Pore-scale Study Research Articles

Three-dimensional pore-scale study of methane hydrate dissociation mechanisms based on micro-CT images

<p>Methane hydrate is a promising source of alternative energy. An in-depth understanding of the hydrate dissociation mechanism is crucial for the efficient extraction. In the present work, a comprehensive set of pore-scale numerical studies of hydrate dissociation mechanisms is presented. Pore-scale lattice Boltzmann (LB) models are proposed to simulate the multiphysics process during methane hydrate dissociation. The numerical simulations employ the actual hydrate sediment pore structure obtained by the micro-CT imaging. Experimental results of xenon hydrate dissociation are compared with the numerical simulations, indicating that the observed hydrate pore habits evolution is accurately captured by the proposed LB models. Furthermore, simulations of methane hydrate dissociation under different sediment water saturations, fluid flow rates and thermal conditions are conducted. Heat and mass transfer limitations both have significant effects on the methane hydrate dissociation rate. The bubble movement can further influence the dissociation process. Dissociation patterns can be divided into three categories, uniform, non-uniform and wormholing. The fluid flow impacts hydrate dissociation rates differently in three-dimensional real structures compared to two-dimensional idealized ones, influenced by variations in hydrate pore habits and flow properties. Finally, upscaling investigations are conducted to provide the permeability and kinetic models for the representative elementary volume (REV)-scale production forecast. Due to the difference in the hydrate pore habits and dissociation mechanisms, the three-dimensional upscaling results contrast with prior findings from two-dimensional studies. The present work provides a paradigm for pore-scale numerical simulation studies on the hydrate dissociation, which can offer theoretical guidance on efficient hydrate extraction.</p>

Pore-scale study of phase transitions and humins coking in cellulose hydrolyzed to levulinic acid

Hydrothermal depolymerization of cellulose has generated tremendous excitement as a promising pathway to sustainable chemistry. Among these biomass products, levulinic acid (LA), as one of the most promising platform chemicals with rich chemistry, can be used as various fuel additives or further refined into sustainable biofuel and high-value chemicals. However, hydrothermal conversion of cellulose into chemicals involves complex physicochemical processes, suffering currently from insufficient understanding of processes coupling mechanisms and carbon deposition caused by coking side reaction. Here, we developed a pore-scale multiphase reactive transport model based on the lattice Boltzmann method (LBM), coupling the multicomponent multiphase flow, heterogeneous reaction, cellulose dissolution, and humins coking, to systematically explore the conversion of cellulose to LA over acid catalyst. A relationship with a monotonic increase between the coking appearance time and H+ concentration has been found when the coking amount shows a linear decrease over acid concentration. Our results underscore the importance of in-depth elucidation of process coupling mechanisms for the establishment of efficient hydrothermal preparation strategies from cellulose to platform chemicals.

Pore-Scale Study of Gas Natural Convection in Confined Porous Media Based on Lattice Boltzmann Method

Abstract Gas natural convection is one common phenomenon in industrial applications, especially for the thermal management of electronic devices. In this study, a numerical model for gas natural convection in a confined porous cavity is constructed based on the lattice Boltzmann (LB) method, which predicts the density-difference-induced flow using a multiple relaxation time (MRT) collision operator. At the gas–solid interfaces, the microscale flow and heat transfer effects are formulated using an effective slip boundary condition. The established LB model is applied to investigate the Nusselt number for heated obstacles arranged in a staggered formation in the cavity. Based on the calculated data, the Nusselt number values obtained for a five-cylinder pore-scale (single pore, SP) domain are analyzed and compared to those for a 13-cylinder (multipore, MP) one. The Nusselt number shows a sharp decrease as soon as the microscale effect is considered at the obstacle walls. It was also observed that the Nusselt number for MP domain achieved lower values than that of SP one. The findings in this work can contribute to the design of thermal management device with confined porous media.

Pore-scale study of the effects of DTPA chelating agent flooding on oil recovery utilizing a clay-coated micromodel

The use of diethylenetriaminepentaacetic acid (DTPA) chelating agent has shown promising results for enhanced oil recovery (EOR) in prior research. Several mechanisms, mainly resulting from rock-fluid interaction, have been proposed for chelating agent flooding; however, little attention has been paid to fluid-fluid interaction thus far. The assessment of these mechanisms has primarily relied on macroscopic techniques such as core flooding. This paper aims to investigate the injection of DTPA brine and its dominant mechanisms at the pore scale using a clay-coated micromodel. The micromodel tests were performed under oil-wet and water-wet states. For a more precise examination of fluid/fluid interactions, the dynamic interfacial tension (IFT) and Zeta potential were measured. It was observed that the injection of DTPA brine in water-wet state changed the saturation distribution and increased oil recovery. Based on visual inspections, this change in saturation distribution could potentially be linked to the formation of micro-dispersions and viscoelastic interfacial phenomena. Micro-dispersions facilitate flow to unswept areas, and viscoelastic interface formation reshapes the interface between oil and brine, causing disconnected oil droplets to coalesce and thus increase recovery. Under the oil-wet state, the micro-dispersion formation and wettability alteration can be the dominant mechanisms, and the amount of recovered oil was higher than that observed in the water-wet state. Furthermore, Zeta potential measurements at the interface between brine and oil showed a more negative value for DTPA brine, which is effective in wettability alteration and micro-dispersions stability. The results indicate that IFT reduction was not significant enough to be considered the dominant mechanism, although it assists in DTPA brine penetration into the crude oil and subsequent micro-dispersion formation.

A pore-scale study on microstructure and permeability evolution of hydrate-bearing sediment during dissociation by depressurization

Research data and visual evidence to understand the interaction between hydrate dissociation and micro-seepage in sediment are currently lacking. In this study, X-ray computed tomography (CT) is used to reveal the dynamic interaction process between sediment pore-scale structural changes and permeability during hydrate dissociation. After Xe hydrate formation in spherical quartz and then decomposed the hydrate by depressurization to obtain the real-time permeability and 3D structure of hydrate-bearing sediments (HBSs) with CT scans. The results show that the heterogeneity of hydrate spatial distribution does not change the power function relationship between permeability and hydrate saturation. The ball-and-stick model shows that the number and size of flow channels dominate the flow resistance of the fluid at high hydrate saturation, while the size of flow channels dominates it at low hydrate saturation. The saturation cut-off point is between 16.35% and 25.37%. The CT scan images show that the hydrate morphology changes from patchy to load-bearing, and then to cementing. The transition process from load-bearing to cementing has the greatest impact on sediment permeability. Further PNM simulations show that the size and number of flow channels and capillary forces are the main factors affecting the ability of gas–water two-phase flow in the sediment. Most importantly, a relative permeability model related to hydrate saturation parameters is proposed based on image processing technology. The good agreement between the model and available laboratory/field measurements proves its practicability. The method of proposing this model provides a new idea to establish new relative permeability models.

Pore-scale study of dynamic surfactant adsorption under immiscible displacement in porous media: effect of wetting conditions

Surfactant flooding is one of the most effective methods of enhanced oil recovery, because it helps to reduce oil-water interfacial tension and mobilize the residual oil. Surfactant molecules can be adsorbed on the particle surface. This paper presents a systematic study of the dynamic adsorption of a water-soluble surfactant during flooding of oil-saturated porous media. The novelty lies in the study of dynamic adsorption with multiphase flows in porous media, which provides new trends. The main goal is to reveal the effect of wetting angle, interfacial tension, flow rate, and adsorption rate constant on the adsorbed amount in the immiscible displacement. Lattice Boltzmann simulations are used to study this problem. The flow regions are represented by digital images of granular porous structures. A map characterizing the adsorbed amount of surfactant versus wetting angle and interfacial tension has been presented. The results showed that increasing the wetting angle positively affects the adsorbed amount, while increasing interfacial tension suppresses the adsorbed value. Different patterns of dynamic adsorption in dependence on flow rate and adsorption rate constant have been identified. At low adsorption rates, regardless of the wetting angle and interfacial tension, a decrease in the flow rate contributes to an increase in the adsorbed amount. At high adsorption rates, the adsorbed amount is determined by the displacement efficiency. The obtained tendencies have been validated by performing a series of simulations on a group of porous structures. It has been found that the prediction reliability of the adsorbed amount increases with increasing wetting angle and capillary number.

Pore-scale study of miscible density instability with viscosity contrast in porous media

The transport of miscible fluids in porous media is a prevalent phenomenon that occurs in various natural and industrial contexts. However, this fundamental phenomenon is usually coupled with interface instabilities (e.g., viscous/density fingering), which has yet to be thoroughly investigated. In this paper, a multiple-relaxation-time lattice Boltzmann method is applied to study the displacement between two miscible fluids in porous media at the pore scale, with the coexistence of density difference (Rayleigh number Ra), viscosity contrast (R), and injection velocity (Utop). A parametric study is conducted to evaluate the impact of Ra, R, and Utop on the flow stability. For a fixed Ra that can trigger density fingering, the increase in R or Utop is found to suppress density fingering. Consequently, under a large Utop and a moderate R, the density fingering is fully stabilized and the flow follows a stabile pattern. Furthermore, as both R and Utop grow to a sufficiently high level, they can jointly trigger viscous fingering. In addition, the increasing Ra shows an enhancing effect on both density fingering and viscous fingering. Finally, by quantitatively analyzing the fingering length (lm) and the fingering propagation time (te), five different flow patterns are classified as viscosity-suppressed (I), viscosity-enhanced (II), viscosity-unstable (III), displacement-suppressed (IV), and stable (V) regimes. In a three-dimensional parameter space spanned by Ra, R, and Utop, the parameter ranges of the five regimes are determined according to lm and te. These findings hold a significant value in providing guidance for controlling the flow stability by selecting appropriate operating conditions.

Effects of carbon aggregates and ionomer distribution on the performance of PEM fuel cell catalyst layer: A pore-scale study

An in-depth understanding of the structure-performance relationship of the catalyst layer is crucial for its optimal design. In this work, the effects of carbon aggregation and ionomer distribution morphologies on the performance of the catalyst layer are investigated by pore-scale simulation. First, an optimized stochastic algorithm that can control the carbon aggregation degree and ionomer morphology is proposed to reconstruct realistic catalyst layer structures. The structural characteristics of the catalyst layer, such as pore size distribution, agglomerate size, and ionomer connectivity, are analyzed. A pore-scale model based on the lattice Boltzmann method is developed to simulate the reactive transport processes in the catalyst layer. The results indicate that the reasonable aggregation degree of carbon supports can balance the pore and cross-ionomer transport resistances of the oxygen to achieve optimal catalyst layer performance. The uniform ionomer coating in the catalyst layer can further improve the performance, and the optimal ionomer content is determined by balancing the electrochemical active surface area and the oxygen transport resistance. Based on the simulation results, a key principle for the optimal design of the catalyst layer is also proposed.

Investigation of pore geometry influence on fluid flow in heterogeneous porous media: A pore-scale study

Brazilian pre-salt reservoirs are renowned for their intricate pore networks and vuggy nature, posing significant challenges in modeling and simulating fluid flow within these carbonate reservoirs. Despite possessing excellent petrophysical properties, such as high porosity and permeability, these reservoirs typically exhibit a notably low recovery factor, sometimes falling below 10%. Previous research has indicated that various enhanced oil recovery (EOR) methods, such as water alternating gas (WAG), can substantially augment the recovery factor in pre-salt reservoirs, resulting in improvements of up to 20%. Nevertheless, the fluid flow mechanism within Brazilian carbonate reservoirs, characterized by complex pore geometry, remains unclear. Our study examines the behavior of fluid flow in a similar heterogeneous porous material, utilizing a plug sample obtained from a vugular segment of a Brazilian stromatolite outcrop, known to share analogies with certain pre-salt reservoirs. We conducted single-phase and multi-phase core flooding experiments, complemented by medical-CT scanning, to generate flow streamlines and evaluate the efficiency of water flooding. Subsequently, micro-CT scanning of the core sample was performed, and two cross-sections from horizontal and vertical plates were constructed. These cross-sections were then employed as geometries in a numerical simulator, enabling us to investigate the impact of pore geometry on fluid flow. Analysis of the pore-scale modeling and experimental data unveiled that the presence of dead-end pores and vugs results in a significant portion of the fluid remaining stagnant within these regions. Consequently, the injected fluid exhibits channeling-like behavior, leading to rapid breakthrough and low areal swept efficiency. Additionally, the numerical simulation results demonstrated that, irrespective of the size of the dead-end regions, the pressure variation within the dead-end vugs and pores is negligible. Despite the stromatolite's favorable petrophysical properties, including relatively high porosity and permeability, as well as the presence of interconnected large vugs, the recovery factor during water flooding remained low due to early breakthrough. These findings align with field data obtained from pre-salt reservoirs, providing an explanation for the observed low recovery factor during water flooding in such reservoirs.

Pore scale study of heat and mass transfer in ice-containing gas diffusion layer of polymer electrolyte membrane fuel cells

The liquid water transport and ice melting process in gas diffusion layer (GDL) are studied using the enthalpy-based multi-component multiphase flow lattice Boltzmann model. Then the heat and mass transfer process under different inlet heating temperatures, inlet flow velocities, rib-channel width ratios and contact angles of carbon fibers are discussed. The results show that heat convection and heat conduction areas are formed in the GDL, and the increase of the inlet heating temperature promotes ice melting, the heating temperature needs to be high enough to promote the reduction of the mixture content of water and ice. The inlet flow velocity has a great influence on both the ice melting efficiency and the discharge of liquid water. The reduction ratio of melting time from 0.5 m/s to 1.0 m/s is larger than that from 1.0 m/s to 1.5 m/s. The wider rib structure reduces the range of heat conduction region, so it has higher ice melting efficiency. The surface wettability of carbon fibers has no significant effect on ice melting rate in the heat convection area. Overall, the more hydrophilic the surface, the sooner the ice melts, but the more hydrophobic the surface, the more favorable the discharge of liquid water.

Pore scale study of ice melting and multiphase flow in gas diffusion layer with porosity gradient in PEMFC

An enthalpy-based multi-component multiphase flow model is established by using the lattice Boltzmann method (LBM), and the ice melting and liquid water flow behavior during cold start process in gas diffusion layers (GDLs) with porosity gradients of proton exchange membrane fuel cell (PEMFC) are captured. GDLs with five porosity gradients are considered to explore the ice melting efficiency, temperature distribution characteristics and liquid water transport. The results show that the ice melting rate is significantly affected by the porosity distribution. The model of porosity decreasing gradually from near catalyst layer to near channel side (M4) greatly improves ice melt efficiency by up to 61.44%, while the 'V' porosity gradient (M2) and the model of porosity increasing gradually from near catalyst layer to near channel side (M3) prolong the heating and melting time of 15.6% and 25.56% respectively. The arrangement of small porosity structure near the channel side enhance the convective and conduction heat transfer. The hydrophilicity of the surface of fibers is beneficial to ice melting but hinders the discharge of liquid water. Considering the ice melting efficiency and drainage performance, the GDL with porosity gradient can be used when selecting the auxiliary measure of inlet preheating.

Pore-scale study of effects of cathode catalytic layer component gradient distribution and perforation on reaction transport processes

An effective catalytic layer structure design can improve the performance of proton exchange membrane fuel cells (PEMFC). The three-dimensional cathode catalytic layer (CL) structure is randomly reconstructed, and the spatial distribution of platinum (Pt) particles, ionomers and carbon particles is analyzed. The effects of the gradient distribution of Pt and ionomer content along the thickness direction and the perforation radius and number on the reaction transport process in the CL are simulated by lattice Boltzmann method (LBM). The results show that compared with the uniform structure, the positive gradient distributions of Pt and ionomer contents can increase the reaction rate by 5.06% and 11.62%, respectively. The larger the perforation radius, the higher the oxygen concentration in the pore and ionomer, and the smaller the activation area. The maximum reaction rate of 3.31 × 10−14 mol·s−1 is achieved for a perforation radius of 28 nm. The structure of two perforations further enhanced the CL performance by 2.76%.

Pore-scale study on the stress jump coefficient in porous composite system

The stress jump coefficient at the fluid/porous interface is a fundamental parameter to study the velocity distribution in a porous composite system. In this paper, a substantial work is carried out to investigate the characteristics of the stress jump coefficient. To this end, the real pore structure of metal foam is constructed using the Weaire–Phelan model, and the macro model and the pore-scale model are presented to simulate a complex three-dimensional porous composite system. Furthermore, a novel method to determine the stress jump coefficient is proposed. The influences of the inlet velocity, the rotation number, the porosity, the free fluid layer thickness, and the flow pattern (the Poiseuille flow, the free boundary flow, and the rotating channel flow) on the stress jump coefficient are studied. The results show that the stress jump coefficient varies with the porosity, which shows that it is dependent on the porous structure. It also found that the stress jump coefficient is independent of the inlet velocity, the rotation number, and the flow pattern. When the thickness of the free fluid layer is large, the stress jump coefficient is also independent of the thickness of the fluid layer.

Pore-scale study of dynamic adsorption of a water-soluble catalyst during drainage displacement in porous media using lattice Boltzmann simulations

This paper presents the first systematic pore-scale study of the dynamic adsorption of a water-soluble catalyst during two-phase flows of immiscible fluids (water and oil) in porous media. The results of this paper are based on mathematical modeling in two-dimensional synthetic porous structures using the lattice Boltzmann simulations for two-phase flow and convective-diffusive transport of catalyst dissolved in a water that cannot penetrate the oil. The results showed that an increase in oil viscosity tends to a decrease in the maximum adsorbed amount of catalyst measured in a quasi-stationary state. An increase in capillary number, on the contrary, promotes an increase in the adsorbed amount. In this paper, for the first time, we have characterized the dynamic adsorption regimes versus the adsorption rate constant and capillary number. At a low adsorption rate, the adsorbed amount is determined by the duration of the reaction between the catalyst and the adsorbent particles, while at a high adsorption rate, the adsorbed amount is controlled by the maximum adsorption value. In addition, it has been established that a prediction accuracy of the adsorbed amount deteriorates significantly with decreasing capillary number.

Pore-scale study of calcite dissolution during CO2-saturated brine injection for sequestration in carbonate aquifers

The calcite reaction with carbonate acid is a prerequisite to release cations providing the reactant for mineral trapping, however, the complicated multiple physicochemical processes during calcite dissolution are hardly precisely captured in carbonate aquifers, impeding the CO2 sequestration technology. In this work, a pore-scale numerical study based on the lattice Boltzmann method coupled fluid flow, mass transport, heterogeneous reactions, and calcite structure evolution is implemented to investigate the calcite dissolution process. Firstly, the impact of pressure difference on this reaction is analyzed to identify two characteristic dissolution patterns. Selecting two typical pressure differences, namely 0.036 and 0.36 Pa, the dissolution dynamics under different temperatures from 25 to 85 °C is then investigated by species concentration and solid distributions elucidating that the effect of pressure difference on calcite dissolution will drop with the increment of temperature. Moreover, the regime diagram of calcite dissolution is plotted based on the relation between temperature and pressure. Finally, to upscale the simulation results to the representative element volume scale, the temporal evolution of normalized permeability, the normalized permeability-porosity relationship and the optimal operations are explored to find that the maximum discrepancy of the normalized permeability with the time among all cases is approximately seven times and the best operating conditions promoting the mineral trapping are a high-temperature formation with a high injection rate.

Reservoir characterization by push-pull tests employing kinetic interface sensitive tracers - a pore-scale study for understanding large-scale processes

The kinetic interface-sensitive (KIS) tracer experiment is a newly developed method to quantify the fluid-fluid interfacial area (FIFA) during drainage processes. The concentration breakthrough curves, obtained by measuring reaction product concentration in brine/water samples after breakthrough, are interpreted with a specialized numerical model to determine the FIFA. However, this method has its own limitations since the volume of the usable water sample (after breakthrough), based on which the analysis takes place, is often insufficient. This happens because this volume strongly depends on the aquifer material, i.e. sand type and system parameters. An alternative is to employ KIS tracers in a “push-pull” experimental set-up, i.e. primary drainage is followed by a consequent main imbibition process, with the flow direction being reversed. This study applies a pore-scale numerical model to study the KIS tracer reactive transport during a push-pull experiment in a 2D porous medium. The advective-dispersive-reactive transport during both primary drainage and main imbibition processes is discussed. The concentration breakthrough curves show a non-linear descending trend with time, until reaching a constant concentration at the steady state. By analyzing the spatial concentration distribution and the concentration breakthrough curve, the macro-scale KIS tracer reactive transport model can be interpreted for the push-pull process. Finally, the new method is applied in a column experiment, where the determined specific interfacial area is found to be close to literature data.

A pore-scale study on the dynamics of spontaneous imbibition for heterogeneous sandstone gas reservoirs

The underlying mechanism for spontaneous imbibition in a water–gas system plays a significant role in hydraulic fracturing in sandstone gas reservoirs. The objective of this study is to characterize the heterogeneity of low-permeability sandstones and investigate their effect on spontaneous imbibition from the perspective of the pore scale. We selected different cores with various pore structures and heterogeneity to evaluate their impact on the dynamics of spontaneous imbibition. The heterogeneities of the cores are contributed from the clay mineral distribution and are characterized through CT scans. The results show that clay minerals have higher CT numbers than the core matrix and that micropores are predominantly distributed in clay particles rather than in the core matrix. Additionally, the water imbibition rate of micropores is larger than that of the macropores, and when the porosities are similar, the water imbibition rate is increased with decreasing permeability. Moreover, the results of 1D frequency scans show that the distribution of water at different locations in the core is governed by the distribution of clay particles. These findings can help us further understand the distribution of fracturing fluids in the heterogeneous low-permeability sandstone reservoirs.

Pore-scale study of microbial hydrogen consumption and wettability alteration during underground hydrogen storage

Hydrogen can be a renewable energy carrier and is suggested to store renewable energy and mitigate carbon dioxide emissions. Subsurface storage of hydrogen in salt caverns, deep saline formations, and depleted oil/gas reservoirs would help to overcome imbalances between supply and demand of renewable energy. Hydrogen, however, is one of the most important electron donors for many subsurface microbial processes, including methanogenesis, sulfate reduction, and acetogenesis. These processes cause hydrogen loss and changes of reservoir properties during geological hydrogen storage operations. Here, we report the results of a typical halophilic sulfate-reducing bacterium growing in a microfluidic pore network saturated with hydrogen gas at 35 bar and 37°C. Test duration is 9 days. We observed a significant loss of H2 from microbial consumption after 2 days following injection into a microfluidic device. The consumption rate decreased over time as the microbial activity declined in the pore network. The consumption rate is influenced profoundly by the surface area of H2 bubbles and microbial activity. Microbial growth in the silicon pore network was observed to change the surface wettability from a water-wet to a neutral-wet state. Due to the coupling effect of H2 consumption by microbes and wettability alteration, the number of disconnected H2 bubbles in the pore network increased sharply over time. These results may have significant implications for hydrogen recovery and gas injectivity. First, pore-scale experimental results reveal the impacts of subsurface microbial growth on H2 in storage, which are useful to estimate rapidly the risk of microbial growth during subsurface H2 storage. Second, microvisual experiments provide critical observations of bubble-liquid interfacial area and reaction rate that are essential to the modeling that is needed to make long-term predictions. Third, results help us to improve the selection criteria for future storage sites.

Pore-Scale Study of Wettability Alteration and Fluid Flow in Propped Fractures of Ultra-Tight Carbonates

The in situ change in oil flow behavior inside propped fractures due to wettability alteration of proppant grains and fracture surfaces was thoroughly investigated for the first time in this study. A series of microscale flow experiments were performed in mixed-wet fractured and propped miniature ultra-tight carbonate cores where the effect of wettability on oil bridging and fracture oil layer integrity was probed during oil production. During the initial production, proppant wettability changed toward an intermediate-wet state (contact angle (CA) = 96°) while that of fracture surfaces became strongly oil-wet (CA = 139°). Consequently, the fracture oil layer grew in size on both fracture surfaces and imbibed into the proppant pack through piston-like displacement and pore body filling until oil bridges were formed during oil injection. However, subsequent waterflooding induced thinning and rupturing of those bridges due to the accompanying reduction in the threshold capillary pressure of the proppant at higher aging times. The in situ chemical treatment of the proppant by a cationic surfactant (dodecyl tri-methyl ammonium bromide) could reverse its wettability toward weakly water-wet state (CA = 78°) after oil solubilization from the sand grains followed by substitutive surfactant adsorption. Surfactant injection also impacted the wettability of the fracture surface due to oil solubilization, reducing its mean contact angle down to an intermediate range (CA = 99°). As a result, the following oil production cycle yielded a smaller fracture oil layer. The surfactant effect on proppant wettability lasted for 2 weeks while its effect on fracture wettability lasted for more than 6 weeks. Similar flow cycles were performed with an anionic nanoparticle (graphene quantum dot) with hydrogen bonding ability. The nanoparticle solution yielded a quick reduction of the proppant and fracture surface contact angles to nearly 77° and 115°, respectively. Proppant wettability alteration occurred because the nanoparticles self-assembled at the three-point contact region between adsorbed oil and quartz surfaces, leading to oil solubilization in intermediate-wet regions while oil-wet regions remained unchanged. Therefore, re-introducing oil into the fracture instantaneously re-instated the initial wettability state of proppant grains (CA = 88°), deeming the nanoparticle solution ineffective. This study revealed that oil production through hydraulic fractures can be enhanced by monitoring the wettability of the proppant pack. If the production has a high water cut, it is beneficial to use chemical agents that reduce the proppant contact angles to a weakly water-wet state in order to preserve the hydraulic conductivity of the oil layer.

Evolution of Water Retention Characteristics in Bio-Geochemically Altered Unsaturated Soils: A Pore-Scale Study

Biogeochemical processes in subsurface can dramatically alter the behavior of multiphase fluid flow and the hydrodynamics of porous media by bio-clogging. There exist analytical solutions such as soilsuction – saturation equations which can be used to predict the water retention curve relations in unsaturated soils. However, due to the complexity of various biogeochemical products, their pore-scale behavior and their interplay with pore structure, such analytical solutions would not provide accurate predictions. In this study, a large database of pore-networks were generated by adjusting the statistical and spatial pore and tube size distribution of the networks resembling various levels of bio-clogging. Numerical simulations including the evolution of pore structure, water retention relationship, and air invasion dynamics during desaturationwere explored. The numerical simulations verified that local pore-clogging leads to the development of isolated pore clusters and impermeable zones. The evolution of impermeable zones results in the formation of preferential flow paths towards the mobile zones for the multiphase flow problems. Using parallel computation, the critical predictors of water retention curves in bio-clogged porous media are found.