Pore Walls Research Articles

Hierarchically Biomimetic Scaffolds with Anisotropic Micropores and Nanotopological Patterns to Promote Bone Regeneration via Geometric Modulation.

Structural engineering is an appealing means to modulate osteogenesis without the intervention of exogenous cells or therapeutic agents. In this work, a novel 3D scaffold with anisotropic micropores and nanotopographical patterns is developed. Scaffolds with oriented pores are fabricated via the selective extraction of water-soluble polyethylene oxide from its poly(ε-caprolactone) co-continuous mixture and uniaxial stretching. The plate apatite-like lamellae are subsequently hatched on the pore walls through surface-induced epitaxial crystallization. Such a unique geometric architecture yields a synergistic effect on the osteogenic capability. The prepared scaffold leads to a 19.2% and 128.0% increase in the alkaline phosphatase activity of rat bone mesenchymal stem cells compared to that of the scaffolds with only oriented pores and only nanotopographical patterns, respectively. It also induces the greatest upregulation of osteogenic-related gene expression in vitro. The cranial defect repair results demonstrate that the prepared scaffold effectively promotes new bone regeneration, as indicated by a 350% increase in collagen I expression in vivo compared to the isotropic porous scaffold without surface nanotopology after implantation for 14weeks. Overall, this work provides geometric motifs for the transduction of biophysical cues in 3D porous scaffolds, which is a promising option for tissue engineering applications.

Dynamic mechanical response, energy absorption capacity, and constitutive modeling of polypropylene fiber-reinforced foamed concrete under high temperature

This study utilized a modified split Hopkinson pressure bar apparatus to subject polypropylene fiber-reinforced foamed concrete (PPFRFC) to substantial deformation loading at high temperatures and strain rates. Based on experimental results, the study systematically investigated the coupled effects of temperature and strain rate on the dynamic mechanical behavior of PPFRFC across a broad range of strain rates (0.001 s−1 to 1300 s−1) and temperatures (25 °C–600 °C). The findings revealed that elevated temperatures significantly affected various mechanical parameters including peak stress, plateau stress, elastic modulus, densification strain, dynamic increase factor (DIF), and energy absorption. Notably, with increasing temperature, the strain rate amplified the peak stress, plateau stress, and energy absorption, whereas its influence on the elastic modulus diminished. Microstructural examination revealed the absence of notable cracks in the pore walls after high-temperatures exposure. However, degradation of the cement matrix results in a loose skeleton structure within the pore walls, leading to a considerable reduction in material strength. Finally, a constitutive model was developed, considering the coupling effects of temperature and strain rate. This model accurately describes the mechanical response of the PPFRFC across various stages, including the elastic, plateau, and densification stages, as well as the stress drop behavior in the transition stage. Moreover, it effectively reflects the influence of strain rate and temperature coupling effects on the material's mechanical properties.

Risk assessment on the impact of Non-Darcy flow on unconventional well performance.

Nano-pores in unconventional reservoirs can cause deviations from Darcy's law where permeability becomes a strong function of pore size and pore pressure. As pores become smaller, the interaction of gas molecules with the pore walls becomes increasingly dominant compared to interactions between gas molecules. This increases gas mobility leading permeabilities measured in the lab not to be accurate at reservoir conditions. Thus, corrections to Darcy flow may be needed when analyzing well production behavior. This paper demonstrates a novel workflow for modeling the impact of non-Darcy flow regimes (i.e., gas slippage, transition flow and Knudsen diffusion) from pore scale characterization to reservoir scale flow simulation. A stochastic approach is used for uncertainty assessment on non-Darcy corrections needed for methane mobility as pore pressure depletes based on reservoir properties, pore sizes, rock compressibility and compaction. Flow simulations are conducted for a range of scenarios to test the impact on the recovery of an unconventional well. Study is conducted using a commercial simulator since workflow modifies the input to the simulator and does not need a new code development. The workflow is demonstrated using field data and shows that non-Darcy flow regimes can have significant impact by increasing gas mobility at low pore pressures near hydraulic fractures. For the field under study, ignoring non-Darcy flow risks up to 20% lower estimations in cumulative gas production under certain scenarios.

Multiscale Model for Hydrogen Chloride Uptake on Aluminum for Spacecraft Fire Safety

An electrical fire within spacecraft environments often produces poisonous gases, such as hydrogen chloride (HCl). These gases adsorb on the interior materials of the spacecraft, primarily treated aluminum surfaces. A safe design of the spacecraft can be facilitated by modeling the spacecraft compartment. In this study, a multiscale model is developed to predict HCl uptake by anodized aluminum, which has a thick oxide layer covering its surface compared to other treatments. X-ray photoelectron spectroscopy revealed that aluminum chloride is found deep in the oxide layer of anodized aluminum, implying that the pore-scale transport and reaction of HCl within the oxide layer covering the aluminum surface must be considered. Hence, a multiscale approach is warranted. A pore-scale model is first developed, wherein the HCl reacts with the aluminum oxide pore walls to create an aluminum chloride layer. The HCl then diffuses through that layer for further reaction, until the layer (diffusion resistance) grows too large and the uptake of HCl stops. This pore-scale model is coupled with a reactor-scale model to predict overall HCl uptake. Results show that this new multiscale approach predicts the uptake of HCl far more accurately when compared to experimental data than previously used single (reactor)-scale models.

Superlubricity Achieved by a Transparent Poly(vinylpyrrolidone) Composite Hydrogel with Glycerol Ethoxylate in Ocular Conditions.

Efficient and stable ocular lubrication is pivotal in safeguarding eye tissues from wear, especially under repetitive strain due to frequent blinking. Hydrogels have been reported to possess adjustable mechanical properties, biocompatibility, durability, and elevated water content and extensive utilization in medical fields. In this work, a kind of visible photo-cross-linking poly(vinylpyrrolidone) (PVP) hydrogel was designed and synthesized using 1-vinyl-2-pyrrolidone (NVP) and poly(ethylene glycol) diacrylate (PEGDA). To optimize the structure and improve the lubrication performance of hydrogels, we prepared and investigated glycerol ethoxylate (GE)-introduced composite hydrogels (GE/PVP). The results show that the addition of 3 wt % GE helped the hydrogel to form a uniform and dense porous matrix and reduce the frictional coefficient (COF) by over 50%, achieving superlubricity (COF ≈ 0.005). However, with the excessive increase of GE (6 wt %), the structure of the hydrogel is destroyed, inducing pore walls to thin and expand. After that, a lubrication mechanism of the GE/PVP composite hydrogel was proposed, in which the addition of GE reduced the surface tension of the hydrogel, enhanced the hydration ability of the hydrogel, and thus decreased the friction between sliding surfaces. Besides, the cytotoxicity tests show that the composite hydrogels possess good biocompatibility. Overall, the as-synthesized hydrogels hold great potential as lubricating medium for use in ocular applications.

High porosity hafnia ceramics by freeze casting

Highly porous hafnia ceramics are prepared by freeze casting. The major challenge resides in the high density of the hafnia particles typically leading to settling in the slurry. To address this, we optimized the suspension formulation to achieve structures with porosities of 68–74 %. Our approach includes a reduction in additive quantities and an adjustment of the pH value for suspension stabilization and prevention of particle settling. The use of PVA and PEG ensures a green body strength during the process yielding in cellular microstructure with pore channels aligned in the freezing direction throughout the sample. With increasing solid loading, the scaffolds become denser and the pore wall thickness increases. Compressive strength tests confirm the enhanced stiffness of the samples with 15 vol% hafnia. Samples with 10 vol% hafnia show a buckling-dominated failure behavior. Our results provide valuable insights to the preparation of suspensions containing high density materials.

Synthesis of MgO-Coated Canna Biochar and Its Application in the Treatment of Wastewater Containing Phosphorus

In order to treat phosphorus-containing wastewater and realize the resource utilization of wetland plant residues, biochar was prepared by the pyrolysis of canna aquatic plant waste at 700 °C, and the adsorption characteristics of phosphorus by MgO-modified biochar (MBC) were explored. The main results are as follows: the adsorption capacity of the MBC was eight times that of unmodified biochar (BC), and the adsorption capacity was up to 244 mg/g. The isothermal adsorption data were consistent with the Langmuir equation, which indicates monolayer adsorption. The functional groups changed little before and after the modification, but a new diffraction peak appeared after the modification. Compared with the standard card, it was suggested that there were MgO crystals with a higher purity. SEM images showed that the BC had a smooth surface, an obvious pore structure, and a thin pore wall, while the MBC had a rough surface and a layered structure, which can provide more adsorption sites for phosphate adsorption. In addition, an XPS analysis showed that Mg3(PO4)2 crystals appeared on the surface of the MBC after adsorption. The mechanism analysis showed that MgO is an important substance for MBC to adsorb phosphorus, and electrostatic adsorption and complex precipitation play key roles. In the test to verify the removal of actual phosphorus-containing wastewater by MBC, it was found that the removal rates for wastewater with 2.06 mg/L and 199.8 mg/L of phosphorus by MBC were as high as 93.4–93.9% and 99.2–99.3%, respectively. MBC can be used as an efficient adsorbent for phosphorus removal.

Photoresponsive Covalent Organic Frameworks: Visible‐Light Controlled Conversion of Porous Structures and Its Impacts

AbstractCovalent organic frameworks are a novel class of crystalline porous polymers that enable molecular design of extended polygonal skeletons to attain well‐defined porous structures. However, construction of a framework that allows remote control of pores remains a challenge. Here we report a strategy that merges covalent, noncovalent, and photo chemistries to design photoresponsive frameworks with reversibly and remotely controllable pores. We developed a topology‐guided multicomponent polycondensation system that integrates protruded tetrafluoroazobenzene units as photoresponsive sites on pore walls at predesigned densities, so that a series of crystalline porous frameworks with the same backbone can be constructed to develop a broad spectrum of pores ranging from mesopores to micropores. Distinct from conventional azobenzene‐based systems, the tetrafluoroazobenzene frameworks are highly sensitive to visible lights to undergo high‐rate isomerization. The photoisomerization exerts profound effects on pore size, shape, number, and environment, as well as molecular uptake and release, rendering the system able to convert and switch pores reversibly and remotely with visible lights. Our results open a way to a novel class of smart porous materials with pore structures and functions that are convertible and manageable with visible lights.

Photoresponsive Covalent Organic Frameworks: Visible-Light Controlled Conversion of Porous Structures and Its Impacts.

Covalent organic frameworks are a novel class of crystalline porous polymers that enable molecular design of extended polygonal skeletons to attain well-defined porous structures. However, construction of a framework that allows remote control of pores remains a challenge. Here we report a strategy that merges covalent, noncovalent, and photo chemistries to design photoresponsive frameworks with reversibly and remotely controllable pores. We developed a topology-guided multicomponent polycondensation system that integrates protruded tetrafluoroazobenzene units as photoresponsive sites on pore walls at predesigned densities, so that a series of crystalline porous frameworks with the same backbone can be constructed to develop a broad spectrum of pores ranging from mesopores to micropores. Distinct from conventional azobenzene-based systems, the tetrafluoroazobenzene frameworks are highly sensitive to visible lights to undergo high-rate isomerization. The photoisomerization exerts profound effects on pore size, shape, number, and environment, as well as molecular uptake and release, rendering the system able to convert and switch pores reversibly and remotely with visible lights. Our results open a way to a novel class of smart porous materials with pore structures and functions that are convertible and manageable with visible lights.

Mechanism of action of Geobacillus stearothermophilus on different shapes of residual oil

Feeding on crude oil and activator in porous medium, microbes remain active on the Oil-water interface under simulated reservoir conditions. At the same time, microbes degrade the residual oil and change the wettability of pore wall. Consequently, the displacement exhibits a decrease of 51.86% in membrane residual oil and an increase of 17.44% in recovery compared with water flooding. The Geobacillus stearothermophilus producing bio-emulsifier and its metabolic products can effectively emulsify reservoir crude oil and reduce the oil-water interfacial tension. In the end and the formed emulsion featured high viscosity can improve the oil flow rate and expanded wave volume of injected fluid. Correspondingly, the cluster-like, columnar residual oil were reduced by 64%, 68% respectively. In the micro porous test cell, the in-situ cultured microbes rely on specific life activities (interfacial tropism and in situ metabolism, etc.) to strip the residual oil off the wall of orifice deep inside the blind-ends, which cannot be achieved by the exogenous injection method. After the in-situ microbial culture process, the residual oil in deep blind-ends was reduced by about 47%, and the recovered oil was increased about 15% compared with exogenous injection method.

A Ferricyanide Anion-Philic Interface Induced by Boron Species within Carbon Framework for Efficient Charge Storage in Supercapacitors.

Carbon materials with hierarchical porous structures hold great potential for redox electrolyte-enhanced supercapacitors. However, restricted by the intrinsic inert and nonpolar characteristics of carbon, the energy barrier of anchoring redox electrolytes on the pore walls is relatively high. As such, the redox process at the interface less occurs, and the rate of mass transfer is impaired, further leading to a poor electrochemical performance. Here, a ferricyanide anion-philic interface made of in situ inserted boron species into carbon rings is constructed for enhanced charge storage in supercapacitors. Profiting from the unique component-driven effects, the polar anchoring sites on the pore wall can be built to grasp the charged redox ferricyanide anion from the bulk electrolyte and promote the redox process; the dynamics process is fastened correspondingly. Especially, the boron atoms in BC2O and BCO2 units with higher positive natural bond orbital values in the carbon skeleton are pinpointed as intrinsic active sites to bind the negatively charged nitrogen atoms in the ferricyanide anion via electrostatic interaction, confirmed by density functional theoretical calculations. This will suppress the shuttle and diffusion effects of the ferricyanide anion from the surface of the electrode to the bulk electrolyte. Finally, the well-designed PC-3 with high content of BC2O and BCO2 units can reach 1099 F g-1 at 2 mV s-1, which is a more than 2-fold increase over boron-free units of carbon (428 F g-1). The work offers a novel version for designing high-performance carbon materials with unique yet reaction species-philic effects.

Microstructure, Mechanical Property and Thermal Conductivity of Porous TiCO Ceramic Fabricated by In Situ Carbothermal Reduction of Phenolic Resin and Titania.

The porous TiCO ceramic was synthesized through a one-step sintering method, utilizing phenolic resin, TiO2 powder, and KCl foaming agent as raw materials. Ni(NO3)2·6H2O was incorporated as a catalyst to facilitate the carbothermal reaction between the pyrolytic carbon and TiO2 powder. The influence of Ni(NO3)2·6H2O catalyst content (0, 5, 10 wt.% of the TiO2 powder) on the microstructure, compressive strength, and thermal conductivity of the resultant porous TiCO ceramic was examined. X-ray diffraction and X-ray photoelectron spectroscopy results confirmed the formation of TiC and TiO in all samples, with an increase in the peak of TiC and a decrease in that of TiO as the Ni(NO3)2·6H2O content increased from 0% to 10%. Scanning electron microscopy results demonstrated a morphological change in the pore wall, transforming from a honeycomb-like porous structure composed of well-dispersed carbon and TiC-TiO particles to rod-shaped TiC whiskers, interconnected with each other as the catalyst content increased from 0% to 10%. Mercury intrusion porosimetry results proved a dual modal pore-size distribution of the samples, comprising nano-scale pores and micro-scale pores. The micro-scale pore size of the samples minorly changed, while the nano-scale pore size escalated from 52 nm to 138 nm as the catalyst content increased from 0 to 10%. The morphology of the pore wall and nano-scale pore size primarily influenced the compressive strength and thermal conductivity of the samples by affecting the load-bearing capability and solid heat-transfer conduction path, respectively.

Effect of Pore Confinement of Ionic Liquids on Solute Diffusion within Mesoporous Silica Microparticles.

The transport properties of the ionic liquid (IL) 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) confined within silica microparticles with well-ordered, accessible mesopores (5.4 or 9 nm diameter) were investigated. [BMIM][PF6] confinement was confirmed by using differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR) spectroscopy. The transport properties of the confined IL were studied using the neutral and cationic fluorescent probes 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM) and rhodamine 6G, respectively, through fluorescence recovery after photobleaching (FRAP) in confocal microscopy. The diffusivity of DCM in 9 nm pores is 0.026 ± 0.0091 μm2/s, which is 2 orders of magnitude less than in the bulk ionic liquid. The pore size did not affect the diffusivity of DCM in unmodified silica nanopores. The diffusivity of the cationic probe is reduced by 63% relative to that of the neutral probe. Diffusivity is increased with water content, where equilibrium hydration of the system leads to a 37% increase in DCM diffusivity. The most dramatic impact on diffusivity was caused by tethering an IL-like methylimidazolium chloride group to the pores, which increased the pore hydrophobicity and resulted in 3-fold higher diffusivity of DCM compared to bare silica pores. Subsequent exchange of the chloride anion from the tethering group with PF6- decreased the diffusivity to half that of bare silica. The diffusion of probe molecules is affected most strongly by the pore wall effects on probe interactions rather than by the pore size itself, which suggests that understanding pore wall diffusion is critical to the design of nanoconfined ILs for separations, catalysis, and energy storage.

Photo‐regulated synergistic catalysis of acid and basic sites in metal–organic framework

AbstractEnzymes have flexible structures and can modulate the multiple catalytic sites efficiently for synergistic catalysis. Inspired by enzymes, various artificial catalysts have been designed, while controllable synergistic effects in artificial catalysts have never been reported. Here, we report the first example of photo‐regulated synergistic catalyst (PRSC) and the synergistic effects between acidic and basic sites can be regulated by light. PRSCs have acidic sites on pore walls and basic sites on pendant azobenzene groups, and photo‐regulation is achieved through azobenzene isomerization. In the cascade reaction of converting benzaldehyde dimethyl acetal (BA) to benzylidenemalononitrile (BM), the PRSC shows a change of up to 33.6% in BM yield. Mechanism exploration indicates that cis azobenzene makes the catalytic system more stable. The promoted adsorption energy of the reactant malononitrile (MN) on basic sites and the activated C atom of MN that connects to methylene groups are responsible for the tunable synergistic effects of PRSCs.

MnO2@CS modified catalytic membrane for simultaneous organics removal and membrane fouling mitigation via peroxymonosulfate activation

The integration of advanced oxidation process and ultrafiltration membrane has been regarded as an effective method to improve pollutants removal and mitigate membrane fouling simultaneously in the field of wastewater treatment. Hence, a novel catalytic membrane microreactor, referred to as MnO2@CS-UF, was synthesized via coupling MnO2 modified carbon spheres (MnO2@CS) with membrane. The mesoporous structure of MnO2@CS guaranteed adequate water transfer channels and also contributed to more exposure of active sites. With MnO2@CS uniformly dispersed, the MnO2@CS-UF membrane showed increased hydrophilicity, higher permeability and superior peroxymonosulfate (PMS)-activation capability. With synergistic effect of pore interception and catalytic degradation, MnO2@CS-UF showed excellent organic pollutants removal (MB removal of about 100 % in 5 min), and had a prominent mitigation effect on membrane fouling caused by model NOM. In addition, MnO2@CS-UF/PMS system showed excellent catalytic filtration performance in the treatment of actual secondary effluent (SE). MnO2@CS-UF/PMS system could effectively remove fluorescent organics, and the removal rates of I-V region were 36.3 %, 72.3 %, 67.7 %, 73.5 % and 71.4 %, respectively. The permeance decline of MnO2@CS-UF was also effectively alleviated, with irreversible and reversible resistances reduced by 61.0 % and 23.2 % than UF membranes, individually. High-valent manganese-oxo species and singlet oxygen played a leading role in contaminants elimination, whereas radical oxidation acted as auxiliary contributors. Standard blocking became the dominant fouling pattern for MnO2@CS-UF/PMS, demonstrating that the MnO2@CS catalytic layer ensured the synthesis of vast active substances, prevented foulant adhesion on pore walls and alleviated the growth of cake layer. These results illustrated that the integration of MnO2@CS catalytic degradation and membrane filtration was feasible to improve the permeate quality and mitigate membrane fouling in wastewater reclamation.

Effect of contact angle on the pressure needed for a liquid to permeate a cylindrical pore

Effective separation of two immiscible liquids with filters requires a difference in the pressures to permeate between the two liquids, and setting the applied pressure between these two pressures. To help design such filters, we present an equation that enables the calculation of the pressure for a liquid phase to permeate through a smooth pore in the shape of a truncated cone as a function of (a) the contact angle of the liquid on the filters and (b) the angle of the pore wall. The equation was derived by considering the interfacial energy required to push a liquid meniscus from the top of the pore to the bottom, and then to exceed the maximum curvature required at the exit. This equation was tested experimentally by adding a hydrostatic head with water on the 3D-printed filters of acrylate polymer while systematically varying the pore radii and contact angle with water. Experimental results showed an increased pressure to permeate with higher contact angles while the equation predicted the opposite. We hypothesized that the reason for the disagreement was the assumption of a smooth pore. For a liquid on a rough pore wall, the curvature of the meniscus is not solely determined from the microscopic contact angle and the pore wall angle, but the liquid would adopt a lower curvature meniscus. Therefore, the developed equation was modified after reflecting the lower curvature, which showed much better agreement with the experimental results. The remaining discrepancy from the theory was attributed to the pressure fluctuation from the fluid flow occurring while adding water.

Enhanced Recovery of Oil Mixtures from Calcite Nanopores Facilitated by CO2 Injection.

Slow production, preferential recovery of light hydrocarbons, and low recovery factors are common challenges in oil production from unconventional reservoirs dominated by nanopores. Gas injection-based techniques such as CO2 Huff-n-Puff have shown promise in addressing these challenges. However, a limited understanding of the recovery of oil mixtures on the nanopore scale hinders their effective optimization. Here, we use molecular dynamics simulations to study the recovery of an oil mixture (C10 + C19) from a single 4 nm-wide calcite dead-end pore, both with and without CO2 injection. Without CO2 injection, oil recovery is much faster than expected from oil vaporization and features an undesired selectivity, i.e., the preferential recovery of lighter C10. With CO2 injection, oil recovery is accelerated and its selectivity toward C10 is greatly mitigated. These recovery behaviors are understood by analyzing the spatiotemporal evolution of C10, C19, and CO2 distributions in the calcite pore. In particular, we show that interfacial phenomena (e.g., the strong adsorption of oil and CO2 on pore walls, their competition, and their modulation of transport behavior) and bulk phenomena (e.g., solubilization of oil by CO2 in the middle portion of the pore) play crucial roles in determining the oil recovery rate and selectivity.

Modeling enzymatic and electrochemical cascade reactions at the three‐phase interface enzyme electrode

AbstractThe enzyme electrode based on enzymatic and electrochemical cascade reactions is a golden approach for detecting various biomarkers. How the interfacial architectures of a promising three‐phase interface enzyme electrode can influence the cascade reactions and electrode performance, however, remains unclear. In this study, a mathematical model has been developed to describe intraphase and interphase mass transfer, coupled with cascade reactions. The results reveal that the interfacial architectures determine the mass transfer of oxygen (substrate of oxidase) and H2O2 (enzymatic product) in the cascade reactions, hence affecting the electrode current. Generally, thinner pore walls facilitate the mass transfer of oxygen, thereby enhancing the enzymatic kinetics and H2O2 production. Meanwhile, smaller pore sizes shorten the diffusion pathway of H2O2 to the electrode surface, thereby increasing the electrocatalytic reaction rate. This work provides an efficient in silico tool for the design of high‐performance three‐phase enzyme electrodes.

Effect of movability of water on the low-velocity pre-Darcy flow in clay soil

Water seepage in soil is a fundamental problem involving various scientific and engineering fields. According to the literature, low-velocity water seepage in low-permeability porous media, such as clay, does not follow Darcy's law, also known as pre-Darcy flow. The formation of immovable water due to water adsorption on the pore wall is believed to be responsible for the formation of pre-Darcy flow. However, this view lacks direct solid evidence. To investigate the pre-Darcy water flow in clay, head permeability experiments are conducted on six clay samples with different densities. The results indicate that water seepage in clay at low hydraulic gradients does not follow Darcy's law. A clear nonlinear relationship between flow velocity and hydraulic gradient is observed. Water flow in clay can be divided into the pre-Darcy flow and Darcy flow regions by the critical hydraulic gradient, which is 10–12 for the Albic soil with dry density between 1.3 g/cm3 and 1.8 g/cm3. According to the disjoining pressure theory, immovable water due to water adsorption on the pore wall is the primary reason for water flow deviating from Darcy's law in clay. The results indicate that the percentage of movable water ranges from 39.7% to 59.3% for the six samples at a hydraulic gradient of 1. As the hydraulic gradient increases, the percentage of moveable water also increases. Additionally, there is a strong correlation between the percentage of movable water and the variation in hydraulic conductivity with the hydraulic gradient. Furthermore, a quantitative relationship between the percentage of movable water and the hydraulic conductivity has been established. The results of this study suggest that water adsorption on the pore wall not only affects the water movability, but is also closely related to the pre-Darcy flow phenomenon in clay.

Study on the Effects of Wettability and Pressure in Shale Matrix Nanopore Imbibition during Shut-in Process by Molecular Dynamics Simulations.

Shut-in after fracturing is generally adopted for wells in shale oil reservoirs, and imbibition occurring in matrix nanopores has been proven as an effective way to improve recovery. In this research, a molecular dynamics (MD) simulation was used to investigate the effects of wettability and pressure on nanopore imbibition during shut-in for a typical shale reservoir, Jimsar. The results indicate that the microscopic advancement mechanism of the imbibition front is the competitive adsorption between "interfacial water molecules" at the imbibition front and "adsorbed oil molecules" on the pore wall. The essence of spontaneous imbibition involves the adsorption and aggregation of water molecules onto the hydroxyl groups on the pore wall. The flow characteristics of shale oil suggest that the overall push of the injected water to the oil phase is the main reason for the displacement of adsorbed oil molecules. Thus, shale oil, especially the heavy hydrocarbon component in the adsorbed layer, tends to slip on the walls. However, the weak slip ability of heavy components on the wall surface is an important reason that restricts the displacement efficiency of shale oil during spontaneous imbibition. The effectiveness of spontaneous imbibition is strongly dependent on the hydrophilicity of the matrix pore's wall. The better hydrophilicity of the matrix pore wall facilitates higher levels of adsorption and accumulation of water molecules on the pore wall and requires less time for "interfacial water molecules" to compete with adsorbed oil molecules. During the forced imbibition process, the pressure difference acts on both the bulk oil and the boundary adsorption oil, but mainly on the bulk oil, which leads to the occurrence of wetting hysteresis. Meanwhile, shale oil still existing in the pore always maintains a good, stratified adsorption structure. Because of the wetting hysteresis phenomenon, as the pressure difference increases, the imbibition effect gradually increases, but the actual capillary pressure gradually decreases and there is a loss in the imbibition velocity relative to the theoretical value. Simultaneously, the decline in hydrophilicity further weakens the synergistic effect on the imbibition of the pressure difference because of the more pronounced wetting hysteresis. Thus, selecting an appropriate well pressure enables cost savings and maximizes the utilization of the formation's natural power for enhanced oil recovery (EOR).