A new chemical technology (i.e. IPC Technology as referred to in this paper) has been developed. A proprietary mixture of surfactants is used in several techniques associated with surface extraction as well as in-situ recovery of heavy oil and bitumen. This formulation of solvents and surfactants is reusable, low foaming, non-flammable, not acutely toxic, and non-carcinogenic. The efficacy of using IPC as an additive for waterflooding was presented earlier through a systematic coreflooding study, which showed increased ultimate recovery of different types of oils when compared with commercial surfactant and alkali flooding. In order to investigate the mechanisms behind the enhanced macro-scale oil recovery, a series of pore-scale tests were designed and conducted in this study. Glass micromodel type of porous media was used in the presence of brines with different ion compositions. The pore-scale chemical-assisted waterflooding tests were carried out in four consecutive stages: brine flooding, chemical flooding, soaking period, and extended brine flooding. High quality pore-scale images were captured for qualitative and quantitative analysis of the IPC-assisted waterflooding at the pore-level. IPC intensified water wettability of grain particles in the presence of NaCl; however in the presence of MgCl 2 , use of IPC makes solid surfaces oil-wet. The phenomenon of water-in-oil and oil-in-water emulsification was observed when IPC was used along with different brine compositions. This phenomenon is weakened for lighter oils. When the acid number of the oil was low, extension of soaking period intensified wettability alteration and emulsification mechanisms. Other pore-scale phenomena were also observed during visualization tests such as coalescence of microdroplets during the extended waterflooding stage, snap-off mechanism creating daughter droplets during the extended waterflooding stage, occasional flow of bulk oil films toward the production end, and wettability reversal during various flooding stages. Attempts were also made to conduct statistical analysis of emulsion droplet sizes under different test conditions. Several parameters, including chemical concentration as well as ion concentration in brines, were found to significantly affect the emulsion droplet size distribution. [ABSTRACT FROM AUTHOR]

Highlights • A critical review on carbonated water injection (CWI) is provided. • Description, key characteristics, and pros and cons of CWI process are discussed. • Parametric sensitivity analysis identifies important factors/mechanisms in CWI strategy. • Main challenges for research/engineering activities in CWI processes are addressed. • Useful guidelines to design, operate, and optimize CWI operations are recommended. Abstract Carbonated water injection (CWI) is a promising enhanced oil recovery (EOR) technique in which the dissolved CO 2 can transfer to the oil phase to improve the oil mobility and to cause oil swelling, both enhancing the sweep efficiency. In addition to serving as an EOR method, CWI promotes a high storage capacity for geological CO 2 storage. A number of laboratory tests and field applications have confirmed the effectiveness of CWI. This paper provides a comprehensive review of CWI to cover its important aspects/features such as displacement mechanisms and recovery performance at various conditions/properties. In this paper, carbonated water injection process and the properties of CO 2 -brine-oil systems are described. The influences of petrophysical properties, fluid properties, and operational parameters on the performance of CWI are also thoroughly addressed. The pore-scale investigations conducted by several researchers are reported in this review. The theoretical and practical challenges associated with the implementation of CWI are also discussed to unveil the fundamental mechanisms of transport phenomena in CWI. Some of the key points that are deduced from the previous theoretical and engineering works include: the exsolution of CO 2 from carbonated water during pressure drop can provide additional energy for an extra oil recovery; detailed mathematical models capable of capturing the effects of dissolution, gravity, capillary pressure, and relative permeability hysteresis in 2-D and 3-D spatial orientation on the CWI performance have not been systematically developed yet; disparities between model results and experimental results are attributed to the instantaneous equilibrium assumptions in the model development; and the occurrence of asphaltene precipitation during CWI operations has not been highlighted in a majority of research works, while it is expected to happen during the CO 2 exsolution due to the pressure decline. [ABSTRACT FROM AUTHOR]

Highlights • Even in a homogeneous porous medium, hydrodynamic transport can be non-Fickian. • Our simulations did not validate the assumptions of the MIM model. • A new theory is needed to couple Darcy-scale transport and two-phase flow. • Relative permeability seems to be a proxy for estimation of the stagnant saturation. Abstract Hydrodynamic dispersion and mixing under two-phase flow can be found in many natural, industrial, and engineering processes such as the modified salinity water flooding (MSWF). In MSWF the injected water displaces the formation brine and will interact with the crude oil and rock to improve the oil recovery. We show throughout numerical simulations that access of the injection water to the available pore space is not homogeneous, even in homogeneous porous media, and it is controlled by the saturation topology and pore-scale velocity field. Under the steady-state two-phase flow in a homogeneous porous medium, the velocity field has a bimodal distribution. The bimodal distribution of pore-scale velocity dictates two different transport time scales spatially distributed over the stagnant and flowing regions at a given saturation topology. These distinctly different transport time scales lead to a non-Fickian transport, which cannot be captured using the conventional advection-dispersion equation. Using the volume-of-fluid method implemented in the OpenFOAM (ver. 4.0), we simulated pore-scale two-phase flow and the hydrodynamic transport at different saturations. We have investigated the impact of stagnant saturation and the tortuosity of flow pathways on the dispersion coefficient and the mass exchange rate - as the two major parameters controlling transport and mixing - under steady-state two-phase flow. At the Darcy scale, different theories such as the mobile-immobile (MIM) theory have been proposed to capture the non-Fickian transport and mixing in two-phase flow through porous media. Based on the simulation results, we assess the validity of the assumptions employed in MIM to define the stagnant saturation and mass exchange rate coefficient. The results of this research provide fresh insights into the potential impact of saturation topology on mixing between the modified salinity water and the formation brine under steady-state flow conditions, which has not been investigated and reported in the literature. [ABSTRACT FROM AUTHOR]

• Remobilization of residually trapped CO2 can occur as a result of reservoir pressure decline. • 3D X-ray microtomography was used to characterize CO2 remobilization under pressure depletion. • Remobilization was observed at a higher saturation than residual saturation. • Ostwald ripening enhanced remobilization by allowing local reconnection of trapped gas. A decrease in reservoir pressure can lead to remobilization of residually trapped CO 2. In this study, the pore-scale processes related to trapped CO 2 remobilization under pressure depletion were investigated with the use of high-resolution 3D X-ray microtomography. The distribution of CO 2 in the pore space of Bentheimer sandstone was measured after waterflooding at a fluid pressure of 10 MPa, and then at pressures of 8, 6 and 5 MPa. At each stage CO 2 was produced, implying that swelling of the gas phase and exsolution allowed the gas to reconnect and flow. After production, the gas reached a new position of equilibrium where it may be trapped again. At the end of the experiment, we imaged the sample again after 30 hours. Firstly, the results showed that an increase in saturation beyond the residual value was required to remobilize the gas, which is consistent with earlier field-scale results. Additionally, Ostwald ripening and continuing exsolution lead to a significant change in fluid saturation: transport of dissolved gas in the aqueous phase to equilibriate capillary pressure led to reconnection of the gas and its flow upwards under gravity. The implications for CO 2 storage are discussed: an increase in saturation beyond the residual value is required to mobilize the gas, but Ostwald ripening can allow local reconnection of hitherto trapped gas, thus enhancing migration and may reduce the amount of CO 2 that can be capillary trapped in storage operations. [ABSTRACT FROM AUTHOR]

For applications of reactive transport in porous media, optimal porous structures should possess both high surface area for reactive sites loading and low mass transport resistance. Hierarchical porous media with a combination of pores at different scales are designed for this purpose. Using the lattice Boltzmann method, pore-scale numerical studies are conducted to investigate diffusion-reaction processes in 2D hierarchical porous media generated by self-developed reconstruction scheme. Complex interactions between porous structures and reactive transport are revealed under different conditions. Simulation results show that adding macropores can greatly enhance the mass transport, but at the same time reduce the reactive surface, leading to complex change trend of the total reaction rate. Effects of gradient distribution of macropores within the porous medium are also investigated. It is found that a front-loose, back-tight (FLBT) hierarchical structure is desirable for enhancing mass transport, increasing total reaction rate, and improving catalyst utilization. On the whole, from the viewpoint of reducing cost and improving material performance, hierarchical porous structures, especially gradient structures with the size of macropores gradually decreasing along the transport direction, are desirable for catalyst application. [ABSTRACT FROM AUTHOR]

Matrix imbibition, which includes spontaneous imbibition (SI) and forced imbibition (FI), is the main mechanism of water-based methods, and can play a significant role in unlocking tight oil potentials as a tremendous amount of oil remains in the matrix following primary production. Previously, SI and FI have been investigated separately in pore-scale studies for several years. However, it is difficult for the results to provide guidance for selecting water-based methods owing to the different core samples and pore classification criteria adopted. Therefore, an integrated study of SI and FI is conducted on tight cores in order to understand the characteristics of oil contributions from different pores. In this work, 68 tight cores from the Chang 8 formation, Ordos Basin (China) are investigated. Nine cores are used to test native wettabilities; then, rate-controlled porosimetry is conducted on a typical tight core. Finally, nuclear magnetic resonance is implemented to determine the oil distributions before and after SI and FI for six cores. Based on the petrophysical properties, the cores are classified into three permeability levels (0.06 mD, 0.1 mD, and 0.22 mD). The SI and FI results demonstrate that FI can always provide more than twice the oil recovery factor of SI in each permeability level. For FI, more than 40% of the produced oil is contributed by mesopores. With increasing permeability, macropores contribute more oil than micropores. For SI, the oil contribution from micropores can reach 53.34%. The permeability of 0.1 mD is a critical point at which the oil contribution of mesopores surpasses that of micropores. [ABSTRACT FROM AUTHOR]

To reduce emissions of the greenhouse gas CO 2 to the atmosphere, sequestration in deep saline aquifers is a viable strategy. Residual trapping is a key containment process important to the success of CO 2 storage operations. While residual trapping affects CO 2 migration over large scales, it is inherently a pore-scale process. Pore-network models (PNMs), capturing such processes, are useful for our understanding of residual trapping, and for upscaling trapping parameters for larger scale models. A PNM for simulation of quasi-static two-phase flow; CO 2 intrusion (drainage) followed by water flooding (imbibition) was developed. It accounts for pore-scale displacement mechanisms, and was used to investigate residual CO 2 trapping. The sensitivity of the residual CO 2 saturation to several parameters was studied, to validate a trapping behavior in agreement with earlier studies. Then the PNM was calibrated to core sample data and used to simulate drainage-imbibition scenarios with different turning point saturations. From these the initial-residual saturation curves of CO 2 in Heletz sandstone were estimated, essential for future macroscopic-scale simulations. Further, the occurrence of different pore-scale mechanisms were quantified and the size distribution of the residual clusters was shown to exhibit a bimodal appearance. The findings improve the understanding of residual trapping in Heletz sandstone. [ABSTRACT FROM AUTHOR]

Physical, chemical and mechanical pore-scale (i.e. micrometer-scale) mechanisms in rock are of key importance in many, if not all, subsurface processes. These processes are highly relevant in various applications, e.g. hydrocarbon recovery, CO 2 geo-sequestration, geophysical exploration, water production, geothermal energy production, or the prediction of the location of valuable hydrothermal deposits. Typical examples are multi-phase flow (e.g. oil and water) displacements driven by buoyancy, viscous or capillary forces, mineral-fluid interactions (e.g. mineral dissolution and/or precipitation over geological times), geo-mechanical rock behaviour (e.g. rock compaction during diagenesis) or fines migration during water production, which can dramatically reduce reservoir permeability (and thus reservoir performance). All above examples are 3D processes, and 2D experiments (as traditionally done for micro-scale investigations) will thus only provide qualitative information; for instance the percolation threshold is much lower in 3D than in 2D. However, with the advent of x-ray micro-computed tomography (μCT) – which is now routinely used – this limitation has been overcome, and such pore-scale processes can be observed in 3D at micrometer-scale. A serious complication is, however, the fact that in the subsurface high pressures and elevated temperatures (HPET) prevail, due to the hydrostatic and geothermal gradients imposed upon it. Such HPET-reservoir conditions significantly change the above mentioned physical and chemical processes, e.g. gas density is much higher at high pressure, which strongly affects buoyancy and wettability and thus gas distributions in the subsurface; or chemical reactions are significantly accelerated at increased temperature, strongly affecting fluid-rock interactions and thus diagenesis and deposition of valuable minerals. It is thus necessary to apply HPET conditions to the aforementioned μCT experiments, to be able to mimic subsurface conditions in a realistic way, and thus to obtain reliable results, which are vital input parameters required for building accurate larger-scale reservoir models which can predict the overall reservoir-scale (hectometer-scale) processes (e.g. oil production or diagenesis of a formation). We thus describe here the basic workflow of such HPET-μCT experiments, equipment requirements and apparatus design; and review the literature where such HPET-μCT experiments were used and which phenomena were investigated (these include: CO 2 geo-sequestration, oil recovery, gas hydrate formation, hydrothermal deposition/reactive flow). One aim of this paper is to give a guideline to users how to set-up a HPET-μCT experiment, and to provide a quick overview in terms of what is possible and what not, at least up to date. As a conclusion, HPET-μCT is a valuable tool when it comes to the investigation of subsurface micrometer-scaled processes, and we expect a rapidly expanding usage of HPET-μCT in subsurface engineering and the subsurface sciences. [ABSTRACT FROM AUTHOR]

Taking advantage of the analogy between hydraulic and electrical flows to facilitate the prediction of porous media characteristics is a longstanding practice in engineering and science applications. The relationships between hydraulic and electrical properties are widely used in the characterization of transport phenomena relying on the strong correlation between electric and hydraulic flow conductance. However, due to the lack of small-scale investigations the similarity among their pathways/tortuosities is still unclear. Here a series of direct finite element numerical simulations are conducted within pore-level microstructures to extract and compare the streamlines of both electric and fluid flow currents and examine the accuracy of the hydraulic-electric analogy by predicting the petrophysical characteristics of the case studies. The fluid flow and electric transports are simulated through low-porosity digital unconsolidated packings of polydispersed grains representing the Athabasca oil sands deposit as the second largest oil reserve in the world. The formation factor, porosity, and absolute permeability of the media under consideration are predicted, and consequently, the streamlines of both electric and hydraulic currents are extracted and compared in terms of length, shape, and pathways. According to the results, the fluid flow streamlines are longer and pose differently than the homogeneous electric current streamlines. The average ratio between the hydraulic and electric tortuosities is 1.15 and a local extremum occurs at the porosity of eighteen percent. Pedotransfer functions for tortuosities, dimensionless permeability, and formation factor are proposed underpinning the rigorous relationships between transport processes in porous media. [ABSTRACT FROM AUTHOR]

In this paper a fully three dimensional, multiphase, micro-scale solid oxide fuel cell anode transport phenomena numerical model is proposed and verified. The Butler-Volmer model was combined with empirical relations for conductivity and diffusivity - notably the Fuller-Shetler-Giddings equation, and the Fickian model for transport of gas reagents. FIB-SEM tomography of a commercial SOFC stack anode was performed and the resulting images were processed to acquire input data. A novel method for estimating local values of Triple Phase Boundary length density for use in a three-phase, three-dimensional numerical mesh was proposed. The model equations are solved using an in-house code and the results were verified by comparison to an analytical solution within the range of its applicability. A limited parametric study was performed to qualitatively assess simulation performance and impact of heterogeneity. Despite the high dependence of the SOFC anode performance on the geometry of its anisotropic, three-phase microstructure there are very few micro-scale numerical models simulating transport phenomena within these electrodes. [ABSTRACT FROM AUTHOR]

Steam assisted gravity drainage is the main technologically and economically feasible method for in situ bitumen extraction. However, SAGD is energy intensive with economic and environmental challenges. Steam-solvent coinjection has proposed to improve SAGD performance, where hydrocarbon solvent is simultaneously injected with steam to increase the production rate and lower the steam-oil-ratio. The addition of solvent, however, complicates an already complex multicomponent thermal-chemical process. Microfluidics is well suited to quantify the pore-scale of steam-solvent coinjection with a tight control over experimental parameters. In this study, a high-pressure high-temperature micromodel combined with optical and thermal imaging is used to probe the pore-scale of steam-solvent coinjection process at relevant reservoir conditions. The effects of butane and hexane, as well as two industrial solvents, condensate and naphtha, on the pore-scale mechanisms are quantified and compared. The in situ thermal data is used to profile and analyze the condensation zone behavior and steam-solvent azeotropic temperature for all steam-solvent cases. We find that overall performance depends on the difference between steam-solvent azeotropic temperature and steam saturation temperature, the degree of solvent-bitumen dilution, and the degree of asphaltene precipitation in the condensing zone. In contrast with pure solvents and condensate, naphtha results in the highest recovery due to a higher steam-solvent azeotropic temperature, effective dilution, with minimal asphaltene deposition. [ABSTRACT FROM AUTHOR]

Turbulent flow characteristics and vortex behaviors in the pore space of real packed beds under different flow conditions is numerically investigated. Real geometric structure of randomly packed beds is modeled using the discrete element software LIGGGHTS. Both LES and RANS methods are employed to predict vortex shape and evolution features at pore scale in actual packed beds. The numerical model is validated corresponding to experimental data, a good agreement is achieved between predicted results and measurements. Based on the computational results, the distribution characteristics of vorticity and velocity in multiple representative special structures are analysed, such as the channel formed by two pellets, similar triangle area formed by compact spherical walls and channel. Besides the effects of special structures on vortex shape and turbulence intensity is studied, such as the guiding and friction effect by the side spherical wall on gas motion, and strong shear force on both sides of the main channel. The Q criterion is used to identify vortex structures and capture distribution and evolution of vortexes in the computational domain. Moreover, the effect of inlet velocity on the position and shape of eddies is investigated, and eddy morphological changes under unsteady inlet conditions are also discussed., At last, a comparison between LES and RANS methods shows that the LES method can capture more details of the distribution and shape of the vortexes in the pore space, and therefore it is more suitable to the complicated flow field in porous media. [ABSTRACT FROM AUTHOR]

To investigate the impact of rock heterogeneity and flowrate on reaction rates and dissolution dynamics, four millimetre-scale Silurian dolomite samples were pre-screened based on their physical heterogeneity, defined by the simulated velocity distributions characterising each flow field. Two pairs of cores with similar heterogeneity were flooded with supercritical carbon-dioxide (scCO 2 ) saturated brine under reservoir conditions, 50 °C and 10 MPa, at a high (0.5 ml/min) and low (0.1 ml/min) flowrate. Changes to the pore structure brought about by dissolution were captured in situ using X-ray microtomography (micro-CT) imaging. Mass balance from effluent analysis showed a good agreement with calculations from imaging. Image calculated reaction rates ( r eff ) were 5-38 times lower than the corresponding batch reaction rate under the same conditions of temperature and pressure but without mass transfer limitations. For both high (Péclet number = 2600-1200) and low (Péclet number = 420-300) flow rates, an impact of the initial rock heterogeneity was observed on both reaction rates and permeability-porosity relationships. [ABSTRACT FROM AUTHOR]

Aqueous based foam injection has gained interest for conventional oil recovery in recent times. Foam can control the mobility ratio and improve the sweep efficiency in oil reservoirs over gas flooding. However, due to the high viscosity of the oil, its application in heavy oil reservoirs is challenging. Moreover, the oil-wet nature of carbonate reservoirs makes it difficult for aqueous based foam to efficiently remove the heavy oil. On the other hand, hydrocarbon solvents have been used for decreasing the heavy oil viscosity and increase its recovery by diffusion and mixing mechanisms. However, the low rate of diffusion/dispersion, inadequate sweep efficiently and asphaltene precipitation, especially in heterogeneous reservoirs, are the major challenges during solvent injection. In this paper, hydrocarbon solvent based foam (SBF) as an alternative to solvent flooding is explored for heavy oil recovery from a fractured porous media. Hydrocarbon solvent-based CO 2 foam was used to increase sweep efficiency during heavy oil recovery. The foam was generated with the help of a fluorosurfactant in a hydrocarbon solvent. Static bulk performances of foam were analyzed at different concentrations of surfactant. Surface tension measurements were also performed to study the adsorption of surfactant into the liquid-gas interface and its effect on foamability and foam stability. A specially designed fractured micromodel (oil wet, representing fractured carbonate reservoirs) was used to visualize the pore-scale phenomena during solvent based foam injection. For the first time, extra heavy oil with a viscosity of about 30,000 cp has been used in this study for pore scale visualization. A high-quality camera was utilized to capture images/movies. According to static experiments, although the value of the surface tension of hydrocarbon solvent was initially low, the addition of surfactant slightly decreased the surface tension further improving the foam stability. However, the mechanisms involved in foamability and foam stability of non-aqueous based foam are more complicated than water based foam and cannot be discussed solely based on surface tension and bulk analysis. In addition, dynamic pore scale observation through this study revealed that solvent based foam can significantly contribute to heavy oil recovery with different mechanisms. At the initial stage, solvent diffuses and mixes with viscous oil and reduce the viscosity. Later, foam bubbles improve the sweep efficiency by diverting the solvent toward the untouched part of the porous media. Foam bubbles partially blocked the larger pores in matrix/swept-area increasing the contact of solvent and heavy oil, providing better mixing. Therefore, oil is swept much faster and more efficiently from the grain in oil-wet porous media compared to that of conventional solvent flooding. [ABSTRACT FROM AUTHOR]

This paper aims at developing a pore-scale numerical model for non-Darcy fluid flow through rough-walled fractures. A simple general relationship between the local hydraulic conductivity and the flow velocity is proposed. A new governing equation for non-Darcy fluid flow through rough-walled fractures is then derived by introducing this relationship into the Reynolds equation. Based on the non-linear finite element method, a self-developed code is used to simulate the non-Darcy fluid flow through fractures. It is found that the macroscopic results obtained by the numerical simulation agree well with the experimental results. Furthermore, some interesting experimental observations can be reproduced. [ABSTRACT FROM AUTHOR]

Steam assisted gravity drainage (SAGD) is the established method for in situ bitumen extraction. However, natural gas powered generation of steam for the SAGD process results in large economic and environmental costs. Alcohol- and alkaline-based additives are emerging strategies to improve SAGD performance, and micromodels are uniquely well-suited to directly compare and quantify the effectiveness of additives with all other variables tightly controlled. The distinct effects of alcohol- and alkaline-based additives on the pore-scale mechanisms were quantified here using a representative high-pressure high-temperature glass micromodel with both optical and thermal imagings ( P =0.7 and 1.3 MPa; T =180 and 200 °C). Alcohol addition resulted in viscous fingering, some solvency, oil-wet trapping in the mobile zone, mainly water-in-oil emulsions, and moderate increases in production. In contrast, alkaline addition resulted in water-wet surface condition, emulsification of the bitumen into discrete oil-in-water droplets, and significantly increased production. Temperature profile data combined with optical results were also used to accurately determine the interface and expansion of discrete regions formed during the SAGD including steam chamber, hot water flow zone, mobile zone, and untouched bitumen zone. Alcohol provided only a slight improvement on the steam chamber growth, while alkaline additive expanded the steam chamber by over 10% and significantly improved the overall recovery. [ABSTRACT FROM AUTHOR]

In the last decade, the study of fluid flow in porous media has developed considerably due to the combination of X-ray Micro Computed Tomography (micro-CT) and advances in computational methods for solving complex fluid flow equations directly or indirectly on reconstructed three-dimensional pore space images. In this study, we calculate porosity and single phase permeability using micro-CT imaging and Lattice Boltzmann (LB) simulations for 8 different porous media: beadpacks (with bead sizes 50 µm and 350 µm), sandpacks (LV60 and HST95), sandstones (Berea, Clashach and Doddington) and a carbonate (Ketton). Combining the observed porosity and calculated single phase permeability, we shed new light on the existence and size of the Representative Element of Volume (REV) capturing the different scales of heterogeneity from the pore-scale imaging. Our study applies the concept of the ‘Convex Hull’ to calculate the REV by considering the two main macroscopic petrophysical parameters, porosity and single phase permeability, simultaneously. The shape of the hull can be used to identify strong correlation between the parameters or greatly differing convergence rates. To further enhance computational efficiency we note that the area of the convex hull (for well-chosen parameters such as the log of the permeability and the porosity) decays exponentially with sub-sample size so that only a few small simulations are needed to determine the system size needed to calculate the parameters to high accuracy (small convex hull area). Finally we propose using a characteristic length such as the pore size to choose an efficient absolute voxel size for the numerical rock. [ABSTRACT FROM AUTHOR]

Pore-scale simulations using a nanoscale FIB-SEM three dimensional microstructure have been used to estimate the absolute permeability and diffusive tortuosity of a representative limestone sample collected from the Arab Formation (Jurassic) carbonate gas reservoir of Abu Dhabi, United Arab Emirates. Estimated absolute permeability (k) values are in excellent agreement with reported experimental permeability values. However, estimated diffusive tortuosity (τ d ) values required to calculate effective diffusivity (D eff ) values of multiple species involved in thermochemical sulfate reduction (TSR) are unrealistically larger than hydraulic tortuosity (τ h ) values. Widely employed empirical correlations (e.g. τ d = φ D m /D eff ) which include only porosity as the factor affecting τ d of natural porous media yield unrealistically smaller values than 1.0. Realistic τ d values were only obtained if a constrictivity factor (δ) was included in the estimation of τ d . Given the difficulty in estimating both δ and τ d values simultaneously, it is proposed that D eff values for reactive transport simulations of TSR, which are commonly calculated as function of φ and τ d (e.g. D eff = (φ/τ d )D m ), should be instead estimated as function of a formation factor (F) (D eff = FD m ). In the proposed expression F accounts for the combined effect of porosity, tortuosity and constrictivity (F = δφ/τ d ). Our pore-scale simulation results shows that F can be accurately estimated from pore-scale simulations using FIB-SEM 3D images. [ABSTRACT FROM AUTHOR]

A CO 2 -brine drainage and imbibition cycle was performed in a Boise sandstone sample at reservoir conditions (1300 PSI, 44 °C) at the Advanced Light Source, LBNL. The sample was repeatedly imaged, at the pore-scale, using synchrotron-based X-ray microtomography. In particular the temporal evolution of residually trapped CO 2 was monitored for about 30 hours in order to quantify fluid stability at early stages following imbibition. The data show that fluid phase distribution distribution was not stable with time over the course of the experiment. We hypothesize that fluid displacement is caused by local capillary equilibration following the imbibition process. [ABSTRACT FROM AUTHOR]

Owing to the complexity of geological formation, a detailed understanding of all transportation phenomenon of CO 2 /water in porous media occurring in CGS is not yet gained. Heterogeneity in wettability and pore structure exists in mostly natural geological formation which has been proof that gives strong effects on porous media flow. Since macroscopic transportation phenomenon in porous media are always determined by pore-scale properties, investigation on heterogeneity at pore-scale gives the most fundamental influence mechanisms of heterogeneity in wettability and pore structure. In this study, we reported a pore-scale study on the influence of heterogeneity in wettability and pore structure in vertical CO 2 /water displacement experiments using a micro-CT machine. Two stratified sand packs, BZ06+BZ02, quarz+dolomite, were employed in displacement experiments. The experiment results indicate that, CO 2 saturation pattern shows a big difference between dolomite layer and quartz layer, CO 2 enters more pore spaces and owns higher CO 2 saturation in dolomite layer than quartz layer in drainage. While in glass beads pack, it is noticeable that the capillary barrier of BZ02 layer significantly affects CO 2 percolation process. CO 2 occupied little void volume of BZ02 porous matrix. In imbibition, residual CO 2 was mainly trapped in more wetting quartz layer in quartz+dolomite pack, and more compact layer in BZ06+BZ02 pack. [ABSTRACT FROM AUTHOR]

We study the impact of brine acidity and initial pore structure on the dynamics of fluid/solid reaction at high Péclet numbers and low Damköhler numbers. A laboratory μ -CT scanner was used to image the dissolution of Ketton, Estaillades, and Portland limestones in the presence of CO 2 -acidified brine at reservoir conditions (10 MPa and 50 °C) at two injected acid strengths for a period of 4 h. Each sample was scanned between 6 and 10 times at ∼4 μm resolution and multiple effluent samples were extracted. The images were used as inputs into flow simulations, and analysed for dynamic changes in porosity, permeability, and reaction rate. Additionally, the effluent samples were used to verify the image-measured porosity changes. We find that initial brine acidity and pore structure determine the type of dissolution. Dissolution is either uniform where the porosity increases evenly both spatially and temporally, or occurs as channelling where the porosity increase is concentrated in preferential flow paths. Ketton, which has a relatively homogeneous pore structure, dissolved uniformly at pH = 3.6 but showed more channelized flow at pH = 3.1. In Estaillades and Portland, increasingly complex carbonates, channelized flow was observed at both acidities with the channel forming faster at lower pH. It was found that the effluent pH, which is higher than that injected, is a reasonably good indicator of effective reaction rate during uniform dissolution, but a poor indicator during channelling. The overall effective reaction rate was up to 18 times lower than the batch reaction rate measured on a flat surface at the effluent pH, with the lowest reaction rates in the samples with the most channelized flow, confirming that transport limitations are the dominant mechanism in determining reaction dynamics at the fluid/solid boundary. [ABSTRACT FROM AUTHOR]

We study the effects of coal wettability on gas bubble/water two-phase flow behaviour in micro-cleats of the coal seam gas reservoir using a free energy based two-phase lattice Boltzmann model. The model is validated by comparison with analytical results and published results, and good agreement is achieved in general. Then we use this model to simulate bubble-water flow in both smooth capillary and a capillary with a narrow throat to systematically study the influences of contact angle, capillary pressure and bubble size on the flow behaviour. The simulation results indicate that both the bubble size and contact angle have significant impacts on the flow capacity of bubble and water, especially in a channel with a narrow throat. A decrease in water flow rate is observed when larger bubbles occur, and the water flow rate increases when the gas wettability becomes stronger. The bubble flow dynamics significantly influence the drainage of water and the further gas production. [ABSTRACT FROM AUTHOR]

Current thermal methods for bitumen recovery are effective but require both environmental and economic performance improvements. Light hydrocarbon solvent injection is an alternative to steam injection that has the potential to reduce greenhouse gas emissions associated with recovery. However, the pore-scale behavior of the solvent-bitumen system is very complex and poorly characterized to date. In this work, a high-pressure high-temperature micromodel with reservoir-relevant geometry combined with imaging tools provides a pore-scale window into solvent injection methods using propane and butane. The combination of two-phase dynamics and solvency at the condensing edge leads to significant bitumen production, similar in both propane and butane injections. In the liquid zone ahead of the condensing edge, butane results in dense, small, and immobile solvent-in-residue emulsions. In contrast, liquid propane produced larger emulsions with some mobility. Spectroscopy combined with fluorescence imaging indicated that the residual immobile emulsions in the butane case are bitumen heavy fractions, and largely asphaltenes. Collectively this work highlights the distinct recovery mechanisms and limitations at both the condensing front and liquid zone for these common solvents. [ABSTRACT FROM AUTHOR]

• A lattice Boltzmann model for modeling the gas hydrate dissociation is proposed. • The gas-water flow during methane hydrate dissociation is analyzed. • Fluid flow promotes hydrate dissociation. • The phenomenon of bubbles formed by liquid curling flows is observed. A multi-phase multi-component lattice Boltzmann model for modeling the gas hydrate dissociation process is proposed, which takes into account the dissociation kinetics, gas-water migration, and heat-mass transfer processes. This model is validated by simulating the xenon hydrate dissociation process. Subsequently, the dissociation mechanism of methane hydrate in microporous media, the gas-water migration, and the effect of fluid flow on the dissociation are investigated by using the present model. The simulation results indicate that methane hydrate dissociation facing the inlet is faster than that away from the direction of inlet flow. Meanwhile, fluid flow promotes methane hydrate dissociation. In addition, the rapid dissociation process within the pore space may lead to a large amount of water production accumulation. Another noteworthy phenomenon is the observation of bubbles formed by liquid curling flows. Liquid curling is one of the reasons for bubble formation near the gas-liquid interface. [ABSTRACT FROM AUTHOR]

There is increasing interest in synthesis of silicon dioxide (SiO 2) nanoparticles from agricultural biomass waste because the process is cost-efficient and eco-friendly. However, the application of rice straw SiO 2 nanoparticles as enhanced oil recovery (EOR) agents have not been examined in detail. Moreover, the pore-scale mechanisms governing the mobilization and displacement of resident oil during rice straw silica nanofluids (RS SNF) flooding is rarely reported in literature. In this study, SiO 2 nanoparticles (nanosilica) were synthesized from rice straw and characterized. Oil-water interfacial tension (IFT) and contact angles were measured to assess the performance of RS SNF as EOR agents. Pore-scale visualization experiments were conducted using 2-dimensional micromodel to identify the prevailing EOR mechanisms of RS SNF flooding. Significant reduction in oil-water IFT and contact angles were obtained in presence of RS SNF. These results were comparable to the performance of commercial silica nanofluids, suggesting that RS SNF could be utilized as favourable substitute to commercial SNF. The ideal rice straw nanosilica concentration for achieving optimum residual oil mobilization and microscopic sweep was identified as 0.1 wt% from pore-scale visualization experiments. At similar conditions, the oil retained (oil saturation) within the micromodel was determined by Image J software as 28.51% after 0.05 wt% RS SNF flooding and 23.73% after 0.1 wt% RS SNF flooding. The oil trapped in the micromodel after rice straw nanosilica–SDS and commercial nanosilica–SDS solutions injections were comparable at 19.35% and 18.33% respectively. The displacement efficiency and microscopic sweep was much higher when the synergy of nanoparticles and 0.2 wt% sodium dodecyl sulfate (SDS) solutions were injected into the micromodel due to the lowest oil-water IFT and contact angles achieved by SiO 2 /SDS solutions. The SiO 2 /SDS fluid front propagated uniformly and in piston-like movement pattern, suggesting that the impact of viscous force over capillary oil retention forces was significantly higher. During nanofluids flooding, uniform invasion of the displacing fluid was observed at the onset of fluid injection, the nanofluid become thicker whereas the oil become thinner with the advancement of the displacing front. However, fingering of the displacing fluid front was noticed with time, resulting in progression of displacement fronts with dendritic and dispersive patterns. Flow behaviours of synergy rice straw nanosilica and SDS as function of time. The rice straw nanofluid concentration is 0.05 wt% and SDS concentration is 0.2 wt%. The resident crude oil is coloured dark yellowish while the nanofluids is milky coloured. Flow direction is from left to right. [Display omitted] • Mechanisms of oil recovery by rice straw nanosilica (RS SNF) was studied. • Enhanced oil recovery performance of RS SNF was comparable to commercial nanosilica. • Optimum nanosilica concentration for maximum oil recovery was identified as 0.1wt%. • Fingering effect was more prominent in absence of surfactant. • SiO 2 /SDS fluids produced uniform and piston-like displacement front. [ABSTRACT FROM AUTHOR]

Continuous increasing of CO 2 emission is observed to a main potential reason of global warning, while CO 2 geological sequestration (CGS) offers a promising technique to solve this huge environmental challenge that the world has to face today. There are, however, still some unsolved problems in CGS procedure, which is determined by pore-scale displacement mechanisms. In this study, we investigate the drainage and imbibition process of CO 2 -brine-glass beads system under reservoir conditions. A micro-focused X-ray computed tomography (micro-CT) machine with high resolution was introduced to imaging CO 2 /brine displacement and analyse the pore-scale mechanisms. The result shows that, CO 2 is continuous and brine phase remains in small pores and throats in drainage. The shape of CO 2 /brine meniscus indicates that glass beads behave intermediate wet. While in imbibition, the effect of pore-scale displacement mechanisms become significant, piston-like front was observed at pore scale, CO 2 phase is cracked and forms isolated ganglia by snap off events. [ABSTRACT FROM AUTHOR]

Effects of heterogeneity of mineral distribution and reaction rate on the rock dissolution process are investigated using a pore-scale reactive transport model based on the lattice Boltzmann method. Coupled fluid flow, species transport, chemical reaction and solid structure alternation due to dissolution are simulated. Effects of mineral distributions and chemical heterogeneity on the dissolution behaviors and evolutions of hydrologic properties are studied under different reactive transport conditions. Simulation results show that the coupling between advection, diffusion and reaction as well as the mineralogical heterogeneity leads to complex reactive transport behaviors and complicated temporal evolutions of hydrologic properties including porosity, permeability and reactive surface. Diverse relationships between surface area and volume are predicted, which cannot be described by simple models such as the spherical-grain model. Porosity-permeability relationships also differ under different mineral distributions and reactive transport conditions. Simulation results indicate that it is extremely challenging to propose general relationships for hydrologic properties for dissolution of rocks with mineralogical heterogeneity, due to the complicated interactions between reactive transport and mineralogical heterogeneity. [ABSTRACT FROM AUTHOR]

Drying process of porous medium is of interest in engineering applications. However, pore-scale studies of wettability effects on the drying process of three-dimensional (3D) porous media remain limited. X-ray microtomography was used to investigate the wettability effects on the drying process of 3D porous medium from a pore-scale perspective. Results show higher wettability increases capillary pressure, which enhances the rearrangement of liquid from large pores to small pores. We also found that liquid clusters are unlikely to disappear for a long time confirming that higher wettability supports the flow through liquid film. This study also provides new insight that capillary flow by liquid film is associated with branch clusters. These clusters maintain the gas–liquid interfacial area that acts as the evaporation surface. As a result, drying is intensive in a water-wet porous medium with a longer constant rate period in which drying is controlled by external diffusion. In a neutral-wet porous medium, an early drop in drying rate occurs. Thus, drying is mainly a falling rate period limited by vapor diffusion through the newly emerging dried region within the medium. [Display omitted] • The first 3D pore-scale study of wettability effects on the drying process of porous medium. • Higher wettability enhances capillary rearrangement and supports the flow through liquid film. • Liquid clusters are morphologically characterized and their distribution was analyzed. • Capillary flow by the liquid film is associated with branch clusters. [ABSTRACT FROM AUTHOR]

Residual trapping of supercritical carbon dioxide ("scCO 2 ") is a key mechanism contributing to the safety and security of geologic sequestration operations. Recent experimental studies have suggested that cycles of scCO 2 and brine injections cause surface chemistry reactions that increase residual trapping. We present results of a new experiment, analyzed with X-ray microcomputed tomography, aimed at pinning down specifics of the alteration mechanism. Four cycles of scCO 2 and brine injections were conducted in a Bentheimer sandstone core with a prominent low permeability region at the base (inlet side) of the core. Multiple successive scCO 2 injections were performed within cycles, and scCO 2 injection flow rate was varied. Saturation profiles near the low permeability layer remain approximately constant, while downstream scCO 2 saturation profiles change shape and generally increase for successive injections. Residual trapping increases over the four cycles, but only in the upper region of the core. Microstructural analysis of the scCO 2 phase indicates increasing presence of relatively high contact angle (i.e. less water-wetting) surfaces as the experiment progresses. These results suggest that surface chemistry alteration occurs during drainage injections, and may also occur during imbibition. However, the effect is only evident in the relatively high permeability region of the core; in the low permeability region, capillary heterogeneity dominates flow patterns and wettability alteration effects are not evident. These results support previous work demonstrating sandstone wettability alteration due to scCO 2 /brine cycling, and provide new clarification as to the conditions under which this wettability alteration will cause shifts in scCO 2 flow and trapping. [Display omitted] • Experiments of repeated scCO 2 injections and scCO 2 -brine cycles in layered sandstone visualized with microCT. • Flow rate, scCO 2 injection iteration, and scCO 2 -brine cycle number influence core saturation profiles, contact angle, and residual trapping. • Wettability alteration manifests differently in preferential pathway region vs. free flow region. [ABSTRACT FROM AUTHOR]

The dynamics of droplet detachment from a surface is a fundamental topic studied in coating engineering, fluid mechanics, and subsurface engineering applications. This topic has direct relevance to wettability alteration using the modified ionic composition of water in contact with oil droplet, low salinity waterflooding (LSWF). Previous experimental studies of LSWF have shown a very long timescale in wettability alteration which cannot be explained using bulk diffusion coefficient. In the present study, we address both the time scale of detachment, as well as the impact of buoyancy and interfacial forces (referred to as Bond number) on droplet detachment by proposing an advanced GPU-enhanced lattice Boltzmann model. In this model, the immiscible two-phase flow has been coupled with wettability alteration due to the salinity dilution. After full validation of the model against the former experiments, a set of computational setups with distinct Bond numbers were designed to investigate the effect of interfacial tension and droplet size on the dynamics of droplet detachment. Results demonstrate that droplet detachment from a surface is not a unique function of Bond number and diffusion length scale is critical in detachment time. In summary, this study provides a fully validated model of LSWF and delineates the significant difference in impacts of interfacial tension and bubble size on detachment time, which can be used to predict capability for LSWF, to determine favorable conditions, as well as to observe and explain low-salinity-effect. • A validated lattice-Boltzmann model for two-phase wettability alteration is proposed. • Diffusion through thin films seems to control the dynamics of wettability alteration. • No unique relation between the Bond number and droplet detachment time was found. [ABSTRACT FROM AUTHOR]

Capillary and gravity forces are considered two important drivers for enhanced oil recovery (EOR) in low-permeability reservoirs. To better understand the residual oil distribution and oil displacement mechanisms of these two driven systems, core- and pore-scale imbibition experiments were conducted in this work. The core-scale results indicated that the imbibition of surfactin (capillary force-driven) and an anionic gemini surfactant (AG, gravity-driven) solutions were comparable (only 0.6% original oil-in-place (OOIP) difference). And pore-scale results showed that surfactin and AG systems exhibited different dominant forces, cocurrent imbibition and countercurrent imbibition. Regarding the residual oil during imbibition, the surfactin solution creates an obvious oil-water-solid interface in the microtube and pushes the residual oil to coalesce into a "cluster" near the outlet. In contrast, the AG solution leaves residual oil films on the microtube wall during imbibition. It was concluded that the interface mechanisms of these differences are revealed by the investigations of contact angles and solid-liquid interfacial forces, which indicated that the original hydrophilic solid wall is exposed by surfactin, leading to capillary force-dominated imbibition, while the AG molecules only absorbed on the oil film that attach to the pore wall, instead of peeling it, could not establish significant capillary forces and must depend on gravity for severe imbibition. These findings explain the micro mechanisms and residual oil distribution features of different imbibition systems, which is helpful to optimize oil producing processes at a given reservoir during given development stages. [Display omitted] [ABSTRACT FROM AUTHOR]