Gas Transport Mechanisms Research Articles

Gas transport mechanisms, mathematical models, and impact factors in low-permeability rocks: A critical review

Gas transport mechanisms, mathematical models, and impact factors in low-permeability rocks: A critical review

Unveiling gas transport mechanisms in tunable MXene nanochannels: Insights from molecular dynamics simulations

MXene-based membranes have shown tremendous potential in gas separation applications. Here, molecular dynamics (MD) simulations are used to investigate the effects of varying the structural parameters of Ti₃C₂O₂ nanochannels on the permeation and separation performance of H₂, CO₂, N₂, and CH₄ gases. The results demonstrate that the interlayer spacing significantly influences gas permeability, with wider channels generally exhibiting higher permeance. Channel length, however, has a relatively minor impact on permeability, varying by gas species. Potential of mean force (PMF) analysis reveals that CO₂ molecules face a notable energy barrier at the channel entrance and have the strongest interactions with the MXene interface within the channel, potentially leading to blockage. Spatial density analysis further confirms this CO₂ blockage phenomenon, which diminishes as the interlayer spacing increases. In terms of gas separation selectivity, H₂/CH₄ and H₂/CO₂ mixtures exhibit high selectivity, with maximum values of 41.08 and 27.06, respectively. Notably, the H₂/CO₂ system exhibits a positive correlation between permeability and selectivity, breaking the traditional permeability-selectivity trade-off. This anomalous behavior can be attributed to the CO₂ blockage effect. This study provides theoretical guidance for the design and optimization of MXene-based membrane materials in practical applications.

A hydraulic-mechanical coupled triple porosity fractal model considering action mechanisms and gas transport mechanisms in carbon-hydrogen reservoirs

The shale formation, a common carbon-hydrogen reservoir, has numerous nanoscale pores that facilitate multiple gas transport mechanisms, including viscous flow, slip flow, Knudsen diffusion, and surface diffusion. Additionally, various action mechanisms involving effective stress, real gas effect, gas adsorption and desorption, and fracture-pore characteristics significantly influence gas transport. In this study, we conceptualize shale as a triple porosity medium comprising organic matrix, inorganic matrix, and microfractures. For each system, its hydraulic-mechanical coupled model with action mechanisms and gas transport mechanisms is derived based on fractal theory. After validating the model with Barnett and Marcellus data, the evolution of apparent permeability and gas pressure is studied, as well as the influences of pore fractal features, the real gas effect, and the number of hydraulic fractures on gas extraction. Simulation results indicate that, in comparison to organic matrix, inorganic pore fractal features exert a more noticeable effect on cumulative production. The real gas effect is critical in gas transport and reservoir reserve estimation; disregarding this effect results in a 17.4% underestimation of cumulative gas production by the 1600th day. Fracture interference attenuates the improvement in cumulative production gains from increased hydraulic fracture numbers.

Theoretical perspectives on CO2 separation by ionic liquids: Addressing crucial questions

The global population explosion necessitates more natural resources for energy, making it a major factor in climate change. CO2 is a leading actor in the greenhouse gas effect, and advancements in CO2 capture processes have attracted researchers in recent years. Numerous technologies are practiced for CO2 separation. Nevertheless, due to the intricate nature of gases and different process conditions, most of them continue to face challenges. To overcome these challenges, researchers focus on finding or designing innovative materials compatible with various processes. From the discovery of high CO2 solubility of ionic liquids (ILs), these unique compounds have rapidly gained popularity for different applications like gas separation. Molecular simulations play a crucial role in supporting researchers in understanding the connection between the properties and chemical structure of newly developed materials as ILs in a time-efficient manner. Despite the growing number of molecular simulation studies, there is a gap in the review papers that effectively highlight the benefits and feasibility of the theoretical approach. Up to date, mostly experimental studies mainly focusing on gas transport mechanisms and the relationship between chemical composition and properties were summarized. Herein, we concentrated on molecular simulation studies investigating the adsorption- and membrane-based CO2 separation performance of confined ILs. We systematically addressed prominent questions posed by researchers in each simulation study, demonstrating how to examine confined ILs and interpret the resulting data in relation to gas separation performance. The most pertinent inquiries encompass the influence of ion type (cation or anion) on gas adsorption at the gas–liquid interface and through the confined IL phase, the significance of the confined form of cation or anion in gas transport, the impact of IL thickness on gas separation, and the discernible effects of the material in which ILs are confined. Therefore, we hope to guide researchers in developing innovative membranes or adsorbents by leveraging insights obtained from the answers to these questions derived from molecular simulations of confined ILs.

Characterization of radon gas transmission rate in pore structures of different rock layers by radon daughters inversion calculation

Determining the transmission rate of radon gas in overburden strata is crucial for conducting a comprehensive study of radon gas's longitudinal and long-distance migration mechanisms. This study investigates the mineral components of rocks in the underground strata of the mining area using the X-ray diffraction method. Additionally, it examines the pore structure parameters of the rocks at different depths using the low-temperature nitrogen adsorption method. This research introduces an approach to inversion calculate the radon gas transmission rate through the activity ratio of radon's characteristic daughters based on the decay law and activity balance of 210Po and 210Pb daughters. In addition, it determines the transmission rates of radon gas in overlying strata at various depths through this method. The relationship between the rock's mineral composition and pore structure is investigated, and the effects of pore structure and mineral composition on the radon gas transmission rate are analyzed. The findings indicated that the pore structure exerts a dual impact on radon gas transport: macropores serve as channels for upward radon gas transport, while micropores offer most of the adsorption area. In contrast, the radon gas transmission rate is indirectly influenced by the mineral composition content associated with the medium's adsorption capacity and pore structure. In the studied lithologies, an increase in quartz content promotes radon gas transmission, while an increase in clay mineral content impedes it. Finally, the mechanisms of radon gas transport, daughter adsorption, and the impacts of rock pore structure and mineral composition on the radon transmission rate are discussed.

Development of coupled fluid-flow/geomechanics model considering storage and transport mechanism in shale gas reservoirs with complex fracture morphology

Field observations frequently demonstrate stress fluctuations resulting from the reservoir depletion. The development of reservoirs, particularly the completion of infill wells and refracturing, can be significantly impacted by stress changes in and around drainage areas. Previous studies mainly focus on plane fractures and few studies consider the influence of complex transport and storage mechanism and irregular fracture geometry on stress evolution in shale gas reservoirs. Based on the embedded discrete fracture model (EDFM) and finite-volume method (FVM), a coupled geomechanics/fluid model has been successfully developed considering the adsorption, desorption, diffusion and slippage of shale gas. This model achieves coupling simulation of natural fractures, hydraulic fractures with complex geometry, storage and transport mechanism, reservoir stress, and pore-elastic effect. The open-source software OpenFOAM is used as the main solver for this model. The stress calculation and productivity simulation of the model are verified by the classical poroelasticity problem and the simulation results of published research and commercial simulator with EDFM respectively. The simulation results indicate that σxx, σyy, σxy and Δσ changes with time and space due to the time effect and anisotropy of formation pressure depletion; Due to the influence of different mechanisms on shale gas storage and transport, the reservoir pressure and stress distribution under different mechanisms are different; Among them, the stress with full mechanisms differs the most compared to the stress without any mechanism. The reservoir with stronger stress sensitivity (smaller Biot coefficient) is less sensitive to formation pressure depletion, and the stress variation range is smaller. For reservoirs with weak stress sensitivity, formation pressure depletion is more likely to lead to stress reversal. Under the influence of fracture geometry, the pressure depletion regions caused by the three types of fracture geometry are approximately rectangular, parallelogram and square, respectively. The corresponding σxx, σyy and Δσ also have great differences in spatial distribution and values. Therefore, the time effect, shale gas storage and transport mechanism and the influence of complex fracture geometry should be considered when predicting pressure depletion induced stress under the condition of simultaneous production. This study is of great significance for understanding the evolution law of stress induced by pressure consumption, as well as the design of infill wells and repeated fracturing.

Joint experimental-computational: Revealing the role of sodium poly(heptazineimide) in the selective separation of CO2 in PEBAX-based mixed matrix membranes

Mixed matrix membranes (MMMs) with the dual properties of porous materials and polymer matrix membranes have become one of the most optimal solutions to the gas separation problem. However, the adapted porous material and the explicit transport mechanism of gases within the mixed matrix membrane hinder its further commercialization. Thanks to their porous nature and tunable gas adsorption properties, heteroatom-doped porous materials, especially nitrogen-doped porous materials, are widely used to design and prepare advanced CO2 separation membranes. Herein, a layered two-dimensional nano-sheet with a nitrogen content of 44.7 % was successfully synthesized via thermal polymerization. And nano-sheets were homogeneously dispersed in a Pebax matrix to make a composite film. Since the CO2 affinity of the nitrogen functional site and the PEO flexible chain, realizing the synergistic improvement of CO2 permeability and selectivity. A comprehensive combined permeation experimental/simulation characterization reveals the CO2 transport pathway within the membrane, indicating the mechanism of action of nitrogen-doped porous packing on CO2 diffusion. Among these, the CO2 permeability of the mixed matrix film containing 5 wt% sodium poly(heptazine imide) (NaPHI) reached 153 Barrer at a feed gas pressure of 9 bar, nearly 149 % compared to the pure Pebax film. The selectivity of CO2/N2 was enhanced to 105, exhibiting a remarkable increase of 159 % and exceeding the 2019 Robeson limit. Importantly, the present work further reveals and demonstrates the gas separation mechanism of nitrogen-doped porous filler/polymer hybrid matrix membranes, which provides new ideas for the design and application of heteroatom-doped porous fillers in the field of gas separation.

Hydrogen bubble evolution and gas transport mechanism on a microelectrode determined by cathodic potential and temperature

Enhancing the efficiency of hydrogen production by optimizing gas product transfer within water electrolysis systems is essential. Employing high-speed photography and electrochemical techniques, the entire process of single hydrogen bubble evolution on a Pt microelectrode surface was measured. Results reveal a notable reduction in both bubble detachment radius and growth time with decreasing absolute potential (from −7 to −3 V) and increasing reaction temperature (from 30 °C to 50 °C). Additionally, a comprehensive model estimating bubble coverage on the microelectrode is presented, incorporating bubble radius and current as key influencing factors. This enables an accurate evaluation of mass transfer coefficients during bubble evolution in the absence of forced flow. Furthermore, findings reveal the dominance of bubble-induced micro-convection as the primary mass-transfer mechanism for gas products at high current densities [O (105–106 A/m2)]. The results also indicate that the mass transfer coefficient increases during the inertia-controlled growth stage of bubbles and decreases during the stage controlled by chemical reactions.

The roles of different migration mechanisms in the transport of H2O–CH4 gas mixture in shales

Studying the roles of different migration mechanisms in shale matrix gas transport is beneficial for shale gas extraction and CO2 sequestration. The roles of different transport mechanisms in shale H2O–CH4 gas mixture transport were investigated considering their contributions to fluid flux. A permeability model coupling multiple gas migration mechanisms was developed by combining the pore network-related effective medium approximation, which was validated using process data from simultaneous H2O–CH4 transport in shales. The results showed that the gas transport mechanisms dominating methane and water vapor migration in shale differed. Slip flow and Knudsen diffusion contributed approximately 99% to methane migration. Water vapor surface diffusion as well as slip flow and Knudsen diffusion primarily contributed to the water vapor permeability before and after a time point, respectively. Water vapor and methane permeability generally decreased with time. The equilibrium water vapor and methane permeabilities ranged from 2.7699×10−22–12.312×10−22 m2 and 0.2640×10−21–4.9036×10−21 m2, respectively. The methane surface diffusion permeability increased slowly with time before approaching methane monolayer adsorption (before ∼100 s). An increase in relative humidity (approximately 300–3600 s) can promote the transport of water vapor. Water vapor and methane adsorption, especially the condensation of adsorbed water, could largely explain the decrease in shale gas permeability.

Metal-Organic-Frameworks Based Mixed-Matrix Membranes for CO2 Separation: An Applicable-Conceptual Approach.

A promising class of porous crystalline materials, metal-organic frameworks (MOFs), have recently emerged as a potential material in fabricating mixed matrix membranes (MMMs) for gas separation applications. Their unique chemistry and structural versatility offer substantial advantages over conventional fillers. This review gives an in-depth exploration of MOF chemistry, focusing on strategies to manipulate their adsorption behavior to enhance separation properties. We scrutinize the impact of various MOF-based MMM components, including polymer matrix, MOFs fillers and polymer/filler interface, on the overall gas separation performance. This involves a detailed analysis of key parameters associated with MMM preparation. Additionally, we offer a comprehensive overview of the determining factors in MOF-based MMM development for gas separation, including MOF structure, synthesis, and chemistry. Moreover, the most advances in modification strategies of MOF for CO2 separation, such as a wide variety of hybrid MOFs will be outlined, which opens the door to an improved CO2 separation process. Finally, the gas transport mechanisms of MMMs are thoroughly discussed to understand the factors affecting the gas permeation through the polymer matrix, MOFs and interface between them.

Research on multi‐mechanism synergistic effects of gas cross‐scale percolation in tight gas reservoirs of Yanchang oilfield

AbstractRevealing the gas transport patterns at the microscale and nanoscale is of great scientific and engineering significance for unconventional oil and gas exploration and development. This paper first systematically analyzes the properties of gas transport mechanism under different Knudsen number conditions. Second, the coupling modes of the cross‐scale multi‐drive mechanism are systematically categorized, and three typical coupling calculation methods are pointed out. Again, the rationality of the cross‐scale multi‐agency coupling is quantitatively calculated. The results show that (a) Under the influence of Fick's diffusion equation, when Knr > 0, the coupled ‘slip flow–Fick diffusion–Knudsen diffusion’ equation does not converge to the Knudson diffusion equation. (b) The ‘Fick diffusion–Knudsen diffusion’ coupled equation is calculated to be more than 102 times as large as the Knudsen diffusion equation.

Comparative Study on Flow Characteristics in Deep Marine and Marine-Continental Transitional Shale in the Southeastern Sichuan Basin, China.

Permeability is a significant characteristic of porous media and a crucial parameter for shale gas development. This study focuses on deep marine and marine-continental transitional shale in the southeastern Sichuan area using the gas pulse decay testing method to systematically analyze the gas permeability, stress sensitivity, and gas transport mechanisms of shale under different pressure conditions and directions. The results show that the porosity and gas permeability of the deep marine shale are greater compared to those of the marine-continental transitional shale. The elevated fluid pressure in the deep marine shale offers superior conditions for the preservation of nanopores, while the high quartz content provides advantageous conditions for fluid transport in nanopore channels. The permeability and stress sensitivity of the deep marine shale are greater than those of the marine-continental transitional shale, and the stress sensitivity is greater in the perpendicular bedding direction than in the parallel bedding direction, possibly related to the mineral composition of shale and the compaction it has undergone. The flow mechanism of the deep marine shale is transition flow and Knudsen flow, while that of the marine-continental transitional shale is transition flow. The deep marine shale possesses smaller nanopore sizes and a higher quantity of micropores, which create advantageous conditions for gas transport within nanopores. During the process of extracting shale gas, the extraction of gas causes a decrease in pore pressure and an increase in effective stress, resulting in a reduction in permeability. However, when the pore pressure reaches a specific value, the enhanced slippage effect leads to an increase in permeability, which is advantageous for gas extraction. In the later stage of shale gas well production, intermittent production plans can be developed considering the strength of the slippage effect, leading to a significant improvement in production efficiency.

Current Progress on Dual‐Layer Hollow Fiber Mixed‐Matrix Membrane in CO2 Capture

AbstractCarbon dioxide (CO2) is a greenhouse gas which is mainly found in natural gas (NG), biogas, and flue gas. Anthropogenic CO2 emissions are the direct result of burning fossil fuels. Meanwhile, pre‐ and postcombustion CO2 separation is a current state of CO2 removal method in an extensive manner. From environmental, economic, and transportation perspectives, removal of CO2 has driven the development of its separation process technology. Among the reported technologies, membrane‐based gas separation technologies have grown substantially, breakthroughs and advances in past decades. This review paper aims to provide an overview on competitive gas separation processes, different types of membranes available, gas transport mechanisms, and fabrication process of hollow fiber membranes, particularly dual‐layer hollow fiber membrane. The performance of the membranes in CO2 separation and effect of spinning parameters on the formation of hollow fiber membranes are highlighted. In addition, approaches to improve the dual‐layer adhesion, strategies to enhance the filler compatibility in the development of dual‐layer hollow fiber mixed‐matrix membranes, and effect of post‐treatments on the gas separation performance of membrane are also discussed. Finally, challenges and future perspectives of dual‐layer hollow fiber mixed‐matrix membranes toward CO2 capture, particularly on CO2/CH4 and CO2/N2 separation, are also included, due to its substantial and direct relevance to the gas separation industry.

Research on triazine-based nitrogen-doped porous carbon/Pebax mixed-matrix membranes for CO2 separation and its gas transport mechanism

Research on triazine-based nitrogen-doped porous carbon/Pebax mixed-matrix membranes for CO2 separation and its gas transport mechanism

A design of gas diffusion media for polymer electrolyte membrane fuel cell: Characterization and water management investigation

Higher power density is a key technological goal for the deployment of polymer electrolyte membrane fuel cells (PEMFCs) in vehicles. Water production is proportional to current density and at high current densities water flooding in catalyst layers can result in significant mass transport limitations. A novel perfluoropolymers material, Teflon™ AF, is introduced as a hydrophobic agent to fabricate microporous layers (MPLs). The MPL ink with varying proportions of Teflon™ AF and Vulcan XC-72 carbon black is prepared and used to coat MPLs into the gas diffusion layers. X-ray computed tomography and mercury intrusion porosimetry are used to characterize the physical properties of the penetrated GDM. The performance of cells with the homemade MPLs are compared with commercial GDM cells under dry and wet operating conditions using varying backpressures. The results at high relative humidity and high backpressure show that the cells with homemade MPLs have less flooding than that with commercial GDM. Oxygen transport resistance measurements using the limiting current method show that the penetration of the MPL into the substrate has an undesirable effect of increasing oxygen transport resistance. The results are of benefit to understanding the water management and gas transport mechanism of MPL for fuel cells operating at both dry and wet conditions.

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.

On Multi-Component Gas Migration in Single-Phase Systems

The present work deals with diffusion of gases in fully saturated porous media. We test and validate the gas transport mechanism of dissolution and diffusion, implemented in the TH2M process class in the open-source finite-element software OpenGeoSys. We discuss the importance of gas diffusion for the integrity of the multi-barrier system. Furthermore, we present a multi-component mass balance equation implementation in Python, which serves as a reference for the two-component TH2M implementation and allows for a discussion of multi-component gas diffusion in liquids. We verify and validate the numerical implementations as follows: First, we come up with a set of numerical benchmarks in which solutions obtained by the two-component TH2M and multi-component implementations are compared. Thus, we show under which conditions predictions made by the TH2M model can be used for multi-component gas systems. Finally, the work is validated using a through diffusion experiment performed at Belgium’s Nuclear Research Centre SCK CEN and a sensitivity analysis is conducted based on the featured experiment. The results of this work illustrate that predictions by both the two- and four-component models match the laboratory findings very well. Therefore, we conclude that also the two-component implementation can reflect the multi-component processes well under the given constraints such as full saturation.

Effect of water film evaporation on the shale gas transmission in inorganic nanopores under viscosity.

Shale gas reservoirs generally have ultra-low water saturation, and the water in reservoirs is closely bound to the walls of inorganic nanopores, forming a water film structure on the hydrophilic surface. When shale gas enters the inorganic nanopores, the water films in the inorganic pores will be removed by evaporation instead of being driven away by the gas, which increases the difficulty of predicting production during shale gas extraction. Based on molecular dynamics simulations, a water film evaporation model is proposed, considering the evaporation of water films during shale gas transport and the influence of water film evaporation on the shale gas transport mechanism. The Green-Kubo method is employed to calculate the viscosity of the water film. The evaporation flux of the water film under the influence of viscosity is discussed in the evaporation model. The transport mechanisms of shale gas in nanopores and the effect of water film evaporation on shale gas transport mechanisms are analyzed in detail. The result indicates that the water films in the inorganic nanopores are constrained on the hydrophilic surface, and the viscosity normal to the surface of the water film of 4Å is 0.005 26 Pa⋅S, which is 6.12 times the reference value of viscosity at 298K. In the process of water film evaporation, the evaporation flux of the water film is influenced by viscosity. In the study of the shale gas transport mechanism, water films in inorganic nanopores can hinder the surface diffusion of the methane molecules adsorbed on boundary and significantly reduce the mass flux of shale gas.

Advective and diffusive gas phase transport in vadose zones: Importance for defining vapour risks and natural source zone depletion of petroleum hydrocarbons

Quantifying the interlinked behaviour of the soil microbiome, fluid flow, multi-component transport and partitioning, and biodegradation is key to characterising vapour risks and natural source zone depletion (NSZD) of light non-aqueous phase liquid (LNAPL) petroleum hydrocarbons. Critical to vapour transport and NSZD is transport of gases through the vadose zone (oxygen from the atmosphere, volatile organic compounds (VOCs), methane and carbon dioxide from the zone of LNAPL biodegradation). Volatilisation of VOCs from LNAPL, aerobic biodegradation, methanogenesis and heat production all generate gas pressure changes that may lead to enhanced gas fluxes apart from diffusion. Despite the importance of the gaseous phase dynamics in the vadose zone processes, the relative pressure changes and consequent scales of advective (buoyancy and pressure driven) / diffusive transport is less studied. We use a validated multi-phase multi-component non-isothermal modelling framework to differentiate gas transport mechanisms. We simulate a multicomponent unweathered gasoline LNAPL with high VOC content to maximise the potential for pressure changes due to volatilisation and to enable the joint effects of methanogenesis and shallower aerobic biodegradation of vapours to be assessed, along with heat production. Considering a uniform fine sand profile with LNAPL resident in the water table capillary zone, results suggest that biodegradation plays the key role in gas phase formation and consequent pressure build-up. Results suggest that advection is the main transport mechanism over a thin zone inside the LNAPL/capillary region, where the effective gaseous diffusion is very low. In the bulk of the vadose zone above the LNAPL region, the pressure change is minimal, and gaseous diffusion is dominant. Even for high biodegradation rate cases, pressure build-up due to heat generation (inducing buoyancy effects) is smaller than the contribution of gas formation due to biodegradation. The findings are critical to support broader assumptions of diffusive transport being dominant in vapour transport and NSZD assessments.

Hollow Fiber Carbon Molecular Sieve Membranes for Gas Separation: A Mini Review

Abstract: Owing to the advantages of rapid adsorption and desorption characteristics, excellent gas separation performance, as well as good thermal and chemical resistance, carbon molecular sieve (CMS) membranes have been developed as a promising gas separation tool. Over the past 30 years, hollow fiber carbon molecular sieve (HFCMS) membranes have become the preferred choice for industrial applications due to their high surface area-to-volume ratio and the ability to assemble lightweight membrane modules. The gas transport mechanism behind the HFCMS is dominated by molecular sieving function. They can be prepared by pyrolysis of the polymeric hollow fiber precursors. Post-treatments can tailor the ultramicropores structure to improve the separation performance. This paper aims to review the recent progress in the preparation of HFCMS membranes from aspects of precursor selection, pyrolysis conditions and post-treatment. Moreover, a brief perspective in terms of future investigation of HFCMS membrane is also proposed.