Gas Diffusion Mechanism Research Articles

Analysis of Gas Diffusion Mechanisms in Shale Matrices during Gas Injection and Production: Model Match and Insights

Analysis of Gas Diffusion Mechanisms in Shale Matrices during Gas Injection and Production: Model Match and Insights

Molecular dynamics simulation of micro mechanisms of gas and water migration and diffusion in shale reservoirs

Clarifying the fluid transport mechanism in nanoporous shale media for shale gas extraction is crucial. In this study, the microscopic mechanism of gas and water migration and diffusion in the nanopores of shale clay minerals is obtained through molecular dynamics simulation. Because of the different compositions and structures of different clay minerals in shale, the adsorption heats of different clay mineral surfaces with CH4 and H2O molecules are different. As the pressure increases, the adsorption heat of the clay mineral surfaces with CH4 and H2O molecules increases. At the same temperature and pressure, the adsorption heat of the clay mineral surfaces with H2O molecules is greater than that with CH4 molecules, and the high-energy adsorption sites on the surface of clay minerals are mainly occupied by H2O molecules, leading to H2O molecules significantly aggregating on the shale clay mineral surfaces, while CH4 molecules are dispersed in an disordered manner in shale clay minerals. The higher the pressure or the lower the temperature is in shale formations, the higher is the degree of aggregation of H2O molecules on the surface of clay minerals. The migration speed and fluidity of gas and water increase with the increase in temperature and decrease with the increase in pressure. At the same temperature and pressure, the migration speed of CH4 molecules is greater than that of H2O molecules. This is because H2O molecules are polar and have stronger intermolecular forces compared to CH4 molecules, resulting in weaker mobility of H2O molecules. The diffusion coefficient of CH4 molecules in kaolinite is the highest, followed by that in illite, and that in montmorillonite is the lowest. When the diffusion coefficient of H2O molecules in shale clay minerals is lower, the corresponding diffusion coefficient of CH4 molecules is higher.

Cross-scale diffusion characteristics in microscale fractures of tight and shale gas reservoirs considering real gas – mixture – body diffusion – water film coupling

The mechanism of solid fluid coupling in unconventional oil and gas reservoirs is complex, and elucidating the characteristics of cross scale gas seepage has important scientific and engineering significance. This article focuses on the gas seepage characteristics in micro fractures of unconventional natural gas reservoirs, with three main innovations: (a) analyzing the coupling mode between real gas effects and mixed gases; (b) Explored the multi factor control mechanism of bulk gas diffusion in microscale fractures; (c) The mechanism of the influence of water film distribution characteristics in microscale fractures on gas seepage was elucidated. Results show that: (a) For the water saturation of 10% and the methane content of 80%, when the gas pressure in the crack increased from 0.2 MPa to 10 MPa, the continuous flow velocity and slippage flow velocity increased. (b) for the water saturation of 10% and the methane content of 80%, when the mixed gas pressure increases from 0.2 MPa to 0.8 MPa, the bulk gas diffusion rate drops relatively rapidly from 8.40E-22 kg/s to 8.18E-23 kg/s (c) for the water saturation of 10% and the methane content of 80%, when the mixed gas pressure increases from 0.2 MPa to 0.6 MPa, the modified slippage flow rate drops relatively rapidly from 7.47E-21 kg/s to 6.96E-21 kg/s.

Leakage and diffusion characteristics of underground hydrogen pipeline

Soil corrosion and hydrogen embrittlement are the main factors of hydrogen pipeline failure. The gas escapes, diffuses and accumulates in the soil and enters the atmosphere when leak occurs. The mechanism of gas diffusion in buried pipelines is very complicated. Mastering the evolution law of hydrogen leakage diffusion is conducive to quickly locating the leakage point and reducing the loss. The leakage model of the underground hydrogen pipeline is established in this paper. The effect of leakage hole, soil type, pipeline pressure, pipeline diameter on hydrogen leakage diffusion were investigated. The results show that when the hydrogen pipeline leaks, the hydrogen concentration increases with the increase of leakage time, showing a symmetrical distribution trend. With the pipeline pressure increase, hydrogen leakage speed is accelerated, and longitudinal diffusion gradually becomes the dominant direction. As the leakage diameter increases, hydrogen leakage per unit of time increases sharply. Hydrogen diffuses more easily in sandy soil, and its diffusion speed, concentration, and range are higher than that in clay soil. The research content provides a reference and basis for the detection and evaluation of buried hydrogen pipeline leakage.

Evaluation of different carbon-modified zeolite derivatives preparation methods as a filler in mixed matrix membrane on their gas separation performance

Carbon-modified zeolite from different preparation methods, including impregnation and chemical vapor deposition (CVD), was investigated and applied as fillers in polysulfone (PSF) membranes in order to improve gas separation performance. Using the impregnation method and CVD, carbon structures were added into the zeolite template. These fillers are zeolite composite carbons (ZCC). After removing the template, zeolite-templated carbons (ZTC) were obtained and analyzed with XRD, SEM, and the N2 adsorption-desorption isotherm. In addition, further investigations were carried out in MMMs embedded by these fillers with XRD, FTIR, TGA, DSC, and gas permeation tests. PSF/ZTC synthesized using CVD demonstrated favorable improvement, which enhanced the gas permeances and CO2/CH4, O2/N2, H2/CH4, and CO2/N2 selectivity up to 107.66%, 24.79%, 140.56%, and 18.55%, respectively. The improvement in gas selectivity was contributed by the high microporosity of the filler. Moreover, the proposed gas diffusion mechanism in MMMs was elucidated, which was dominated by Knudsen and surface diffusion. It was interesting to note that the use of a very small percentage of ZTC-C loading was sufficiently considered to provide a high enhancement compared to those for a high percentage of loading with different fillers. Therefore, the CVD process is more promising for preparing zeolite-carbon fillers to boost gas separation performance and opens up possibilities for further development of advanced membranes.

Investigating the Parameter-Driven Cathode Gas Diffusion of PEMFCs with a Piecewise Linearization Model

Improving mass transfer in gas diffusion layers is critical to achieving high-performance proton-exchange membrane fuel cells (PEMFCs). Leaks through the interface between the gas and the membrane electrode assembly frame have been widely investigated, and the controllability of the cathode gas diffusion has not been achieved in most studies. In this study, we develop a structural parameter to investigate the controllability of the gas diffusion mechanism in the cathode in order to improve upon the design and performance of PEMFCs. This parameter accounts for the cathode gas diffusion layer porosity and carbon loading inside the catalyst layer. It is comprehensively calculated to relax the two segments’ distribution along three directions of the coordinate axis. The experimental and simulation results show that the obtained values of the parameter vary and change during voltage stabilization. According to the results, regardless of the materials in the cathode gas diffusion layer, the same steady-state voltage is obtained when the parameter is fixed. The cell could be controllably operated for a wide range of diffusion layer thicknesses by selecting the optimal parameter.

Reduction Behavior and Characteristics of Metal Oxides in the Nanoscale

The development of nanomaterials and nanotechnology enables the production of nanosized metallic alloys with advanced characteristics from their oxides via a thermal reduction technique. The aim of the present work was to produce metallic iron, nickel, and tungsten through the gaseous reduction of nanosized metal oxide powders as a preliminary step towards the fabrication of nanosized heavy tungsten alloys with unique properties. Nanosized NiO, Fe2O3, and WO3 were isothermally and non-isothermally reduced with H2, and the oxygen weight loss was continuously recorded as a function of time. The Thermogravimetric TG-DTA technique was applied in the non-isothermal reduction up to 1000 °C. The reduction extents were calculated from the TG curve, whereas the accompanying heat of the reaction was measured from the DTA curve. The results revealed that NiO was reduced at <420 °C, Fe2O3 was reduced at <600 °C, and WO3 was reduced at >950 °C. In the isothermal process, metal oxides were reduced with H2 at 700–1000 °C; a micro-force balance was used and the O2 weight loss was continuously recorded. At a given temperature, the rate of reduction increased in the order NiO > Fe2O3 > WO3. The nano-oxide powders and the reduced products were physically and chemically characterized. The activation energy (Ea) values were computed from the isothermal reduction in the initial and later stages to elucidate the corresponding reduction mechanism. The Ea values indicated that the reduction of metal oxides was controlled by the gas diffusion mechanism at both the initial and later stages of reduction. The results of the present study determined the optimal operation parameters at which the thermal gaseous reduction technique could be applied for preparing metallic alloys from nanosized metal oxides.

Experimental investigation of Kr diffusion in UO2+x: Slight deviations from stoichiometry, significant effects on diffusion kinetics and mechanisms

Thermo-Desorption Spectrometry (TDS) was employed to study the release of fission gases from a Kr implanted UO2+x during 1050–1300 °C isotherms. The stoichiometry of the sample has been regularly increased by O2 additions in the vacuum setup to cover an hyperstoichiometry x range from ∼1 × 10−6 to ∼0.1. We used an analytical model based on Fick's second law to determine krypton diffusion kinetics (DKr) from the Kr cumulated release profiles. We demonstrate that even little x deviations from stoichiometric UO2 down to the 10−6 order are sufficient to quantitatively enhance fission gas diffusion. This likely explains a fair amount of data scatter in literature for experimental fission gases diffusion rates in (supposedly) perfectly stoichiometric UO2. A model was developed from the experimental results and well-accepted data for UO2 to estimate DKr (and DXe equivalently) as a function of T and x in the ranges of the present study.For moderate hyperstoichiometries, the present results suggest a fission gas diffusion mechanism identical (or similar to the very least) to that largely accepted for stoichiometric UO2, with the increased concentration of oxygen interstitials in the material kinetically enhancing Kr diffusion. At x ≈ 0.001 the diffusion-limiting mechanism changes, materialized by an increase of the activation energy (Ea). This Ea increase seemingly rules out a diffusion of fission gases controlled by VU (U vacancies). Instead, the rise of oxygen interstitial clusters, becoming the dominant type of defects in UO2+x for similar x ≥ 0.001 threshold, makes these defects prime suspects in controlling Kr (and Xe) diffusion in hyperstoichiometric uranium dioxide.

Adsorption and Diffusion Behaviors of CO2 and CH4 Mixtures in Different Types of Kerogens and Their Roles in Enhanced Energy Recovery

CO2 geological sequestration in subsurface shale formations is a promising strategy to store CO2 and to increase shale gas production. The understanding of gas adsorption and diffusion mechanisms in microporous media is critical for CO2 storage-enhanced gas recovery (CS-EGR). The type of kerogens is one of the important factors that influence the adsorption and diffusion behaviors of gases. In this work, the Grand Canonical Monte Carlo and Molecular Dynamics simulations were utilized to develop kerogen models and further investigate gas and water adsorption and diffusion behavior on the type IA, IIA, and IIIA kerogen models. The results indicated that the adsorption and diffusion capacities of CO2 are larger than those of CH4. The adsorption and diffusion capacity decreased with increasing water content. However, the CO2/CH4 adsorption selectivity increased with the increase in water content. Type IIIA demonstrated the best potential for adsorption and diffusion. This study provides insights into the role of the adsorption and diffusion behavior of CO2 and CH4 mixtures on kerogens of different types under different water contents at a microscopic scale, and can facilitate further understanding of the processes involved in CO2 storage coupled with enhanced energy recovery.

A FRACTAL MODEL FOR GAS DIFFUSION IN DRY AND WET FIBROUS MEDIA WITH TORTUOUS CONVERGING–DIVERGING CAPILLARY BUNDLE

In this paper, a fractal model is proposed for gas diffusion in dry and wet fibrous media with tortuous converging–diverging capillary bundle on the basis of the fractal theory. The proposed theoretical model for the normalized gas diffusivity (NGD) can be expressed as an explicit functional relation of porosity, [Formula: see text], fluid saturation, [Formula: see text], fractal dimensions, [Formula: see text] and [Formula: see text], the minimum average radius, [Formula: see text], the maximum average radius, [Formula: see text], the straight capillary length of a unit cell [Formula: see text] as well as fluctuation amplitude [Formula: see text]. The predictions of the proposed model have been compared with the existing experimental data and the available model predictions, and a good agreement can be observed. The effect of various parameters on the NGD is studied alone. It is observed that the NGD decreases with an increase in the fluctuation amplitude. Also, it is seen that the NGD decreases with an increase in the tortuosity fractal dimension. Moreover, it is found that the NGD in wet fibrous media decreases with an increase in the fluid saturation. The present model has no empirical constant and each parameter contains clear physical meaning. These may better reveal the physical mechanisms of gas diffusion in fibrous media.

Ti3C2 MXene Membranes for Gas Separation: Influence of Heat Treatment Conditions on D-Spacing and Surface Functionalization.

Two-dimensional (2D) MXene materials have recently been the focus of membrane research due to their unique properties, such as their single-atomic-layer thickness, flexibility, molecular filtration abilities and microstructural similarities with graphene, which is currently the most efficient precursor material for gas separation applications. In addition, the potential to process nanoscale channels has motivated investigations of parameters which can improve membrane permeability and selectivity. Interlayer spacing and defects, which are still challenging to control, are among the most crucial parameters for membrane performance. Herein, the effect of heat treatment on the d-spacing of MXene nanosheets and the surface functionalization of nanolayers was shown regarding its impact on the gas diffusion mechanism. The distance of the layers was reduced by a factor of over 10 from 0.345 nm to 0.024 nm, the defects were reduced, and the surface functionalization was maintained upon treatment of the Ti3C2 membrane at 500 °C under an Ar/H2 atmosphere as compared to 80 °C under vacuum. This led to a change from Knudsen diffusion to molecular sieving, as demonstrated by single-gas permeation tests at room temperature. Overall, this work shows a simple and promising way to improve H2/CO2 selectivity via temperature treatment under a controlled atmosphere.

Modeling Interfacial Interaction between Gas Molecules and Semiconductor Metal Oxides: A New View Angle on Gas Sensing.

With the development of internet of things and artificial intelligence electronics, metal oxide semiconductor (MOS)-based sensing materials have attracted increasing attention from both fundamental research and practical applications. MOS materials possess intrinsic physicochemical properties, tunable compositions, and electronic structure, and are particularly suitable for integration and miniaturization in developing chemiresistive gas sensors. During sensing processes, the dynamic gas-solid interface interactions play crucial roles in improving sensors' performance, and most studies emphasize the gas-MOS chemical reactions. Herein, from a new view angle focusing more on physical gas-solid interactions during gas sensing, basic theory overview and latest progress for the dynamic process of gas molecules including adsorption, desorption, and diffusion, are systematically summarized and elucidated. The unique electronic sensing mechanisms are also discussed from various aspects including molecular interaction models, gas diffusion mechanism, and interfacial reaction behaviors, where structure-activity relationship and diffusion behavior are overviewed in detail. Especially, the surface adsorption-desorption dynamics are discussed and evaluated, and their potential effects on sensing performance are elucidated from the gas-solid interfacial regulation perspective. Finally, the prospect for further research directions in improving gas dynamic processes in MOS gas sensors is discussed, aiming to supplement the approaches for the development of high-performance MOS gas sensors.

Mechanism and influence factor of hydrocarbon gas diffusion in porous media with shale oil

Due to the compactness of shale reservoir matrix and the high conductivity of fractures, the hydrocarbon gas injection huff and puff method or displacement is the most realistic technology to improve shale oil recovery. The diffusion mechanism plays an important role in shale oil development; therefore, it is crucial to figure out the factors influencing diffusion, which could enhance shale oil recovery. In this paper, a physical simulation experiment is designed to evaluate the diffusion ability of hydrocarbon gas. Diffusion experiments are conducted to simulate diffusion in the bulk fluid and in the porous media, to learn about how the pressure, permeability and fracture affect the diffusion behavior. The diffusion coefficients between the bulk diffusion and core sample diffusion are compared. The experimental results show that the diffusion coefficient and mass transfer capacity are positively correlated with permeability and pressure: increasing these parameters can promote the diffusion process. The diffusion coefficient of hydrocarbon gas in a saturated oil core is significantly less than that in crude oil, which indicates that the porous media seriously affects the process of gas diffusion in crude oil. Fractures have little impact on the diffusion behavior. Combined with numerical simulation, the influencing factor of diffusion on the development effect of hydrocarbon gas injection is clarified. The recovery enhances and then decreases with the increasing diffusion coefficient. Cited as: Wanyan, Z., Liu, Y., Li, Z., Zhang, C., Liu, Y., Xue, T. Mechanism and influence factor of hydrocarbon gas diffusion in porous media with shale oil. Advances in Geo-Energy Research, 2023, 7(1): 39-48. https://doi.org/10.46690/ager.2023.01.05

A NOVEL FRACTAL MODEL FOR GAS DIFFUSION COEFFICIENT IN DRY POROUS MEDIA EMBEDDED WITH A DAMAGED TREE-LIKE BRANCHING NETWORK

A novel gas diffusivity model for dry porous media with a damaged tree-like branching network is proposed by using the fractal theory in this study. We systematically investigated the effects of the number of damaged channels and the other structural parameters on the dimensionless gas diffusivity (DGD) and concentration drop. As the number of damaged channels increases, the DGD presents a decreasing trend, while the ratio of concentration drop shows a rising tendency. Meanwhile, the DGD is negatively correlated to the length exponent, the total number of branching levels, and the branching angle, respectively. On the other hand, the DGD is positively correlated with the diameter exponent. Besides, the ratio of concentration drop is negatively correlated with the length exponent and the total number of branching levels. However, it is positively associated with the diameter exponent and branching levels. In addition, during the calculation of the value of concentration drop, the total concentration drop can be disassembled into two equal-ratio sequences. And the scale factors in sequences are constants that are independent of the number of damaged channels. The reliability of the model predictions was verified by a comparison with the experimental data available in the literature. The physical mechanism of gas diffusion in the damaged network may be well explained by the proposed model.

Microscopic modelling of gas diffusivity in unsaturated cementitious materials considering multiple diffusion regimes

Common degradations of concrete including reinforcement corrosion and carbonation are highly associated with the transport of aggressive gases. However, it is still challenging to accurately understand the gas diffusion behaviour and determine the gas diffusivity in unsaturated cementitious materials. This study proposes a pore-scale model for simulating gas diffusivity in unsaturated cement paste covering multiple gas diffusion mechanisms. The representative volume elements of colloidal calcium silicate hydrate and heterogeneous cement paste with various water saturation degrees are established, capturing the structural attributes of hierarchical cement paste and water-gas distribution with minimum surface energy. A finite difference model for diffusion considering Knudsen effect and molecular diffusion is developed to simulate the diffusion process of various gases through partially saturated cement pastes with different pore structures. Results indicate that gas diffusivity decreases with the increasing water saturation level, the evolution of which can be divided into stable, decreasing and non-diffusive stages. The gas diffusion in partially saturated cement paste is highly dependent on the pore structure and diffusive gas type. The results obtained from this study are expected to provide a deeper insight into the solution of diffusion-related durability problems and the prediction of service life of concrete.

A new approach for modeling adsorption kinetics and transport of methane and carbon dioxide in shale

AbstractIn this article, a new approach is proposed to investigate adsorption kinetics and transport of gases in shale. Due to co‐existence of pores with different size in the shale, a set of adsorption processes happened in pores of different sizes are considered. A first‐order multi‐process model is developed, which can perfectly fit the adsorption kinetic data of CH4 and CO2 obtained at different temperatures. The modeling and pore characterization results indicate that an adsorption process happens in micropores/mesopores (<50 nm) and another adsorption process happens in macropores (>50 nm) in the Wufeng shale. Gas diffusion mechanism is dominant in micropores/mesopores, and gas seepage mechanism is dominant in macropores. The effective diffusivity of CO2 is smaller than that of CH4, because the adsorption of large amount of CO2 in the pores hinders its diffusion. The coefficients related to the diffusion and seepage have no obvious trend with temperature.

Facets of coalbed methane reservoir in East Bokaro Basin, India

The East Bokaro Basin of the Damodar valley is a potentially prospective CBM (coalbed methane) play having significant cumulative coal seam thickness, in-situ gas content, vitrinite percentage, and adequate thermal maturity. Successful CBM recovery needs a detailed understanding of the organic content, pore structures/networks, storage properties and gas flow mechanism. The present work attempts to systematically investigate East Bokaro coal for organo-petrographic controls on gas content and generation, variations in sorption capacity and saturation, pore mechanisms, cleat intensity, cleat aperture distribution and spacing. The values of in-situ gas, sorption capacity and methane concentration (C1) vary from 3.52 to 30.93 cc/g (dry ash-free basis), 15.40–32.40 cc/g (dry ash-free basis), and 66–93 vol%, respectively. The atomic ratios H/C and O/C indicate that thermally matured coal seams contain type III-IV kerogen positioned in the dry gas window. The decrease of hydrogen-containing liptinite with increasing depth reveals the function of thermal gradient on the cracking of liptinitic compounds with successive evolution of hydrocarbons and the development of a carbon-rich pore matrix. The H/C ratio is also influenced by the increasing content of vitrinite and reflectance values of deeper coal. More than 63 % of desorbed gas was determined from desorption measurement and low sorption time (τ, mainly <10 days). This demonstrates good diffusion characteristics of the studied coal. It shows the tendency of desorbed gas diffusion from pores reaches to cleat-fractures with negligible influence of secondary mineral infillings. The high-pressure sorption studies of methane on various samples indicate substantial open-pore characteristics, supporting adsorption, diffusion and gas release. The relationship of C3/C1 and C2/C1 ratios demonstrates that the hydrocarbons in coal primarily originated from the thermogenic transformation of organic matter. Such an assessment is also supported by the stable isotope (δ13C1) value that ranges between −22.70 ‰ and −57.30 ‰. However, some of the lighter isotope values (<–50 ‰) indicate a mixed origin of gases, which may be due to the influx of fresh-water to coal associated aquifers carrying bacteria received from local drainage. Geochemically and thermally altered dissolved and partially filled pores, shown by SEM photographs, negligibly influence gas sorption, diffusion, and flow mechanism in coalbeds. The pore network model signifying that the studied coal seams are microstructurally different comprises a lateral difference in pore and cleat/fracture.

Coupled modeling of irradiated fuel thermochemistry and gas diffusion during severe accidents

In this paper, a novel approach where irradiated fuel thermochemistry and gas release are coupled is presented in details and illustrated by the simulations of some tests of the VERCORS program characterized by increasing temperatures and varying gas composition in the furnace (oxidizing or reducing conditions). At each step of the tests, the oxidation/reduction of the nuclear fuel and the fission product chemical speciation are precisely assessed thanks to a thermochemical equilibrium calculation relying on the OpenCalphad thermochemical solver and on a built-in thermochemical database derived from the SGTE database and completed by a solid solution model for the U-O-fission product system. Fission product releases are estimated from the chemically reactive gases that form in the fuel (according to the thermochemical calculation) and from a gas diffusion model based on the equivalent sphere model. The gas diffusion model takes into account not only the noble gases available in the fuel prior to the test but also the chemically reactive gases that form during the test. It is shown that the proposed coupled approach provides a consistent estimation of fission product release (I, Te, Cs, Mo, Ba) during the VERCORS tests in spite of the simple gas diffusion mechanism considered in the simulations (no distinction between the fission products). The proposed coupled approach is used to test some thermochemical hypotheses to improve the calculated release of some fission products (Ba, Mo).

Experimental study on methane adsorption and time-dependent dynamic diffusion coefficient of intact and tectonic coals: Implications for CO2-enhanced coalbed methane projects

CO2-enhanced coalbed methane (CO2-ECBM) technology can be applied not only to extract coal seam gas, but also to geologically store a large amount of CO2 produced in various industries, thus reducing the CO2 concentration in the atmosphere and protecting the earth’s environment from the adverse effects of climate change. For CO2 injection and methane recovery from coal seams, the differences in the gas adsorption and diffusion characteristics due to the properties of the coal seam are crucial. In this study, first, intact and tectonic coals were collected to conduct a proximate analysis and high-pressure methane adsorption/desorption tests. Subsequently, the adsorption characteristics of the intact and tectonic coals with different particle sizes were analyzed. A time-dependent dynamic diffusion coefficient was introduced to investigate the diffusion characteristics of the two coals. The results showed that the intact and tectonic coals have higher adsorption capacity for CO2 than methane under the same particle size, demonstrating the potential for storing a large amount of CO2 in coal seams. The time-dependent dynamic diffusion coefficient decreased significantly in the initial stages and then tended to stabilize as the desorption progressed. To better explain the gas diffusion characteristics of the coal, a gas diffusion mechanism model was established to analyze the variation in the diffusion coefficient over time. In addition, a new method for reducing the pressure difference was proposed to maximize CO2 storage and methane recovery. Finally, the environmental implications and safety of CO2 storage in coal seams during the implementation of a CO2-ECBM project was analyzed.

Modelling of multiple gas transport mechanisms through coal particle considering thermal effects

Multiple gas diffusion mechanisms coexist in the nanopores of coal particle, including Knudsen diffusion, slip flow and transition flow of bulk gas and surface diffusion of adsorbed gas. The contribution of each mechanism to the total gas diffusion quantity varies with gas pressure and pore radius. In this study, a coupled thermal-diffused-mechanical model was constructed considering the multiple gas transport mechanisms. The complex coupling relations among gas adsorption/desorption, gas diffusion field, mechanical deformation field and thermal field during gas diffusion of coal were considered. The contribution of each diffusion mechanisms to the total gas diffusion quantity under variable pressure and pore radius was discussed. Thermal effects of gas diffusion were studied by considering the whole diffusion process as adiabatic. The results show that Knudsen diffusion is dominant in the low-pressure region, regardless of the pore size. In the high-pressure region, the slip flow is dominant when the pore radius is large, while the surface diffusion is dominant when the pore radius is relatively small. The gas diffusion velocity and total diffusion quantity under adiabatic conditions are lower than that under isothermal conditions. But the rate of pressure drop is opposite, which can be explained by the ideal gas equation of state. In reality, the gas diffusion process is neither isothermal nor adiabatic, but between the two states. The thermal effect should be considered in the study of gas diffusion properties.