Critical Capillary Pressure Research Articles

Unveiling the Role of PTFE Surface Coverage on Controlling Gas Diffusion Layer Water Content.

Gas diffusion layers (GDLs) are usually coated with a hydrophobic agent to achieve a delicate balance between liquid and gas phases to maximize mass transport. Yet, most GDL numerical models to date have assumed an average contact angle for all materials, thereby eliminating the possibility of studying the role of the polytetrafluoroethylene (PTFE) content. This study introduces two mixed wettability algorithms to predict the mixed wetting behavior of GDLs composed of multiple materials. The algorithms employ contact angle and distance to solid materials to determine the critical capillary pressure for each pore voxel. The application of the algorithms to the estimation of capillary pressure vs saturation curves for two GDLs, namely, a micro-computed tomography (μ-CT) reconstructed SGL 39BA GDL and a stochastically reconstructed Toray 120C GDL, showed that, in agreement with experimental data, the addition of PTFE resulted in a decrease in saturation at a given capillary pressure. For Toray-120C, the mixed wettability model was capable of reproducing experimentally observed features in the intrusion curve at low saturation that could not be reproduced with a single wettability model, providing a clear link between PTFE coverage and intrusion at low saturation. Numerical results also predicted an increased breakthrough pressure and a decrease in saturation with increasing PTFE, in agreement with experimental observations. The decreased saturation at breakthrough improves gas transport through the layer while maintaining the layer's ability to remove water. Diffusivity simulations confirm the increase in diffusivity at breakthrough with increasing PTFE, thereby providing a rationale for the addition of PTFE, as well as for the optimal amount. This study emphasizes the importance of multimaterial wetting models and calls for more detailed investigations into PTFE and ionomer distributions in GDLs and catalyst layers, respectively.

Directional drying of a colloidal dispersion: quantitative description with water potential measurements using water clusters in a poly(dimethylsiloxane) microfluidic chip.

We have developed a poly(dimethylsiloxane) (PDMS) microfluidic chip to study the directional drying of a colloidal dispersion confined in a channel. Our measurements on a dispersion of silica nanoparticles once again revealed the phenomenology commonly observed for such systems: the formation of a porous solid with linear growth in the channel at short times, slowing down at longer times as the evaporation rate decreases. The growth of the solid is also accompanied by mechanical stresses that are released by the delamination of the solid from the channel walls and the formation of cracks. In addition to these observations, we report original measurements using hydrophilic filler in the PDMS formulation used (Sylgard-184). When the PDMS matrix is in contact with water, water molecules pool around these hydrophilic sites, resulting in the formation of microscopic water clusters whose size depends on the water potential ψ. In our work, we have used these water clusters to estimate the water potential profile in the channel as the porous solid grows. Using a transport model that also takes into account solid delamination in the channel, we then linked these water potential measurements to the hydraulic permeability of the porous solid. These measurements finally enabled us to show that the slowdown in the evaporation rate is due to the invasion of the porous solid by air/water nanomenisci at a critical capillary pressure ψcap.

Coupled hydro-mechanical-chemical simulation of CCUS-EOR with static and dynamic microscale effects in tight reservoirs

Tight reservoir can be potentially stimulated via CO2 injection and serves as potential sites for CO2 storage. However, simplifying flow mechanisms in hydro-mechanical-chemical coupling simulations can cause significant deviations in forecasting productivity and storage performance. In this study, a coupled hydro-mechanical-chemical model with both static and dynamic microscale effects was established for the first time. We characterized the impact of micro- and nanopores on the multicomponent thermodynamics and multiphase flow of CO2 and in-situ oil by considering the critical property shift, capillary pressure, and boundary slip. The matrix and hydraulic fracture system were constructed based on the embedded discrete fracture model (EDFM). The effect of mineral reaction in the matrix and time-varying width and permeability of propped fractures were also involved in the model to address practical field-scale problems. An iteratively coupled scheme was used in a more comprehensive model to compute the flow and mechanical parameters at each time step, with the impact of different degrees of microscale effects studied by changing the proportions of micropores and nanopores. Two fluid models with different minimum miscible pressures (MMPs) were used to simulate the miscible and immiscible production processes. The stress sensitivity was analyzed based on the Biot poroelasticity theory and time-varying fracture conductivity, with the results suggesting that stress sensitivity can reduce overall production by slower outward pressure movement but can inhibit gas channeling and breakthrough to a certain extent. The detailed analysis of microscale effects, miscibility, and stress sensitivity in this work is of practical significance for tight reservoir development and CO2 geological storage.

Computational pulmonary edema: A microvascular model of alveolar capillary and interstitial flow.

We present a microvascular model of fluid transport in the alveolar septa related to pulmonary edema. It consists of a two-dimensional capillary sheet coursing by several alveoli. The alveolar epithelial membrane runs parallel to the capillary endothelial membrane with an interstitial layer in between, making one long septal tract. A coupled system of equations uses lubrication theory for the capillary blood, Darcy flow for the porous media of the interstitium, a passive alveolus, and the Starling equation at both membranes. Case examples include normal physiology, cardiogenic pulmonary edema, acute respiratory distress syndrome (ARDS), hypoalbuminemia, and effects of PEEP. COVID-19 has dramatically increased ARDS in the world population, raising the urgency for such a model to create an analytical framework. Under normal conditions fluid exits the alveolus, crosses the interstitium, and enters the capillary. For edema, this crossflow is reversed with fluid leaving the capillary and entering the alveolus. Because both the interstitial and capillary pressures decrease downstream, the reversal can occur within a single septal tract, with edema upstream and clearance downstream. Clinically useful solution forms are provided allowing calculation of interstitial fluid pressure, crossflows, and critical capillary pressures. Overall, the interstitial pressures are found to be significantly more positive than values used in the traditional physiological literature. That creates steep gradients near the upstream and downstream end outlets, driving significant flows toward the distant lymphatics. This new physiological flow provides an explanation to the puzzle, noted since 1896, of how pulmonary lymphatics can function so far from the alveoli: the interstitium is self-clearing.

Coupling effect of critical properties shift and capillary pressure on confined fluids: A simulation study in tight reservoirs

Critical properties shift and large capillary pressure are important contributors for the phase behavior altering of nanopore fluid. However, the effects of critical properties shift and large capillary pressure on the phase behavior are ignored in traditional compositional simulators, leading to inaccurate evaluation results of tight reservoirs. In this study, phase behavior and production of confined fluid in nanopores are studied. First, we developed a method for coupling the effect of critical properties shift and capillary pressure into the vapor-liquid equilibrium calculation base on Peng-Robinson equation of state. Second, a novel fully compositional numerical simulation algorithm considering effect of critical properties shift and capillary pressure on phase behavior is accomplished. Third, we have discussed the alterations of critical properties shift effect, capillary pressure effect and coupling effect on the composition of oil and gas production in detail. The critical properties shift and capillary pressure effects on oil and gas production in tight reservoirs are analyzed quantitatively through four cases, and the influences of the two effects in oil/gas production are compared. Based on the fully compositional numerical simulation, the simulator can rigorously simulate the impacts of component changes during production. The simulation results show that both the critical properties shift effect and the capillary pressure effect reduce the bubble point pressure of Changqing shale oil, and the influence are more prevalent in pores of smaller radius. In pores is larger than 50 nm, the phase behavior altering of the fluid can be ignored. In addition, we devised four cases to comprehensively investigate the effects of critical properties shift and large capillary pressure on production performance of tight reservoirs. The comparisons between the four cases show that the capillary pressure effect impacts the reservoir production performances greater than the critical properties shift effect, such as higher oil production, higher GOR, and lower content of lighter component and higher content of heavier component in the residual oil/gas. The results of coupling effects indicate that the critical properties shift effect would suppress the effect of the capillary pressure effect. In particular, the difference between the simulation results of the coupling effects and the base case is smaller than that between the simulation results of the capillary pressure effect and the base case.

Experimental study of strong imbibition in microcapillaries representing pore/throat characteristics of tight rocks

Imbibition pervasively exists in many industrial processes and is fundamental in developing unconventional resources in tight formations. Micromodels have been widely used as powerful tools for real-time observations of imbibition dynamics to support numerical modeling developments and understanding of macroscopic patterns. However, the inherent limitation of 2D micromodels in presenting 3D features in the pores/throats of tight rocks may reduce the reliability of the microfluidic results. Here, we quantitively investigate imbibition dynamics in single pore geometries that consider 2D pore-scale characteristics of tight rocks with an etching depth higher than the critical geometric criteria for including 3D features. We discuss the experimental data based on varying parameters, such as five pore shapes, a wide range of pore-throat aspect ratios (6.25 to 100), two typical throat widths (3 and 8 μm), and local capillary numbers (10-6 to 10-4). The results show that the wetting phase saturations increase linearly with the increasing inlet flow rate, consistent with previous findings in cores and 2.5D micromodels, but only in macropores. Snap-offs are found when the moved meniscus front pressure is higher than the critical capillary pressure of snap-off. An etching depth higher than the critical geometric criteria is necessary but insufficient to mimic imbibition dynamics in tight rocks by 2D micromodels with a pattern that the most pore-throat aspect ratios are less than 30 at a strong imbibition condition.

Acoustic responses of underwater superhydrophobic surfaces subjected to an intense pulse

Underwater stability of an air layer trapped in a micro-structure, plastron, is critical in drag reduction applications. Here, we investigate the wetting state and plastron stability of underwater superhydrophobic surfaces (SHS) under an intense acoustic drive. Flat surfaces and SHS are subjected to short acoustic pulses of different intensities. At low amplitude, the comparison between the results of various surfaces shows that plastron behaves like a water–air interface, whose presence can be detected from the phase of the reflected acoustic waves. At moderate intensity, a wetting transition towards a completely wetting state is observed and shown to be triggered by a sufficiently large acoustic radiation pressure. This wetting transition is well captured by a simplified model by balancing radiation pressure with the critical capillary pressure for the interface sliding. Cavitation clouds appear under strong excitation; their sizes and positions greatly depend on the surface acoustic boundary condition. For SHS in a Cassie–Baxter state (with an air layer), cavitation clouds appear at specific locations (from the solid surface) corresponding to the pressure anti-node of the transient standing wave generated by the reflection. This study unprecedentedly demonstrates the capability of acoustic waves to monitor and characterize plastron stability with low and moderate amplitudes, respectively.

From Individual Liquid Films to Macroscopic Foam Dynamics: A Comparison between Polymers and a Nonionic Surfactant.

Foams can resist destabilizaton in ways that appear similar on a macroscopic scale, but the microscopic origins of the stability and the loss thereof can be quite diverse. Here, we compare both the macroscopic drainage and ultimate collapse of aqueous foams stabilized by either a partially hydrolyzed poly(vinyl alcohol) (PVA) or a nonionic low-molecular-weight surfactant (BrijO10) with the dynamics of individual thin films at the microscale. From this comparison, we gain significant insight regarding the effect of both surface stresses and intermolecular forces on macroscopic foam stability. Distinct regimes in the lifetime of the foams were observed. Drainage at early stages is controlled by the different stress-boundary conditions at the surfaces of the bubbles between the polymer and the surfactant. The stress-carrying capacity of PVA-stabilized interfaces is a result of the mutual contribution of Marangoni stresses and surface shear viscosity. In contrast, surface shear inviscidity and much weaker Marangoni stresses were observed for the nonionic surfactant surfaces, resulting in faster drainage times, both at the level of the single film and the macroscopic foam. At longer times, the PVA foams present a regime of homogeneous coalescence where isolated coalescence events are observed. This regime, which is observed only for PVA foams, occurs when the capillary pressure reaches the maximum disjoining pressure. A final regime is then observed for both systems where a fast coalescence front propagates from the top to the bottom of the foams. The critical liquid fractions and capillary pressures at which this regime is obtained are similar for both PVA and BrijO10 foams, which most likely indicates that collapse is related to a universal mechanism that seems unrelated to the stabilizer interfacial dynamics.

Modeling and monitoring of foam propagation in highly permeable porous media under lateral water flow

Foam injection is a promising option for soil remediation applications. However, predicting how it will propagate in highly permeable aquifers under groundwater flow is challenging. Here, we have studied pressure and saturation variations during foam propagation. A 2D tank packed with 1 mm glass beads was used to study foam injection in highly permeable porous media under lateral flow. Specifically, we evaluated the efficiency of pressure and time-domain reflectometer (TDR) sensors to predict foam propagation using an imaging technique. A numerical model coupling two-phase flow and surfactant transport was developed to simulate the experimental results. This model takes into account the effect of non-Newtonian behavior of foam, surfactant concentration, and critical capillary pressure through the definition of the mobility reduction factor (MRF). The experimental results show that the foam injection pressure first increases with a logarithmic law and then stabilizes. This pressure stabilization can be related to the state of pseudo-equilibrium between foam generation and destruction. We observed an asymmetrical foam propagation due to water lateral flow. Comparisons of the liquid saturation fields calculated by analysis of TDR probes and estimated by imaging show that the TDR sensors monitor foam propagation well in saturated porous media. They can predict the shape of the injected foam. Contrary to pressure sensors, it is possible to capture weak foam behavior using TDR sensors. Finally, the numerical model we have developed correctly captures the shape of foam propagation and its ability to divert water flows. This model produces the propagation of the strong foam well and predicts the saturation and pressure fields with good precision.

Aspiration-mediated hydrogel micropatterning using rail-based open microfluidic devices for high-throughput 3D cell culture

Microfluidics offers promising methods for aligning cells in physiologically relevant configurations to recapitulate human organ functionality. Specifically, microstructures within microfluidic devices facilitate 3D cell culture by guiding hydrogel precursors containing cells. Conventional approaches utilize capillary forces of hydrogel precursors to guide fluid flow into desired areas of high wettability. These methods, however, require complicated fabrication processes and subtle loading protocols, thus limiting device throughput and experimental yield. Here, we present a swift and robust hydrogel patterning technique for 3D cell culture, where preloaded hydrogel solution in a microfluidic device is aspirated while only leaving a portion of the solution in desired channels. The device is designed such that differing critical capillary pressure conditions are established over the interfaces of the loaded hydrogel solution, which leads to controlled removal of the solution during aspiration. A proposed theoretical model of capillary pressure conditions provides physical insights to inform generalized design rules for device structures. We demonstrate formation of multiple, discontinuous hollow channels with a single aspiration. Then we test vasculogenic capacity of various cell types using a microfluidic device obtained by our technique to illustrate its capabilities as a viable micro-manufacturing scheme for high-throughput cellular co-culture.

Analysis of Liquid Bridge Characteristics in a Horizontal Fracture: Critical Fracture Aperture and Fracture Capillary Pressure

Analysis of Liquid Bridge Characteristics in a Horizontal Fracture: Critical Fracture Aperture and Fracture Capillary Pressure

Model for Interfacial Tension of Nanoconfined Lennard-Jones Fluid

Understanding and controlling the interfacial tension (IFT) of nanoconfined fluids has tremendous implications in scientific research and engineering applications. On the basis of the physical meaning of the equimolar dividing surface and the density distribution characteristic at the interface, we propose a simple model for the density profile at the vapor–liquid interface. The equimolar dividing surface and the surface of tension are assumed to coincide with each other in this work since the inhomogeneity of the interface is characterized by the proposed density profile model. Then, on the basis of the density distribution model, the density function theory (DFT) and the extended Peng–Robinson equation of state (PR EOS, which considers the effects of critical properties shift and capillary pressure) are used to estimate the confined interfacial properties and phase behavior in nanopores. Besides, the influences of temperature, pore radius, and wettability on IFT and capillarity are addressed. The developed model is validated as reliable for IFT calculation through comparison with the experimental data in the literature. Results show the following: (i) The IFT decreases with the reduction of pore size and the increase of temperature, and the decreasing rate is larger in smaller pores and higher temperatures. (ii) Capillary pressure increases with the decreasing pore size and the decreasing temperature, and it is more sensitive to pore size compared with the IFT. (iii) In the liquid phase-wet condition, as the contact angle decreases, the IFT decreases, while the capillary force increases, and the change rate is more obvious in smaller pores. Compared with other methods, the model for nanoconfined IFT proposed in this work, which is derived from the underlying physics mechanism, has a simper formulation with the similar reliability. Particularly, this work sheds light on the variation of IFT of hydrocarbons confined in nanopores of shale gas reservoirs, which provides an insight into the means of enhanced gas recovery in petroleum engineering.

Dynamic analysis of deformation and start-up process of residual-oil droplet on wall under shear flow

This study focuses on the dynamic process of deformation and start-up of an oil droplet on a wall driven by an external shear flow. A mesoscopic numerical method is developed based on the lattice Boltzmann framework for multiphase coupled with the color-gradient model. After validations of the numerical method, we performed force analyses based on the flow field, considering the pressure difference and the shear stress inside the droplet simultaneously. It was intriguing to find that the shear stress is a driving force instead of a resistance due to the interesting phenomenon that a vortex was formed inside the droplet and the direction of reaction shear stress acted by solid wall accorded with the direction of the start-up. A force-balance model was established on the basis of the force analyses to predict the yield criteria in terms of the critical capillary number and pressure difference. Finally, by comparing the flow field before and after the start-up of the droplet, it was found that the proportion of the wall applying shear driving force decreases but the values of the shear stress increase significantly after the start-up compared to before the start-up.

Simulation of quasi-static drainage displacement in porous media on pore-scale: Coupling lattice Boltzmann method and pore network model

Lattice Boltzmann method (LBM) and pore network model (PNM) are two types of simulation methods for modelling the fluid flow in porous media at the pore scale. LBM is accurate in representing the pore structures but is computationally expensive, while PNM is very efficient but cannot capture the details of pore structures. In this work, we propose to couple these two methods to simulate two-phase drainage flow in porous media. To describe the throat geometry more accurately, an improved pore network extraction method is proposed based on watershed method, where throat bonds with real throat cross sections are extracted. A multi-relaxation-time color-gradient lattice Boltzmann model is adopted to simulate the flow properties for each throat bond, namely critical entry capillary pressure, capillary pressure – saturation relationship, single-phase conductance and relative conductance – saturation relationship for both phases, which are the input parameters for PNM simulations. To further improve the computational efficiency, five artificial neural network models are developed which relate the five flow properties to the actual shape of throat cross sections, where the shape is characterized by 9 parameters. The database consists of LBM simulations of the flow properties of 1421 throat cross sections extracted from 5 sandstone digital rocks. The trained artificial neural network models show much better performance than the conventional PNM when predicting the flow properties for each throat bond. Finally, the accuracy of the improved PNM coupled with LBM is validated by both the single-phase flow and two-phase drainage flow in a Berea sandstone.

Foam Generation in Flow Across a Sharp Permeability Transition: Effect of Velocity and Fractional Flow

Summary Foam reduces gas mobility and can help improve sweep efficiency in an enhanced-oil-recovery (EOR) process. For the latter, long-distance foam propagation is crucial. In porous media, strong foam generation requires that the local pressure gradient exceed a critical value (▿Pmin). Normally, this happens only in the near-well region. Away from wells, these requirements might not be met, and foam propagation is uncertain. It has been shown theoretically that foam can be generated, independent of pressure gradient, during flow across an abrupt increase in permeability (Rossen 1999). The objective of this study is to validate theoretical explanations through experimental evidence and to quantify the effect of fractional flow on this process. This article is an extension of a recent study (Shah et al. 2018) investigating the effect of permeability contrast on this process. In this study, the effects of fractional flow and total superficial velocity are described. Coreflood experiments were performed in a cylindrical sintered-glass porous medium with two homogeneous layers and a sharp permeability jump in between, representing a lamination or cross lamination. Unlike previous studies of this foam-generation mechanism, in this study, gas and surfactant solution were coinjected at field-like velocities into a medium that was first flooded to steady state with gas/brine coinjection. The pressure gradient is measured across several sections of the core. X-ray computed tomography (CT) is used to generate dynamic phase-saturation maps as foam generates and propagates through the core. We investigate the effects of velocity and injected-gas fractional flow on foam generation and mobilization by systematically changing these variables through multiple experiments. The core is thoroughly cleaned after each experiment to remove any trapped gas and to ensure no hysteresis. Local pressure measurements and CT-based saturation maps confirm that foam is generated at the permeability transition, and it then propagates downstream to the outlet of the core. A significant reduction in gas mobility is observed, even at low superficial velocities. Foam was generated in all cases, at all the injected conditions tested; however, at the lowest velocity tested, strong foam did not propagate all the way to the outlet of the core. Although foam generation was triggered across the permeability boundary at this velocity, it appeared that, for our system, the limit of foam propagation, in terms of a minimum-driving-force requirement, was reached at this low rate. CT images were used to quantify the accumulation of liquid near the permeability jump, causing local capillary pressure to fall below the critical capillary pressure required for snap-off. This leads to foam generation by snap-off. At the tested fractional flows, no clear trend was observed between foam strength and fg. For a given permeability contrast, foam generation was observed at higher gas fractions than predicted by previous work (Rossen 1999). Significant fluctuations in pressure gradient accompanied the process of foam generation, indicating a degree of intermittency in the generation rate—probably reflecting cycles of foam generation, dryout, imbibition, and then generation. The intermittency of foam generation was found to increase with decreasing injection velocities and increasing fractional flow. Within the range of conditions tested, the onset of foam generation (identified by the rise in ▿P and Sg) occurs after roughly the same amount of surfactant injection, independent of fractional flow or injection rate.

Determination of confined fluid phase behavior using extended Peng-Robinson equation of state

Phase behavior of confined fluids deviate significantly from that of bulk fluids because of the strong confinement effect in nanopores. The classical equation of state (EOS) based on bulk fluids is inapplicable for fluids in shale reservoirs because it cannot reflect the unique thermodynamic properties of confined fluids, such as the suppressed bubble-point pressure. The objective of this work is to extend the existing Peng-Robinson EOS (PR EOS) for unconventional reservoir fluids. In this work, techniques have been developed to compute the phase behavior of confined fluids by incorporating critical property shift and capillary pressure in the PR EOS, where a new term representing the molecule-wall interaction has been introduced. On the basis of data collected from both experiments and molecular simulations, a correlation which is a function of dimensionless pore size has been developed and incorporated in the extended PR EOS to correct the shifted critical properties. The capillary pressure effect is considered in phase equilibrium computation to account for the strong capillary force in nanopores, where the pressure in vapor phase is larger than that in liquid phase. The newly extended PR EOS is validated by reproducing the molecular simulation data from a previous literature, yielding an overall error of 7.71%. Both critical property shift and capillary pressure are found to be important factors influencing the phase behavior of confined fluids. The combined effect has been incorporated in the extended PR EOS which is successfully used to compute the phase behavior of three unconventional reservoir fluids. The bubble-point pressures under reservoir temperatures for Bakken, Eagle Ford, and Wolfcamp fluids at 10 nm are found to be reduced by 31.98%, 23.15%, and 37.77%, respectively.

Estimating local capillary trap volume capacities using a geologic criterion

Capillary pressure heterogeneity causes local capillary trapping of CO2 in saline aquifers. However, quantifying local capillary trapping using conventional reservoir simulation is computationally intensive. This work employs a geologic criterion (GC) to rapidly estimate the volume capacity of local capillary traps in static geologic models. The criterion refers to the ‘critical capillary entry pressure’ that is used to indicate flow barriers and flow paths during buoyant flow. A previous study (Saadatpoor, 2012) found that issues exist in the criterion method: unknown physical critical capillary entry pressures and boundary barriers. This work addresses the two issues. It is shown that the criterion method gives a close upper bound estimation of local capillary trap volume capacities. Several fine-scale geostatistical realizations of capillary entry pressure fields are considered, and the effects of reservoir heterogeneity, system sizes, aspect ratios, and boundary types are examined. The results from the GC are also strictly interpreted through invoking a simple counting argument. The overall work enhances our mechanistic understanding of the geological controls of local capillary traps.

Determination of Effective Parameters of Gas Injection in Naturally Fractured Reservoirs by Combination of Reservoir Simulation and Design of Experiment Techniques

Screening analysis is a useful guideline which helps us with proper field selection for different enhanced oil recovery processes. In this work, reservoir simulation is combined with experimental design to estimate main effects and possible interactions of reservoir rock and fluid properties on performance of different gas injection processes in naturally fractured reservoirs (NFRs). Studied parameters include reservoir thickness (h), oil viscosity μo), pore size distribution (λ), horizontal permeability (Kh), storage capacity (ω), reservoir dip, critical water saturation (Swc) and threshold capillary pressure (pct). The recovery factors of different simulation designs are analyzed by use of fractional factorial design (FFD) approach. Finally, the statistical significance of results are evaluated by hypothesis testing and ANOVA, and presented by Pareto and tornado plots. Main effect analysis showed that effective parameters are ordered with respect to their importance as follows: For Methane and nitrogen injection: Kh, dip, Swc, ω, h, μo and λ; threshold capillary pressure showed a minor effect on recovery factor. For Carbon dioxide injection: μo, dip, Swc, ω, h and Kh; Pore size distribution (λ) and threshold pressure were not shown to be statistically significant in this process. Likeness of main effects and parameter interactions for methane and nitrogen injection prove the similarity of dominant mechanisms and characteristics of these processes compared to CO2 injection which seems almost different.

Cyclic CH4 Injection for Enhanced Oil Recovery in the Eagle Ford Shale Reservoirs

Gas injection is one of the most effective enhanced oil recovery methods for the unconventional reservoirs. Recently, CH4 has been widely used; however, few studies exist to accurately evaluate the cyclic CH4 injection considering molecular diffusion and nanopore effects. Additionally, the effects of operation parameters are still not systematically understood. Therefore, the objective of this work is to build an efficient numerical model to investigate the impacts of molecular diffusion, capillary pressure, and operation parameters. The confined phase behavior was incorporated in the model considering the critical property shifts and capillary pressure. Subsequently, we built a field-scale simulation model of the Eagle Ford shale reservoir. The fluid properties under different pore sizes were evaluated. Finally, a series of studies were conducted to examine the contributions of each key parameter on the well production. Results of sensitivity analysis indicate that the effect of confinement and molecular diffusion significantly influence CH4 injection effectiveness, followed by matrix permeability, injection rate, injection time, and number of cycles. Primary depletion period and soaking time are less noticeable for the well performance in the selected case. Considering the effect of confinement and molecular diffusion leads to the increase in the well performance during the CH4 injection process. This work, for the first time, evaluates the nanopore effects and molecular diffusion on the CH4 injection. It provides an efficient numerical method to predict the well production in the EOR process. Additionally, it presents useful insights into the prediction of cyclic CH4 injection effectiveness and helps operators to optimize the EOR process in the shale reservoirs.

Buckling of a drying colloidal drop.

It is well known that drying drops of colloidal dispersions undergo complex morphological transitions involving buckling of a particle-packed outer shell during drying. Although capillary stresses generated during drying have been identified as the cause for buckling, the exact conditions for buckling and its relation to the particle size, rigidity, and nature of packing have not been understood. Here, we derive explicit expressions for the critical capillary pressure for buckling of droplets based on the mechanical properties of the particle network formed during drying and the conditions under which buckling can be avoided. We anticipate our results to form the basis for design of formulations for the pharmaceutical, food and ceramics industry.