Gas Transport Behavior Research Articles

Transport Behavior of Pure and Mixed Gas through Thermoplastic-Lined Pipes Materials

Abstract To determine the permeability properties of lining materials for the design of nonmetallic pipes in oil and gas fields, we conducted a study on the transport behavior of pure and mixed gases (carbon dioxide [CO2] / methane [CH4]) through commonly used thermoplastic-lined pipe materials. The material tested were high-density polyethylene (HDPE), polyamide (PA), and polyvinylidene fluoride (PVDF). It was observed that the permeability coefficient of the pure gas through HDPE, PA12, and PVDF increased with increasing temperature. The permeation of mixed gas with different volume fractions was also tested at different temperatures, in which the deviation between the test and the ideal state was founded. For HDPE and PVDF, the permeability coefficient of the mixed gas is higher than that of pure gas because of the plasticization effect of CO2. However, for PA12, the permeability coefficient of mixed gas is lower than that of the pure gas. This is because of the tight interaction between the chains and the competition between CO2 and CH4 for a limited number of active sites.

Transient analytical solution for coupled water–gas transport in unsaturated soil cover of landfill

A new analytical solution for coupled water–gas transport in unsaturated landfill cover system is proposed, which can consider transient diffusive-advective transport of gas under transient water transport condition. The proposed analytical solution is first verified against the laboratory measurements and numerical simulations. Using the proposed solution, parametric studies are conducted on the coupled transient behaviors of water–gas transport. The results show that ignoring the coupling effect can lead to significant error in gas transport results under various conditions, e.g., the surface gas emission flux is underestimated by about 28.3%∼79.8% due to ignoring the spatial and temporal variations of water content induced by transient water transport. The influencing parameters (i.e., initial saturation degree, rainfall patterns, and evaporation rate) all have significant impact on the coupled water–gas transport behaviors. The magnitude of the error induced by ignoring the coupling effect increases with increasing initial saturation degree and increasing evaporation rate (e.g., for initial saturation degree increasing from 0.35 to 0.95, the accumulated percolation at cover bottom increases by 10.9 times and the gas emission flux at cover surface decreases by 64.7%). The results highlight that it’s imperative to consider the effect of transient water transport when one needs more accurate assessment of gas transport results in the cover. This analytical solution can also be utilized to aid landfill cover design and to verify other numerical models.

Gas mass transfer characteristics in shales: Insights from multiple flow mechanisms, effective viscosity, and poromechanics

Accurate characterization of gas transport behavior is indispensable for successful shale gas reservoir development. In this study, a fractal-theory-based apparent permeability (AP) model is developed to better characterize real gas mass transfer in shales. The proposed model involves multi-scale organic matter (OM) and inorganic matter (IOM) flow channels and incorporates poromechanics-related dynamic pore size, multiple gas transport mechanisms, and dynamic viscosity. The discrete integration method (DIM) is employed to address viscosity changes during fractal scaling up. Experimental data, molecular dynamics simulations, and published theoretical models were used to verify the proposed model and reasonable agreements have been achieved. Results indicate that as the pore pressure and size increase, dominant mechanisms for gas transport change from surface diffusion at low pressure (lower than switch pressure) to viscous flow at higher pressure and larger pore sizes. Furthermore, higher organic matter content makes the permeability curve more similar to OM type. Ignoring effective viscosity results in underestimation of permeability, which is more noticeable when pressure rises and pore shrinks. Poromechanical properties primarily affect pore size, larger elastic modulus and smaller Poisson's ratio leads to significantly lower permeability. The real gas effect on AP reduction cannot be ignored, especially at high pressure. This work provides insights into the gas transport characteristics in shales, which have practical implications for shale gas reservoir simulation and development.

Experimental Study of the Effect of Molecular Collision Frequency and Adsorption Capacity on Gas Seepage Flux in Coal

Summary The differences in the transport behavior and adsorption capacity of different gases in coal play crucial roles in the evolution of coal permeability. Previous studies of coreflooding experiments failed to explain the mechanism of gas flow and have attributed the variation in gas seepage flux (flow rate) at the beginning of the experiment to the change in effective stress, while the differences in the microscopic properties of different gases, such as molar mass, molecular diameter, mean molecular free path, and molecular collision frequency, were ignored. To research the effect of these gas properties on seepage flux while circumventing the effective stress, coreflooding experiments with helium (He), argon (Ar), nitrogen (N2), methane (CH4), and carbon dioxide (CO2) were designed. The results show that the gas transport velocity in coal is affected by the combination of molecular collision frequency and dynamic viscosity, and the transport velocities follow the order of ν (CH4) > ν (He) > ν (N2) > ν (CO2) > ν (Ar). A permeability equation corrected by the molecular collision frequency is proposed to eliminate differences in the permeabilities measured with different gases. The adsorption of different gases on the coal matrix causes different degrees of swelling, and the adsorption-induced swelling strains follow the order of ε (CO2) > ε (CH4) > ε (N2) > ε (Ar) > ε (He). The reduction in seepage flux and irreversible alterations in pore structure caused by adsorption-induced swelling are positively correlated with their adsorption capacities. The gas seepage fluxes after adsorption equilibrium of coal follow the order of Q (He) > Q (CH4) >Q (N2) > Q (Ar) > Q (CO2). Like supercritical CO2 (ScCO2), conventional CO2 can also dissolve the organic matter in coal. The organic molecules close to the walls of the cleats along the direction of gas flow are preferentially dissolved by CO2, and the gas seepage flux increases when the dissolution effect on the cleat width is greater than that on adsorption swelling.

Adsorption–desorption characteristics of coal-bearing shale gas under three-dimensional stress state studied by low field nuclear magnetic resonance spectrum experiments

The micro-scale gas adsorption–desorption characteristics determine the macro-scale gas transport and production behavior. To reveal the three-dimensional stress state-induced gas adsorption–desorption characteristics in coal-bearing shale reservoirs from a micro-scale perspective, the coal-bearing shale samples from the Dongbaowei Coal Mine in the Shuangyashan Basin were chosen as the research subject. Isothermal adsorption–desorption experiments under three-dimensional stress state were conducted using the low field nuclear magnetic resonance (L-NMR) T2 spectrum method to simulate the in-situ coal-bearing shale gas adsorption–desorption process. The average effective stress was used as the equivalent stress indicator for coal-bearing shale, and the integral of nuclear magnetic resonance T2 spectrum amplitude was employed as the gas characterization indicator for coal-bearing shale. A quantitative analysis was performed to examine the relationship between gas adsorption in coal-bearing shale and the average effective stress. And a quantitative analysis was performed to examine the relationship between the macroscopic and microscopic gas quantities of coal-bearing shale. Experimental findings: (1) The adsorption–desorption process of coal-bearing shale gas follows the L-F function model and the D-A-d function model respectively with respect to the amount of gas and the average effective stress. (2) There is a logarithmic relationship between the macroscopic and microscopic gas quantities of coal-bearing shale during the adsorption–desorption process. This quantitatively characterizes the differences in the curves, which may be related to the elastic–plastic deformation, damage and fracture of the micropores in coal-bearing shale, as well as the hysteresis of gas desorption and the stress field of the gas occurrence state.

A theoretical investigation on gas transport behaviors in naturally geological reservoirs for the Hausdorff fractal derivative model

A theoretical investigation on gas transport behaviors in naturally geological reservoirs for the Hausdorff fractal derivative model

Gas transport mechanisms through MOF glass membranes

The study provides an in-depth study on the gas adsorption and transport behaviors of MOF glass membranes for the first time. Temperature dependance of gas permeability, adsorption coefficients and diffusion coefficients between 298 ​K and 313 ​K for TIF-4 and ZIF-62 glass membrane were evaluated. The CO2 permeability was dominated by the adsorption process, while CH4 transport was mainly driven by the activated diffusion. Further, the MOF glass membranes exhibited significant entropic selectivity in adsorption, along with notable enthalpic selectivity in diffusion.

Quantification of Gas Transport Behavior During Coalbed Methane Extraction in A Coal Seam Considering a Dual-Porosity/Single-Permeability Model

Quantification of Gas Transport Behavior During Coalbed Methane Extraction in A Coal Seam Considering a Dual-Porosity/Single-Permeability Model

(Invited) Leveraging Imaging Techniques to Accelerate the Advancement of Polymer Electrolyte Membrane Water Electrolyzers

Polymer electrolyte membrane water electrolyzers (PEMWEs) are critically needed to meaningfully incorporate clean renewable energy into our energy, transportation, and manufacturing sectors, for without this energy conversion mechanism, the intermittency of renewable energy will always pose a prohibitive cost barrier to practical use. However, in addition to overcoming the intermittency issue of renewable energy, energy storage also has the power to transform current energy infrastructure, particularly when plagued by older systems that lack modularity and flexibility. For example, electricity grids are often constrained to move electricity in and out of a jurisdiction with careful predictions based on past use and future needs. This comes at a cost when electricity must be sold at a loss to prevent a catastrophic surplus of electricity in the grid. While the potential for PEMWEs is tremendous, we must reduce their capital and operating costs in order to reach widespread adoption. In recent years, important work in the field has contributed to advancing PEMWEs in a variety of areas from new materials to flow field design; however, experimental validation, particularly spatially resolved, is necessary to take a fully informed approach to tailoring these materials and architectures.In this talk, I will discuss how we use X-ray and neutron imaging, both ex situ and operando, to provide high spatial and temporal resolution experimental performance and materials characterization for PEMWEs to inform our design process for new materials fabrication, new architectures, and operating parameters. We obtained nano-scale, 3D imaging of PEMWE catalyst layers to inform the stochastic simulations used to uncover the influence of pore size and ionomer distributions on electrical and ionic conductivities. We also used operando X-ray imaging to elucidate the influence of catalyst layer and porous transport layer interfacial conditions on liquid and gas transport behaviour. By applying neutron imaging, we elucidated the impact of temperature on the distribution heterogeneity of liquid water reactant at the catalyst layer. I will also discuss how we are using soft X-rays to perform synchrotron-based scanning transmission X-ray microscopy with absorption spectroscopy to uncover heterogeneity across performance and material characteristics for the PEMWE. Through these highly resolved techniques (in terms of time and space), we aim to advance our predictive models for designing next generation materials, interfaces, and flow configurations for high performance water electrolyzers.

Multi-scale investigation on gas transport behaviour of compacted granular bentonite under partially saturated states

Compacted bentonites with high swelling potential and low permeability are under consideration as buffer/backfill materials in deep geological repositories of radioactive nuclear waste [1]. Within the short-term service period, the bentonite undergoes progressive saturation developing the swelling pressure, due to the water received from the host rock. Since the progressive saturation first occurs at the boundaries near the host rock, the bentonite close to the fuel canister could remain partially saturated for a long time. Within the long-term running stage, bentonite will withstand the accumulation and transport of gases such as H2 (mainly generated by the anaerobic corrosion of the metallic overpack of canisters) [2]. The accumulated volume of gas can produce a gas pressure build-up that could break through the bentonite, damaging its microstructural integrity and isolation capacity for radionuclides and contaminants [3]. This issue may be improved by adopting granular bentonite (GB) with an extended particle size distribution (maximum grain sizes of the order of mm) [4]. The compacted GB has a high initial macroporosity, which might permit gas transport at low pressures with less impact on the microstructure. However, up to now, few investigations have focused on the gas transport behaviour of partially saturated compacted GB within a multi-scale perspective.
 To this end, gas injection tests were performed on partially saturated compacted MX-80 type GB at different hydro-mechanical states. Effective gas permeability was measured in samples at the as-compacted state and in samples prepared at the as-compacted state and then partially saturated under isochoric conditions without additional vertical stress apart from the generated due to swelling. Additionally, the tests were also conducted on these two types of samples but subjected to initial vertical stress of 3.8 MPa. X-ray micro-tomography in combination with mercury intrusion porosimetry was employed to observe the microstructural characteristics of samples before and after gas injection. The results highlight that the pore network of compacted GB is constituted of inter-granular, inter-aggregate and intra-granular/aggregate pores. Increasing the as-compacted degree of saturation Sr reduces the proportion of inter-granular pores and the effective gas permeability Ka.eff (Figure 1(a)). For the as-compacted sample, gas transport can induce the size of inter-granular/aggregate pores to extend (Figure 1(b)). When the initial vertical stress is applied on the as-compacted sample, Ka.eff decreases, with the closure of inter-granular/aggregate pores. Although the progressive saturation of the as-compacted samples under constant volume causes the reduction of inter-granular/aggregate pores, they are mainly filled with the low-density bentonite gel and the accumulated gas pressure is able to reopen and connect them to form fissures (Figure 1(b)). Thereby, at a given Sr resulting from this saturation process, Ka.eff is similar to that at the same as-compacted Sr (Figure 1(a)). However, the decrease in Ka.eff is significant, when the progressive saturation is conducted under vertical stress. This is because many inter-granular/aggregate pores collapse during the saturation process, and the vertical stress restrains the formation of fissures driven by gas pressure. On the other hand, gas transport does not affect the distribution of intra-granular/aggregate pores within the studied hydro-mechanical states.
 The current outcomes provide multi-scale insights into the gas transport behaviour of partially saturated compacted GB and emphasise its microstructural response to gas transport under different hydro-mechanical states. The values of Ka.eff measured in the tested GB are higher than those obtained in powder bentonites but comparable to those of sand/bentonite mixtures at an equivalent porosity [5, 6]. The reason is the GB material presents the granular-type microstructure, with many interconnected inter-granular/aggregate pores, even at high Sr. Once the progressive saturation increases Sr to a value close to 1, Ka.eff will sharply drop, attributed to the significant microstructural modification [7]. Therefore, the initial microstructure (before gas injection) of compacted GB plays a critical role in the gas transport behaviour. This microstructural effect was also confirmed in previous investigations of other soil types [8]. Furthermore, the present work underlines the microstructure of compacted GB is modified during gas transport, dependent on hydro-mechanical states, which can further affect the gas transport behaviour.

Investigation of methane mass transfer and sorption in Marcellus shale under variable net-stress

Natural gas in shale exists as free and adsorbed gas, subject to prevailing pore pressures and stress conditions. Accordingly, to accurately estimate/predict the shale gas recovery potential, a central requirement is to represent gas transport and sorption behavior under varying stress conditions. The objective of this work is to facilitate the interpretation of laboratory-scale experiments, at relevant conditions, in an attempt to bridge the gap in scales between laboratory- and field-scale observations.We have conducted a series of high-pressure experiments on a full-diameter core sample from the Marcellus shale. These include gas loading (pressure-decay) and depletion (production) experiments with pure methane (CH4) at variable stress conditions to characterize transport and sorption behavior under reservoir-relevant conditions. We have formulated and applied a novel integral model for mass transfer and storage in multi-porosity shale systems that allows us to effectively investigate transport and sorption phenomena: We delineate gas transport by interpreting helium (He) pressure-decay experiments and demonstrate how to use the information gained to calculate the relevant transport coefficients of CH4 and other gases. A separate measurement of the CH4 sorption isotherm on a smaller sample (a shale cube) was interpreted and combined with the transport description to predict the production behavior of CH4 from the experiments with the full-diameter core.Our experiments demonstrate that the representation of sorption hysteresis is crucial for predicting and guiding shale gas production: At the end of both gas production experiments, approximately 20% of the initial gas in place remained in the core. Without accounting for sorption hysteresis, our modeling demonstrates that the CH4 production could be overestimated by 10%. We demonstrate that our integral, triple-porosity model provides an effective approach for the interpretation and prediction of gas transport and sorption behavior during loading and production experiments on shale cores under variable net-stress conditions.In summary, our work combines measurements and modeling of mass transfer and sorption in shales at different scales to validate a characterization approach that facilitates an improved understanding of shale gas production. Furthermore, the triple-porosity model utilized in our work defines a potential pathway for the translation of laboratory-scale experimentation to larger-scale applications.

An Apparent Permeability Model in Organic Shales: Coupling Multiple Flow Mechanisms and Factors.

It is of great significance to study shale apparent permeability under the action of multiple flow mechanisms and factors because shale reservoirs possess complex pore structures and flow mechanisms. In this study, the confinement effect was considered, with the thermodynamic properties of gas being modified, and the law relating to the conservation of energy adopted to characterize bulk gas transport velocity. On this basis, the dynamic change of pore size was assessed, from which shale apparent permeability model was derived. The new model was verified by three steps: experimental and molecular simulation results of rarefied gas transport, shale laboratory data, and comparison with different models. The results revealed that, under the conditions of low pressure and small pore size, the microscale effects became obvious, which significantly improved gas permeability. Through comparisons, the effects of surface diffusion and matrix shrinkage, including the real gas effect, were obvious in the smaller pore sizes; nevertheless, the stress sensitivity effect was stronger in larger pore sizes. In addition, shale apparent permeability and pore size decreased with an increase in permeability material constant and increased with increasing porosity material constant, including internal swelling coefficient. The permeability material constant had the greatest effect on gas transport behavior in nanopores, followed by the porosity material constant; however, the internal swelling coefficient had the least effect. The results of this paper will be important for the prediction and numerical simulation of apparent permeability relating to shale reservoirs.

Study on gas transport behavior in coal matrix based on free gas density gradient diffusion: Comparison with fick model

Understanding the mechanism of gas transport in coal matrix is crucial for coalbed methane production and gas disaster prevention. The conventional Fick's law may have certain theoretical flaws because its content gradient includes both adsorbed and free gases, ignoring the fact that free gas dominate gas transport. In this study, enclosed isothermal adsorption tests with adsorbable methane (CH4) and non-adsorbable helium (He) were done first. Then, combined with Fick's law and free gas density gradient diffusion (FGDGD), the corresponding mathematical models of gas diffusion-adsorption were numerically established and solved with finite difference method (FDM). Finally, the numerical simulation results of these two models were validated. The results show that: 1) whether CH4 or He, the simulation curves based on the FGDGD model are always consistent with the experimental data, verifying the correctness of the FGDGD model; 2) the Fick model fits the experimental data with a substantial error when simulating the CH4 diffusion and adsorption process, but fits well when modeling He, demonstrating that the model's failure can be attributed to the misconception that adsorbed gas is involved in transport; and 3) the microchannel diffusion coefficient of free gas (Dm) shows that Dm (He) is greater than Dm (CH4), which can be attributed to the pore structure and gas adsorption-desorption. Consequently, the FGDGD model is reasonable in assuming that gas migration is mainly driven by free gas density gradient, and it also offers a novel method for further research into gas flow in coal matrix.

Multiscale pore characterization of coal measure reservoirs and gas storage and transport behavior in Yanchuannan gas field of China

Multiscale pore characterization of coal measure reservoirs and gas storage and transport behavior in Yanchuannan gas field of China

Study on the Dynamic Behaviors of PEMFCs with the Effect of the Interfacial Transfer Process

The dynamic response of proton exchange membrane fuel cells (PEMFCs) to power demand remarkably influences the energy management system (EMS) for hybrid hydrogen vehicle applications. This study established a one-dimensional PEMFC model considering the interfacial gas transport behaviors to investigate the most fundamental dynamic process—step loading. The model provided rigorous analytic equations to describe the transient response of output voltage, and a dimensionless variable, was found to link the appearance of undershooting quantitatively with the structural characteristics of components and operating processes. The analytic equations showed that the main reason for undershooting was the weaker capacity of remaining oxygen in the catalyst layer (CL) than the diffusion capacity of gas diffusion media (GDM). Deductions from analytic equations were confirmed by experimental results further and indicated that the degree of undershooting () was linear to the loading step (), which expressed as to some extent. The quantitative expressions based on the fundamental theories were achieved in this study and provide new perspectives and methods on the studies of dynamic controlling of the PEMFCs power system.

Fast light-switchable polymeric carbon nitride membranes for tunable gas separation

Switchable gas separation membranes are intriguing systems for regulating the transport properties of gases. However, existing stimuli-responsive gas separation membranes suffer from either very slow response times or require high energy input for switching to occur. Accordingly, herein, we introduced light-switchable polymeric carbon nitride (pCN) gas separation membranes with fast response times prepared from melamine precursor through in-situ formation and deposition of pCN onto a porous support using chemical vapor deposition. Our systematic analysis revealed that the gas transport behavior upon light irradiation is fully governed by the polarizability of the permeating gas and its interaction with the charged pCN surface, and can be easily tuned either by controlling the power of the light and/or the duration of irradiation. We also demonstrated that gases with higher polarizabilities such as CO2 can be separated from gases with lower polarizability like H2 and He effectively with more than 22% increase in the gas/CO2 selectivity upon light irradiation. The membranes also exhibited fast response times (<1 s) and can be turned “on” and “off” using a single light source at 550 nm.

A molecular simulation study on amine-functionalized silica/polysulfone mixed matrix membrane for mixed gas separation

Polysulfone (PSF) based mixed matrix membranes (MMMs) are one of the most broadly studied polymeric materials used for CO2/CH4 separation. The performance of existing PSF membranes encounters a bottleneck for widespread expansion in industrial applications due to the trade-off amongst permeability and selectivity. Membrane performance has been postulated to be enhanced via functionalization of filler at different weight percentages. Nonetheless, the preparation of functionalized MMMs without defects and its empirical study that exhibits improved CO2/CH4 separation performance is challenging at an experimental scale that needs prior knowledge of the compatibility between the filler and polymer. Molecular simulation approaches can be used to explore the effect of functionalization on MMM's gas transport properties at an atomic level without the challenges in the experimental study, however, they have received less scrutiny to date. In addition, most of the research has focused on pure gas studies while mixed gas transport properties that reflect real separation in functionalized silica/PSF MMMs are scarcely available. In this work, a molecular simulation computational framework has been developed to investigate the structural, physical properties and gas transport behavior of amine-functionalized silica/PSF-based MMMs. The effect of varying weight percentages (i.e., 15–30 wt.%) of amine-functionalized silica and gas concentrations (i.e., 30% CH4/CO2, 50% CH4/CO2, and 70% CH4/CO2) on physical and gas transport characteristics in amine-functionalized silica/PSF MMMs at 308.15 K and 1 atm has been investigated. Functionalization of silica nanoparticles was found to increase the diffusion and solubility coefficients, leading to an increase in the percentage enhancement of permeability and selectivity for amine-functionalized silica/PSF MMM by 566% and 56%, respectively, compared to silica/PSF-based MMMs at optimal weight percentage of 20 wt.%. The model's permeability differed by 7.1% under mixed gas conditions. The findings of this study could help to improve real CO2/CH4 separation in the future design and concept of functionalized MMMs using molecular simulation and empirical modeling strategies.

Novel Shale Permeability Model Coupling Sorption-Induced Differential Deformation and Multiple Flow Regimes

Under certain stress conditions, tight shale is characterized by the features of gas storage rocks, such as low permeability, low porosity, multiscale pore network, and elastic deformation. Flow-regime-based permeability models can reflect the behavior of non-Darcy gas transport but cannot characterize the elastic performance of tight shale. In this study, we introduce a novel permeability model that couples the sorption-induced differential deformation and multiple flow regimes in shale. The model not only determines flow-regime-based permeability for low pore pressures and micro–nano flow paths but also reflects the differential-swelling-index-based poroelasticity for high pore pressures and macro flow paths. The parameter sensitivity of the proposed model is analyzed. The results show that, with the increase in pore pressure at a constant confining pressure, the model is more sensitive to initial permeability than to initial porosity. The compressibility coefficient, which quantifies shale deformability, significantly affects the permeability evolution predicted by the model. The sorption-capacity-controlled differential deformation does not affect the overall permeability evolution but affects the upper and lower limits of permeability distribution in the intermediate process.

Enrichment of biogas through composite membrane of PEBA-1657/ hierarchical T-type zeolite

Presently, composite membranes emerged as a promising approach to overcome the limitations of polymeric and inorganic membranes particularly in acid gas separation. In the present work, composites membranes were fabricated by combining hierarchical T-Type (h-zeolite T) zeolite and PEBA-1657 at different filler composition that ranging from 5 wt% – 30 wt% for the CO2/CH4 separation. The physicochemical properties of the resultant inorganic filler and membranes were investigated by using Brunauer-Emmett- Teller (BET), field emission scanning electron microscopy (FESEM), Fourier Transform infra-red (FTIR), x-ray diffraction (XRD), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). FESEM and EDX analysis revealed that the formation of voids and agglomeration of particles is pronounced as the fillers loading was increased up to 30 wt%. The single gas permeation test demonstrated that amalgamation of h-zeolite T particles into PEBA-1657 has resulted in the improvement of CO2 permeability up to 122% and CO2/CH4 selectivity up to 31%. Hybrid membrane encapsulated with 25 wt% of h-zeolite T displayed a maximum separation efficiency with the highest CO2 permeability of 164.83 Barrer and CO2/CH4 selectivity of 19.37. However, further increment of fillers composition up to 30 wt% resulted in a sharp reduction of CO2/CH4 selectivity to 15.80 due to the particles sedimentation and agglomeration. Overall, the favorable gas transport behavior of PEBA-1657/h-zeolite T composite membrane indicates its promising prospect for CO2/CH4 separation especially in biogas and natural gas purification application. Future research efforts are directed on the optimization of the fabrication parameters and performance investigation at different operating condition to further enhance the CO2 separation and extend its operability under various environment.

Multiscale modeling of gas flow behaviors in nanoporous shale matrix considering multiple transport mechanisms.

This study proposes a multiscale model combining molecular simulation and the lattice Boltzmann method (LBM) to explore gas flow behaviors with multiple transport mechanisms in nanoporous media of shale matrix. The gas adsorption characteristics in shale nanopores are first investigated by molecular simulations, which are then integrated and upscaled into the LBM model through a local adsorption density parameter. In order to adapt to high Knudsen number and nanoporous shale matrix, a multiple-relaxation-time pore-scale LBM model with a regularization procedure is developed. The combination of bounce-back and full diffusive boundary condition is adopted to take account of gas slippage and surface diffusion induced by gas adsorption. Molecular simulation results at the atomic scale show that gas adsorption behaviors are greatly affected by the pressure and pore size of the shale organic nanopore. At the pore scale, the gas transport behaviors with multiple transport mechanisms in nanoporous shale matrix are explored by the developed multiscale model. Simulation results indicate that pressure exhibits more significant influences on the transport behaviors of shale gas than temperature does. Compared with porosity, the average pore size of nanoporous shale matrix plays a more significant role in determining the apparent permeability of gas transport. The roles of the gas adsorption layer and surface diffusion in shale gas transport are discussed. It is observed that under low pressure, the gas adsorption layer has a positive influence on gas transport in shale matrix due to the strong surface diffusion effect. The nanoporous structure with the anisotropy characteristic parallel to the flow direction can enhance gas transport in shale matrix. The obtained results may provide underlying and comprehensive understanding of gas flow behaviors considering multiple transport mechanisms in shale matrix. Also, the proposed multiscale model can be considered as a powerful tool to invesigate the multiscale and multiphysical flow behaviors in porous media.