Gas Transport Model Research Articles

Analytical Model for Real Gas Transport in Shale Reservoirs with Surface Diffusion of Adsorbed Gas

Shale is a kind of heterogeneous porous medium, which has a large number of nanopores. A model for shale gas flow is proposed and validated with published results. The transport mechanisms and the effect of surface diffusion are investigated. The results show that when the diameter of shale nanopore is 1 nm, the surface diffusion is dominant. With the increase in Knudsen number, the surface diffusion permeability decreases gradually. When the pore diameter is 8 nm, the Knudsen diffusion is dominant at a high Knudsen number. At low pressure, the apparent permeability predicted by the present model considering the surface diffusion is larger than that without considering the surface diffusion. At high pressure, the apparent permeability decreases when surface diffusion is considered. The study offers theoretical support for numerical simulations of shale gas development.

In silico study of the role of cell growth factors in photosynthesis using a virtual leaf tissue generator coupled to a microscale photosynthesis gas exchange model.

Computational tools that allow in silico analysis of the role of cell growth and division on photosynthesis are scarce. We present a freely available tool that combines a virtual leaf tissue generator and a two-dimensional microscale model of gas transport during C3 photosynthesis. A total of 270 mesophyll geometries were generated with varying degrees of growth anisotropy, growth extent, and extent of schizogenous airspace formation in the palisade mesophyll. The anatomical properties of the virtual leaf tissue and microscopic cross-sections of actual leaf tissue of tomato (Solanum lycopersicum L.) were statistically compared. Model equations for transport of CO2 in the liquid phase of the leaf tissue were discretized over the geometries. The virtual leaf tissue generator produced a leaf anatomy of tomato that was statistically similar to real tomato leaf tissue. The response of photosynthesis to intercellular CO2 predicted by a model that used the virtual leaf tissue geometry compared well with measured values. The results indicate that the light-saturated rate of photosynthesis was influenced by interactive effects of extent and directionality of cell growth and degree of airspace formation through the exposed surface of mesophyll per leaf area. The tool could be used further in investigations of improving photosynthesis and gas exchange in relation to cell growth and leaf anatomy.

Analysis of gas transport behavior in organic and inorganic nanopores based on a unified apparent gas permeability model

Different from the conventional gas reservoirs, gas transport in nanoporous shales is complicated due to multiple transport mechanisms and reservoir characteristics. In this work, we presented a unified apparent gas permeability model for real gas transport in organic and inorganic nanopores, considering real gas effect, organic matter (OM) porosity, Knudsen diffusion, surface diffusion, and stress dependence. Meanwhile, the effects of monolayer and multilayer adsorption on gas transport are included. Then, we validated the model by experimental results. The influences of pore radius, pore pressure, OM porosity, temperature, and stress dependence on gas transport behavior and their contributions to the total apparent gas permeability (AGP) were analyzed. The results show that the adsorption effect causes Kn(OM) > Kn(IM) when the pore pressure is larger than 1 MPa and the pore radius is less than 100 nm. The ratio of the AGP over the intrinsic permeability decreases with an increase in pore radius or pore pressure. For nanopores with a radius of less than 10 nm, the effects of the OM porosity, surface diffusion coefficient, and temperature on gas transport cannot be negligible. Moreover, the surface diffusion almost dominates in nanopores with a radius less than 2 nm under high OM porosity conditions. For the small-radius and low-pressure conditions, gas transport is governed by the Knudsen diffusion in nanopores. This study focuses on revealing gas transport behavior in nanoporous shales.

Dynamic pore network modelling of real gas transport in shale nanopore structure

Gas transport in shale nanopores is controlled by the complex transport mechanisms and pore structure characteristics. So far much work has been done on the single pore and regular structure based pore network but little is known on real gas transport behavior on a realistic shale pore structure model. In this work, a dynamic pore network model is proposed to describe single component methane transport in nano-scale porous media. Gas transport behavior takes transport mechanisms of slip flow, transition flow, surface diffusion and ad/desorption into account. Real gas effect under high pressure and temperature is considered when calculating gas properties. A three dimensional pore network model is built from a three dimensional image that is reconstructed using multi-point statistics from a small organic-rich area on a two dimensional shale SEM image. This pore network model is used to analyze dynamic gas transport and pressure drop transmission process. Our simulated results reveal that there exists a time period that gas permeability on the pore network is influenced by the pressure drop transmission process and pore structure that flows on. When pressure drop reaches outlet, gas transport turns steady state. At steady state period gas permeability becomes constant and is influenced by the effective stress, pressure, temperature and shale rock property.

Rain and wind affect chamber measurements

Chamber methods are widely used to measure soil-atmosphere fluxes of different gases. To obtain reliable flux estimates it is important to avoid changes in the natural soil-atmosphere system as far as possible. While issues like chamber dimensions, closure time and calculation approaches have been intensively discussed, there is still uncertainty about other possible biases like wind and rain affecting the measurement.To study the effect of local airflow and rain on chamber flux measurements, we used inert tracer gas (Tetrafluoromethane, CF4) that was continuously injected into the subsoil and concurrently measured during the automatic soil respiration measurements. While the variability of the CO2 fluxes included the changing signal of the biological source and the physical effect of the changing environment, the CF4 measurements allowed isolating the latter. Finite element modeling of gas transport within the soil-chamber-atmosphere system was used to further analyze the effect of rain and soil moisture.Rain events had a strong effect on the measured CF4 fluxes with a decrease shortly after rainfall which was then super-compensated by elevated fluxes for 1–2 days. This pattern could be addressed as an artefact of automatic chamber measurements that has been overlooked so far. Since the chamber closed automatically, it excluded a part of the rain amount. This resulted in a temporarily preferential venting of soil gas through the chamber surface that remained drier than the surrounding soil outside the chamber. Thus, we recommend leaving automatic chambers open during rain to strictly avoid rain exclusion.Local wind speed near the chamber was generally low since we worked in a forest. Yet, CF4 flux and CO2 flux decreased with increasing wind speed, what can be interpreted as an artefact due to the chamber modifying the natural airflow conditions. The wind effect, however, was weaker than the effect of rain.

Technical Note: Isotopic corrections for the radiocarbon composition of CO<sub>2</sub> in the soil gas environment must account for diffusion and diffusive mixing

Abstract. Earth system scientists working with radiocarbon in organic samples use a stable carbon isotope (δ13C) correction to account for mass-dependent fractionation, but it has not been evaluated for the soil gas environment, wherein both diffusive gas transport and diffusive mixing are important. Using theory and an analytical soil gas transport model, we demonstrate that the conventional correction is inappropriate for interpreting the radioisotopic composition of CO2 from biological production because it does not account for important gas transport mechanisms. Based on theory used to interpret δ13C of soil production from soil CO2, we propose a new solution for radiocarbon applications in the soil gas environment that fully accounts for both mass-dependent diffusion and mass-independent diffusive mixing.

Evaluation of passive and active lattice Boltzmann method for PEM fuel cell modeling

Modeling of multi-component gas transport ( O2, N2, H2O) in a Proton exchange membrane fuel cell (PEMFC) cathode channel with heterogeneous porous gas diffusion layer (GDL) has been carried out using a two different lattice Boltzmann models. Generally, two passive and active methods are used for modeling single-phase, multi-component fluid flows in the Lattice Boltzmann method. In this study, the implementation of both models is described completely and their results are compared with experimental study. The results demonstrate that maximum error in active approach is only 2.5% at the common operating voltage of PEMFC and this model reaches to steady solution 33.3% faster than passive model. Also, that is found that the active method is more appropriate for complex geometry by using bounce back boundary condition on the obstacles in GDL and modify new bounce back on the reaction surface where the implementation of this boundary condition is so easy for complicated geometries.

Mathematical modeling of gas transport in porous geological media with contrast of properties and irregular distribution of pores

AbstractIn this work, we perform multiscale modeling of gas transport through the geological media having irregular pore structure and contrast of properties on different spatial scales. We assume that the medium consists of inorganic matrix with organic inclusions imbedded into it. There exist a contrast of properties and spatial scales between the matrix and organic inclusions. The pore sizes vary from micro to nanometers, permeability and diffusivity can differ by several orders of magnitude. We consider filtration and molecular diffusion as mechanisms for free gas transport in both inorganic and organic materials, and surface diffusion as the main mechanism for sorbed gas transport through nanoporous organic inclusions. The irregularities of porous structure we characterize by their deviations from the periodic distribution. We implement multiscale homogenization together with an averaging with respect to random deviations of distribution of pores to derive the macroscopic equation for evaluating the free gas amount in‐place. It turns out that macroscale parameters characterizing gas transport depend on diffusivity, permeability, and porosity of the components of the system, the amount of inclusions and their spatial distribution. We determine the distribution of gas concentration through the production time and investigate its sensitivity to irregularities of pores distribution. We are also interested in the effect of bottom‐hole pressure and study how depletion can be affected by the interchange of gas between kerogen inclusions and inorganic material.

EnKF-based estimation of natural gas release and dispersion in an underground tunnel

With the rapid industrialization and urbanization in China, underground tunnels, as an effective way to construct and maintain energy supplies, have been widely utilized to transport natural gas in urban chemical plants and industrial parks. Natural gas transport in an underground tunnel really facilitates the repair and maintenance of gas pipelines, but could pose a high risk of causing fire or explosion in case of natural gas pipeline leakage. The reasonable estimation of natural gas leakage and subsequent spatiotemporal distribution of natural gas dispersion is of great importance for effective emergency response and loss prevention. In this study, a numerical gas transport model in conjunction with the ensemble Kalman filter (EnKF) was proposed to realize the inversion of natural gas leakage rate and predict gas dispersion in a ventilated underground tunnel. The EnKF-based model can improve gas release estimation and dispersion prediction by assimilating observation data into simulation results. The proposed model-data assimilation system was firstly validated by twin experiment, and furthermore the sensitivity of the initial-guess release rate was analyzed to verify the practicability of the proposed model. Moreover, the reasonable estimation of dynamic natural gas release rate was realized and strategic decisions on methane sensors arrangement in tunnel were provided. The reasonable results indicate that the proposed EnKF-based model is an effective method for estimating natural gas release rate and predicting real-time natural gas spatiotemporal distribution in case of natural gas release accident in an urban underground tunnel.

An improved transport model of shale gas considering three-phase adsorption mechanism in nanopores

For the shale gas transport model in nanopores, researchers recently pay more and more attention to the existence of water phase. However, the effect of water film on the gas transport is still not clearly understood. In this paper, we considered the existence of water film on inorganic shale pore surface and introduced the adsorption mechanism of methane-water film-shale clay three-phase (gas-liquid-solid) to the shale gas transport model. Then, we modified the expression of porosity and derived a new transport equation of shale gas considering the adsorption mechanism of three-phases in nanopores. Using the finite element method, we solved the equation and analyzed the effect of water film. Finally, the range of water film effect was analyzed by defining the offset ratio. The results show that: (1) The existence of water film has negative effects on physical quantities, such as effective porosity and apparent permeability. When the water molecular coverage ratio reaches to 1, the decrease of porosity can be about 18.1%; (2) The existence of water film reduces the gas production rate and accumulative gas production. When the water molecular coverage ratio reaches to 1, the gas accumulative production decreases about 49.6%; (3) The pore radius which has significant water film effect is very small. Furthermore, the smaller of the pore size, the more significant the water film effect is. Also, it can be found that the effect of water film on porosity can be ignored with when pore radius exceeds 29 nm. When the pore radius is larger than 11 nm, the effect of water film on kapp/kd can be ignored.

Nanoscale confined gas and water multiphase transport in nanoporous shale with dual surface wettability

Fluid transport in nanoporous shale is known to be affected by the nanoscale fluid transport mechanisms, surface wettability and heterogeneous pore structure. The pores of shale are believed to be dual surface wettability with gas-wet organic pores and water-wet inorganic pores. Investigation on the nanoscale multiphase transport behavior in dual surface wettability nanoporous shale has practical implication in understanding inject water distribution during injected water flow in and flow back process. In this study, we propose a nanoscale gas and water multiphase pore network transport model to study nanoscale confined gas and water transport behavior in dual wettability nanoporous shale. A 3-D shale pore network model is constructed from 3-D image that is reconstructed from 2-D shale SEM image of organic-rich sample. Water transport considers the boundary slip length determined by the contact angle. Bulk gas transport in inorganic pores considers slip effect while bulk gas transport and surface diffusion for adsorbed gas are both considered in organic pores. Injected water flow in process is modeled by water displacing gas process while injected water flow back process is modeled by gas displacing water process. Gas and water relative permeabilities during injected water flow in and flow back process at different TOC volumes and inorganic pore contact angle are analyzed in detail and are compared with relative permeabilities without nanoscale transport mechanisms. Study results reveal that nanoscale gas and water relative permeabilities are influenced by the total organic carbon (TOC) in volumes and inorganic pore water contact angle while nanoscale transport mechanisms influence on the relative permeabilities can be neglected.

Gas transport study in the confined microfractures of coal reservoirs

Microfractures are commonly observed in coal reservoirs. During the coalbed gas production process, stress sensitivity and coalbed gas viscosity changes are significant factors that affect gas transport. By using research methods based on desorption theory and elastic-plastic mechanics, a coal confined microfracture gas transport model that considers dynamic microfracture width variations and gas effective viscosity is established in this paper. This model comprehensively fuses the Knudsen diffusion model, the slip flow model, the surface diffusion model, and the cubic grid model. The reliability of this model is verified via molecular simulations, and the influence factors of gas transport capacity in confined microfractures of coal reservoirs are then discussed in detail. The results demonstrate the following findings. (1) The analyzed flows are well simulated in coal confined microfractures by the model established in this paper, which considers stress sensitivity and coalbed methane viscosity change. (2) Under low formation pressure (less than 5 MPa), the effective gas viscosity rapidly decreases with the decrease in formation pressure, and the negative contribution of gas viscosity change to the microfracture permeability of coal is large. The smaller the initial fracture width of coal, the more obvious this negative effect. (3) When the initial fracture width is fairly small (near 1 nm), Knudsen flow and surface diffusion greatly contribute to the microfracture permeability of coal. However, the larger the initial microfracture width, the smaller the contribution of the two flow regimes to the microfracture permeability, and the permeability is mainly provided by slip flow. (4) Under given conditions, the microfracture permeability of coal is positively related to rock mechanical parameters (Young's modulus and Poisson's ratio) and negatively related to fracture compressibility. Under low formation pressures (less than 15 MPa), the microfracture permeability of coal is positively related to gas desorption. When the formation pressure exceeds 15 MPa, the influence of gas desorption performance on permeability is nearly constant.

A multi-scale model of gas transport in the lung to study heterogeneous lung ventilation during the multiple-breath washout test.

The multiple-breath washout (MBW) is a lung function test that measures the degree of ventilation inhomogeneity (VI). The test is used to identify small airway impairment in patients with lung diseases like cystic fibrosis. However, the physical and physiological factors that influence the test outcomes and differentiate health from disease are not well understood. Computational models have been used to better understand the interaction between anatomical structure and physiological properties of the lung, but none of them has dealt in depth with the tracer gas washout test in a whole. Thus, our aim was to create a lung model that simulates the entire MBW and investigate the role of lung morphology and tissue mechanics on the tracer gas washout procedure. To this end, we developed a multi-scale lung model to simulate the inert gas transport in airways of all size. We then applied systematically different modifications to geometrical and mechanical properties of the lung model (compliance, residual airway volume and flow resistance) which have been associated with VI. The modifications were applied to distinct parts of the model, and their effects on the gas distribution within the lung and on the gas concentration profile were assessed. We found that variability in compliance and residual volume of the airways, as well as the spatial distribution of this variability in the lung had a direct influence on gas distribution among airways and on the MBW pattern (washout duration, characteristic concentration profile during each expiration), while the effects of variable flow resistance were negligible. Based on these findings, it is possible to classify different types of inhomogeneities in the lung and relate them to specific features of the MBW pattern, which builds the basis for a more detailed association of lung function and structure.

The influence of closed pores on the gas transport and its application in coal mine gas extraction

The contribution of closed pores to total porosity in most cases exceeds 60%. It plays a critical role in gas storage and transport in coal seam. However, few studies about gas transport model considering the closed pores were reported. In this paper, we tried to construct a constitutive equation of coal considering the closed pore effect. Then, the model of gas transport in coal seam with closed pores was proposed, including gas diffusion between closed pores and open pores as well as gas seepage in fracture. Then, 4 cases were carried out to investigate the influence of various factors on the gas transport in coal seam. Results show that the total gas production decreased with the rising closed porosity, desorption time and stress, while increased with the initial permeability. Furthermore, the closed porosity and desorption time shows a remarkable effect on the contribution of pore to total gas production, followed by the initial permeability and stress. Then, their influence degree on gas pressure difference shows the same trends with that on gas production contribution. Furthermore, the stage characteristics of various curves were analyzed. Finally, the field data from gas extraction of Yangliu coal mine in Huaibei coalfield was used to validate our model, indicating that our model can match the field data better compared with the model without closed pores effect.

A novel numerical model of gas transport in multiscale shale gas reservoirs with considering surface diffusion and Langmuir slip conditions

AbstractMultiflow mechanisms coexist in shale gas reservoirs (SGRs) due to the abundant nanopores and the organic matter as a medium of gas souring and storage. The gas transport mechanisms in nanopores including bulk gas transfer and adsorption‐gas surface diffusion were already investigated in pore‐scale models, but their effects on actual gas production of multistage fractured horizontal wells in SGRs are not clearly understood, which are crucial for the economic development of unconventional resources. Therefore, a comprehensive apparent permeability (AP) model which couples the surface diffusion of adsorbed gas, slippage flow considering the additional flux generated by surface diffusion based on Langmuir's theory, and Knudsen diffusion is established. The presented model is validated with the experimental data and lattice Boltzmann method (LBM) simulation results. Then, we propose a numerical model which combines multiflow mechanisms in microscale pores and a multistage fractured horizontal well (MSFHW) in macroscale shale gas reservoirs together. The effects of different transport mechanisms on both AP of nanopores and gas production are analyzed thoroughly. The results show that the effect of surface diffusion on the apparent permeability of nanopores is much greater than that on the actual gas production of MSFHW, and the influence of high‐pressure condition must be considered when calculating the surface diffusion coefficient. The presented numerical model has important implications for accurate numerical simulation and efficient development of shale gas reservoirs.

The application of a drift model of fission gas transport to unstable gas release in irradiated uranium dioxide

A previous paper has discussed the application to stable fission gas release of an enhanced Fick's Law transport process which also includes a so-called drift term arising from gas atom drag by moving dislocations. The new transport law was shown to be effective at describing stable fission gas release from the onset of grain-boundary tunnel interlinkage to the appearance of high burnup fuel restructuring. In the present work this law is applied to the release of unstable fission gases and is shown to be incapable of explaining the observed data. An explanation for this anomaly has been sought in the PIE of dislocations in irradiated fuel. The ultimate rationale focuses on the recognition not only that successfully released unstable gas atoms are created in the surface region of the fuel grain but also that the solute drag mechanism conspires to favour gas atom motion in the depth of the fuel grain rather than the surface.

Effect of Pore Shape on Nanoconfined Gas Flow Behavior: Implication for Characterizing Permeability of Realistic Shale Matrix

Good knowledge of nanoconfined gas flow behavior will significantly contribute to advance ultimate gas recovery from shale gas reservoirs. However, up to now, it has still been extremely challenging to have a clear understanding of the key issue. In particular, little attention is drawn to the effect of pore geometry on nanoconfined gas flow behavior. By means of developed experimental technology, realistic images reflecting pore characteristics can be obtained, which demonstrate that various pore shapes are exhibited in the shale matrix. To my knowledge, current related research is focused on gas transport through several conventional pore shapes, including circle, rectangle, and slit. As a result, the urgent issue, i.e., research on gas flow behavior through unconventional pore shapes, is highlighted. In this work, a model for gas transport through elliptical nanopores is established, which possesses an excellent varying-shape nature with changing aspect ratio (AR) and therefore covers lots of pore shapes and shares great application value. Both effects of gas slipperiness and surface diffusion are incorporated in this model. Moreover, the Langmuir isotherm is utilized to quantify the thickness of the adsorption layer of methane. When AR is equal to 1 and an infinite number, the proposed model can achieve excellent agreement compared with gas transport data through circular and slit-like nanopores. Results show that (a) the gas transport capacity will decrease with increasing AR value at a specific pressure point; (b) contribution of surface diffusion will become more prominent with higher AR value; (c) under different gas adsorption/desorption characteristics, there is no difference in gas transport capacity through nanopores with different ARs at high pressure and relatively large difference at low-pressure atmosphere; (d) current production prediction models or commercial software require consideration of the effect of various pore shapes to enhance application accuracy. In sum, the findings of this study can help in better understanding the methane flow feature through nanopores with various cross-sections, which further serve as the necessary theoretical attempt with regard to accurate characterization for flow capacity of a realistic shale matrix.

Gas transport in shale matrix coupling multilayer adsorption and pore confinement effect

Accurate prediction of gas transport in shale formations mainly consisting of micro- and nano-scale pores is a great challenge since most existing models only consider monolayer adsorption and the underlying multi-physics are either partially or completely overlooked. In this work, we have proposed a comprehensive single-component gas transport model integrating multilayer adsorption, surface diffusion, real gas effect, and pore confinement effect. The new analytical model is developed based on Bravo layer-sequence-model and then upscaled using an Effective Medium Approximation (EMA) method, where the generalized Brunauer-Emmett-Teller (BET) model, the modified Peng-Robinson equation of state (EoS) model, and the Sutton’s viscosity model are innovatively incorporated. The newly developed model has been successfully validated against simulation and experimental results with the assistance of an efficient iterative ensemble smoother (ES) algorithm. It is observed that the pore confinement effect is of importance when the pore size is smaller than 50 nm. The increase of gas viscosity due to the real gas effect is significant under reservoir conditions. The apparent permeability is found to increase greatly as the adsorption layer number increases, implying that the application of Langmuir model in existing gas transport models may substantially underestimate it. Given inorganic nanopores, the viscous flow is important at any pore sizes and the Knudsen diffusion plays a role in the pores that are less than 5 nm under reservoir conditions. As for organic nanopores, the contribution of surface diffusion is tangible when the pore size is below 150 nm and the Knudsen diffusion is negligible under high pressures.

Influence radius of gas extraction borehole in an anisotropic coal seam: Underground in‐situ measurement and modeling

AbstractCoal seam degasification by in‐seam drilling borehole is one of the popular techniques for coal mine methane (CMM) control. How to address the conflict between borehole numbers and drainage durations always arouses researchers’ interests. Furthermore, since coal has anisotropy structures, how the anisotropy feature of coal associated with bedding and cleat structure affects the influence radius of the drainage borehole has rarely been considered. In order to address the above‐mentioned issues, this work not only conducted underground in‐situ measurement of the borehole influence radius within 200 days in an anisotropy coal seam at Jiulishan coal mine in China, but also modeled the influence radius evolvement around borehole in an anisotropy coal seam using finite‐element based COMSOL package. The underground in‐situ measurement revealed an anisotropy feature of borehole influence radius associated with coal bedding and cleat structures. The influence radius of borehole parallel to the bedding plane in butt cleat direction is much larger than that perpendicular to the bedding plane. Ten permeability measurements of coal show that the permeability of coal parallel to the bedding plane in butt cleat direction is higher than that perpendicular to the bedding plane. Simulation results of the gas transport model in coal are consistent with underground in‐situ measurement in determining the influence radius of drainage borehole in the anisotropy coal seam. Based on modeling results, the ellipse influence area of borehole in an anisotropy coal seam is proposed, and the relationship between the influence radius of borehole and drainage time is obtained. On the basis of these findings, the proper in‐seam borehole arrangements for a specific mining panel at different drainage durations are determined. The finding of this work will be fundamental for addressing the conflict between minimizing the cost of borehole arrangement and increasing coal mining productions by adjusting CMM drainage durations.

Gas transport modelling at different spatial scales of a geological repository in clay host rock

The work presented in this article is a part of the international European Commission FORGE project which studied the behaviour of repository gases in the context of radioactive waste disposal. Experimental work is essential to understand the main transport mechanisms for gas and to determine the main geophysical parameters. However, while laboratory and rock experiments can help to investigate the short- and partly the middle-term behaviour of gas in a rock formation, long-term predictions have to be based on numerical simulations. Numerical simulation of long-term gas behaviour in a deep geological repository was one of the aims in the FORGE project. The objectives of the FORGE modelling were to describe the state-of-the-art consideration of gas in performance assessment, and to propose an updated treatment of gas issues in long-term safety assessments for high-level radioactive waste and spent nuclear fuel. Three benchmark exercises for a hypothetical geological repository in clay host rock ranging in scale from a single disposal cell to the whole repository were defined. To provide added value to this benchmark, a feature not yet well represented in typical gas simulations was introduced: the explicit representation of the interfaces between the excavation disturbed zone and the engineered elements within the excavation, such as waste canisters, bentonite plugs, and access drifts. In order to assess gas transport behaviour at the whole repository scale, models were developed with ‘mathematical’ or ‘numerical’ upscaling techniques for small-scale features. The assessment across different modelling scales revealed the main long-term gas migration pathways and led to the conclusion that the explicit representation of interfaces is not important.