Sub- and super-critical carbon dioxide flow behavior in naturally fractured black coal: An experimental study

A proper understanding of super-critical carbon dioxide (CO₂) flow behavior in coal is essential, as CO₂ normally exists in its super-critical state in deep coal seams and studies are lacking. The main objective of this study is to distinguish the permeability behavior of coal for sub-critical and super-critical CO₂ flows. Therefore, a series of triaxial experiments was conducted on naturally fractured black coal specimens. Permeability tests were carried out for 15, 20 and 25 MPa confinements at 33.5 °C temperature. Three test scenarios were conducted to investigate, (1) variation of the permeability behavior of coal with CO₂ phase condition, (2) the swelling effect on sub- and super-critical CO₂ permeability patterns, and (3) the potential of nitrogen (N₂) to reverse CO₂-induced swelling. According to the test results, the permeability of super-critical CO₂ is significantly lower than sub-critical CO₂ due to the higher viscosity and swelling associated with super-critical CO₂. Moreover, at super-critical state there is a higher decline of CO₂ permeability with increasing injecting pressure due to the higher increments in the associated viscosity and swelling. Although CO₂ adsorption-induced swelling causes permeability of both CO₂ and N₂ to be reduced at low injection pressures the poro-elastic effect becomes more dominant and may cause CO₂ permeability to increase for higher injecting pressures, because CO₂ flow behavior may transfer from super-critical to sub-critical after the swelling due to the decline of downstream pressure development. Moreover, N₂ has the potential to reverse some swelling effects due to CO₂ adsorption, and this recovery rate is higher at lower injecting pressures and higher confining pressures.     

A new triaxial apparatus to study the mechanical and fluid flow aspects of carbon dioxide sequestration in geological formations

Climate scientists are practically unanimous in the belief that anthropogenic greenhouse gas contributions have added to the thickness and thus the effectiveness of the greenhouse gas layer, leading to a warming of the planet (IPCC, 2005 [1]). Engineers and scientists around the globe are researching and developing measures to reduce greenhouse gas emissions. These measures have included proposals to sequester carbon dioxide (CO₂) in deep geological formations (Perera et al., in press [18]). For CO₂ sequestration in deep geological reservoirs to become a feasible strategy to reduce greenhouse gas emissions, a sound understanding of the manner by which mechanical properties and permeability changes with the introduction of CO₂ to the geological reservoir will influence the stability of that reservoir is required. Thus there is a need to develop laboratory equipment capable of simulating the CO₂ injection and storage process for deep geological CO₂ sequestration under the expected in situ pressure (confinement and fluid) and temperature conditions. Triaxial experiment has been identified as the best method for this purpose (Perera et al., 2011b [19]). Therefore, we present a new high-pressure triaxial apparatus which can provide the high confining and fluid injection pressures and elevated temperatures expected for deep geological CO₂ sequestration. The new setup can be used to conduct mechanical and permeability testing on intact or fractured natural rock samples or synthetic rock samples subjected to high-pressure injection of up to three fluid phases (gas and/or liquid) at high pressures and temperatures corresponding to field conditions. The equipment is capable of delivering fluids to the sample at injection pressures of up to 50 MPa, confining pressures of up to 70 MPa and temperature up to 50 °C and will continuously record fluid injection and confining pressures, axial load and displacement, radial displacement and independent outflow rates for liquid and gas fluid phases (under drained conditions).Leakage tests have confirmed the effectiveness of the device at pressures up to its maximum capacities. Additionally the temperature-pressure relationship for the hydraulic oil used to apply confining pressure to the sample has been calibrated to account for the influence of changes in temperature on confining pressure. Several permeability tests (using N₂ and CO₂ as the injection fluid and 10 MPa confining pressure) and one strength test are reported for black coal samples from the Sydney Basin, New South Wales. According to the results of the permeability tests, coal mass permeability decreases with increasing effective stress for both gases. However, the permeability for N₂ gas is much higher than CO₂. Moreover, test results are consistent with matrix swelling due to the adsorption of CO₂ in coal. The strength testing results are in agreement with the results of testing carried on similar black coal samples from literature, certifying the ability for the new device to accurately measure strength and deformation properties of rock under deep ground conditions.     

The mechanical behaviour of coal with respect to CO2 sequestration in deep coal seams

Carbon dioxide displays a strong affinity for coal due to its propensity to adsorb to the coal surface. The process of CO₂ adsorption on coal causes lowering of surface energy and, it is hypothesised that an associated decrease in surface film confinement results in a decrease in material tensile resistance. Following the results of work carried out on the mechanical influence of CO₂ on brown coal under in situ conditions [Viete DR, Ranjith PG. The effect of CO₂ on the geomechanical and permeability behaviour of brown coal: implications for coal seam CO₂ sequestration. Int J Coal Geol 2006;66(3):204–16], a theoretical explanation is proposed for the perceived lack of a weakening effect with the adsorption of CO₂ to coal at significant confining pressures. We propose that at significant hydrostatic stresses, resistance to failure is otherwise provided (by external confinement) and the effects of adsorptive weakening are concealed. Our model predicts that adsorptive weakening, fracturing under in situ stresses, and associated permeability increases are not an issue for coal seam CO₂ sequestration for sufficiently deep target seams. Lowering of the elastic modulus of coal upon introduction of CO₂ may proceed by means other than surface energy lowering and could well occur irrespective of the depth of sequestration. The effect of elastic modulus lowering under in situ conditions would be beneficial for the long-term retention of sequestered gases.     

In situ diagnostics of Victorian brown coal combustion in O2/N2 and O2/CO2 mixtures in drop-tube furnace

Experimental investigation of the combustion of an air-dried Victorian brown coal in O₂/N₂ and O₂/CO₂ mixtures was conducted in a lab-scale drop-tube furnace (DTF). In situ diagnostics of coal burning transient phenomena were carried out with the use of high-speed camera and two-colour pyrometer for photographic observation and particle temperature measurement, respectively. The results indicate that the use of CO₂ in place of N₂ affected brown coal combustion behaviour through both its physical influence and chemical interaction with char. Distinct changes in coal pyrolysis behaviour, ignition extent, and the temperatures of volatile flame and burning char particles were observed. The large specific heat capacity of CO₂ relative to N₂ is the principal factor affecting brown coal combustion, which greatly quenched the ignition of individual coal particles. As a result, a high O₂ fraction of at least 30% in CO₂ is required to match air. Moreover, due to the accumulation of unburnt volatiles in the coal particle vicinity, coal ignition in O₂/CO₂ occurred as a form of volatile cloud rather than individual particles that occurred in air. The temperatures of volatile flame and char particles were reduced by CO₂ quenching throughout coal oxidation. Nevertheless, this negative factor was greatly offset by char-CO₂ gasification reaction which even occurred rapidly during coal pyrolysis. Up to 25% of the nascent char may undergo gasification to yield extra CO to improve the reactivity of local fuel/O₂ mixture. The subsequent homogeneous oxidation of CO released extra heat for the oxidation of both volatiles and char. As a result, the optical intensity of volatile flame in ∼27% O₂ in CO₂ was raised to a level twice that in air at the furnace temperature of 1273 K. Similar temperatures were achieved for burning char particles in 27% O₂/73% CO₂ and air. As this O₂/CO₂ ratio is lower than that for bituminous coal, 30-35%, a low consumption of O₂ is desirable for the oxy-firing of Victorian brown coal. Nevertheless, the distinct emission of volatile cloud and formation of strong reducing gas environment on char surface may affect radiative heat transfer and ash formation, which should be cautioned during the oxy-fuel combustion of Victorian brown coal.     

Evolution of coal permeability from stress-controlled to displacement-controlled swelling conditions

When a coal sample is constrained either by displacements or by a confining stress, additional force and resulting stress develop within the coal. A simple “free expansion + push back” approach is developed in this work to determine the magnitude of this stress and its effect on permeability evolution. In this approach, the coal is allowed to expand freely due to gas sorption, and then it is pushed back by the applied effective stress to the original constrained conditions. The total “push-back” strains are used to calculate the change in coal permeability. This free expansion plus push back approach is applied to examine the variety of permeability responses observed in the laboratory and the veracity of their representation by theoretical models linking this behavior to gas sorption-induced swelling/shrinkage. These cases include (1) coal swelling tests under the uniaxial strain condition; (2) coal swelling tests under the displacement controlled condition; (3) coal swelling tests under the stress controlled condition. These responses are verified against other coal permeability models available in the literature and against experimental data and field data where few analytical solutions are currently available. In particular, this approach has led to a new coal permeability model that can be used to explain stress-controlled experimental observations. Stress-controlled swelling tests are normally conducted in the laboratory to characterize the evolution of coal permeability under the influence of gas sorption. Typically reductions in permeability are observed from gas-sorption-induced swelling even where effective stresses remain constant. This behavior remains enigmatic as the permeability of the porous coal is determined by the effective stress only. Our model is capable of replicating this apparently anomalous behavior.     

Effects of matrix moisture on gas diffusion and flow in coal

Gas production from coal is a complex process whereby gas, initially adsorbed in the coal matrix, desorbs and diffuses through the matrix into the cleat and eventually flows through the cleat system into a production well or a drainage borehole. Hence, the gas production rate is mainly controlled by the gas diffusivity in the matrix and gas permeability in the cleat system. Moisture in the coal matrix has significant impact on gas adsorption capacity and would also play a key role in desorption and migration of gas. However, how moisture affects gas desorption and diffusion in the coal matrix is still poorly understood. In this work, experimental study is performed to investigate effects of moisture on gas sorption rate for an Australian coal. Coal seam gases, CH₄ and CO₂, are used in the study. The experimental results show that moisture content in the matrix has significant impact on the gas sorption rate and the impact of moisture content on the diffusion rate is stronger for CH₄ than CO₂. Moreover, the impact of moisture on gas diffusivity in pores with different size is different, suggested from the modelling results using the bidisperse approach. Furthermore, moisture in coal matrix would cause coal swelling/shrinkage and mechanical properties change that could impact on coal permeability under reservoir conditions. Experimental measurements of coal matrix swelling and Young’s modulus on the same coal sample show that matrix moisture content has significant impact on those properties and may have significant implications on coalbed methane recovery and CO₂ storage in coal.     

Performance of an entrained-flow gasification technology of pulverized coal in pilot-scale plant

Performance of an entrained-flow gasification technology of pulverized coal in pilot-scale plant is introduced. The gasifier was operated for a throughput of 30–45 t coal per day at pressures of 1–3 MPa. Dense-phase pneumatic conveying was employed for coal's feeding to the gasifier using nitrogen and carbon dioxide as carrier gas, respectively. Effects of the operating conditions including oxygen/carbon ratio and steam/carbon ratio on gasification results were investigated, and the concentration of (CO + H₂) in gaseous products reached up to about 97% (vol., dry basis) when CO₂ was employed as carrier gas. Moreover, performances of some important instruments in the conveying system of pulverized coal, such as the level indicator and the solid mass flow meter, were also investigated. The typical operating results in this plant such as (CO + H₂) concentration, oxygen consumption, coal consumption, carbon conversion and cold gas efficiency were almost as good as those of some well-known dry-fed entrained-flow coal gasification plants.     

Thermoplastic properties of coals at elevated pressures: 3. Effects of low-temperature preoxidation and subsequent heat-treatment on behaviour in H2 and He

The thermoplastic properties of a mildly preoxidized Lower Kittanning seam low volatile coal have been examined at elevated pressures of H₂ and He utilizing a high-pressure microdilatometer. It was observed that the maximum swelling parameter (V_s, vol%) of the preoxidized coal was significantly restored at elevated pressures of He. The thermoplastic properties of the preoxidized coal were even further restored at high pressures of H₂. The results indicate that carbonization of this coal at elevated H₂ pressures reduces the effect of preoxidation by removing some of the oxygen introduced during preoxidation and replacing it with reactive donatable hydrogen. It was shown that subsequent heat-treatment of the preoxidized coal at a relatively mild condition (in vacuum at 403 K) results in dramatic reductions in the thermoplastic behaviour of coal when subsequently carbonized at elevated pressures of H₂ or He.     

Thermoplastic properties of coal at elevated pressures: 1. Evaluation of a high-pressure microdilatometer

Thermoplastic behaviour of a Pittsburgh seam hvA coal (PSOC1099) was characterized by the use of a high-pressure microdilatometer. Phenomena such as softening, swelling, final resolidification, and the temperatures at which they occur were measured as functions of heating rate (25 ° and 65 °C min^?1), particle size (= 75 μm and 250 × 425 μm), gaseous atmosphere (N₂, H₂, $\text{CO}\text{H}{}_2$) and applied gas pressure (atmospheric to 2.8 M Pa). The results obtained illustrate several important aspects of thermoplastic properties of this coal under the conditions utilized. It is observed that pressure alone can play a major role in determining its overall thermoplastic behaviour. Compared to that at atmospheric pressure, swelling is significantly reduced at 2.8 MPa of pressure for any given heating rate or particle size. In these experiments, the chemical composition of the gaseous atmospheres ($\text{CO}\text{H}{}_2$, H₂ and N₂) does not appear to alter significantly the plastic phenomena at any given pressure. Increasing the heating rate or decreasing the particle size results in increased swelling at all applied pressures and atmospheres.     

Swelling and plastic properties of coal devolatilized at elevated pressures of H2 and He: Influence of potassium and silicon additives

The influences of SiO₂ and potassium additives on the swelling and plastic properties of a low volatile bituminous coal have been characterized at elevated pressures of H₂ and He using a high-pressure microdilatometer. The results suggest that non-porous SiO₂ serves as a ‘diluent’ as it has only a minor influence on the nature of the thermoplastic properties of coal. In marked contrast, K₂CO₃ or KOH significantly reduced the swelling of coal at low pressures (< 1.0 MPa). The effectiveness of K₂CO₃ and KOH as de-caking additives increased with the increase in loading of the additives. In addition, the effectiveness of potassium additives depended on the type of anions present. While K₂CO₃ or KOH served as strong decaking additives, KCl showed little effect. At elevated pressures of H₂ or He (>2.0 MPa), the swelling behaviour of K₂CO₃ or KOH loaded coal was reduced only slightly compared with the behaviour without additives. The influence of potassium additives is a function of coal particle size, heating rate and mode of addition (dry-mixed or solution impregnation). The presence of K₂CO₃ or KOH resulted in increased coke yield (i.e., reduced weight loss of coal during pyrolysis). This increase in coke yield was accompanied by a slight reduction in total light gases monitored (primarily C₂C₄). It is suggested that char-forming crosslinking reactions that can be catalysed by K₂CO₃/KOH facilitate increased thermosetting solid yield. At elevated pressures of H₂, the maximum swelling parameter in the presence of these potassium compounds was further increased compared with that noted in He. This behaviour is explained by suggesting that a hydrogen atmosphere reduces the extent of cross-linking reactions.     

An investigation of the causes of the difference in coal particle ignition temperature between combustion in air and in O2/CO2

The ignition temperatures of a Loy Yang brown coal and a Datong bituminous coal were investigated in a wire-mesh reactor where the secondary reactions of the evolved volatiles were minimised. An increase in the average particle ignition temperature of 21 °C was observed for the brown coal when air (21% O₂ + 79% N₂) was replaced with a mixture of 21% O₂ + 79% CO₂. Combustion was also carried out in the mixtures of 21% O₂ + 79% argon and 21%O₂ + 79% helium in order to determine the effects of heat transfer on the observed particle ignition temperature. It is concluded that the thermal conductivity of gas atmosphere surrounding the particles greatly influences the observed particle ignition temperature while the effects of the heat capacity of the gas atmosphere was very minor under our experimental conditions. The structure of char and the reactions involving the char (char-O₂ and char-CO₂) can greatly affect the observed particle ignition temperature. In particular, the char-CO₂ reactions were largely responsible for the observed difference in particle ignition temperature in air and in 21% O₂ + 79% CO₂. Alkali and alkaline earth metallic (AAEM) species in the brown coal also significantly affect the observed particle ignition temperature.     

Effects of solvent and iron-compounds on the liquefaction of brown coal

Hydrogen-donor solvents such as hydrophenanthrene are the most effective aromatic solvents for the liquefaction of brown coal. The hydrogen-donating ability of the solvent is more important for brown coals than for bituminous coals, because the thermal decomposition and subsequent recombination of the structure of the brown coals occurs rapidly. Three-ring aromatic hydrocarbons are more effective solvents than two-ring aromatics, and polar compounds are less effective solvents with brown coals than with bituminous coals. The thermal treatment of brown coal, accompanied by carbon dioxide evolution at temperatures > 300°C, in the presence of hydrogen-donating solvent did not affect the subsequent liquefaction reaction. However, thermal treatment in the absence of solvent strongly suppressed the liquefaction reaction, suggesting that the carbonization reaction occurred after the decarboxylation reaction in the absence of hydrogen donor. To study the effect of various iron compounds, brown coal and its THF-soluble fraction were hydrogenated at 450°C in the presence of ferrocene or iron oxide. The conversion of coal and the yield of degradation products are increased by the addition of the iron compounds, particularly ferrocene, and the yield of carbonaceous materials is decreased.     

Use of tracers to characterize the effects of a CO2-saturated brine on the petrophysical properties of a low permeability carbonate caprock

To assess the long-term safety of a geological carbon dioxide storage site, the confining properties of the rocks sealing an underground reservoir (caprocks) and their evolution inthe presence of CO₂ must be characterized. The present study consists in the measurement of the transport parameters of dissolved CO₂ through low permeability carbonate-rich caprocks. The properties of interest are the effective permeability and the diffusion coefficients of carbon dioxide dissolution products in water. The impact of carbon dioxide has been evaluated when altering rock samples by diffusion of a CO₂-saturated brine under reservoir thermodynamic conditions, and by comparison of the pre- and post-alteration measured values. Permeability was measured by a gas-tracing method and, to study diffusion, radioactive isotopes of carbon (¹⁴C) and hydrogen (³H) were used. Despite a porosity increase observed for all the studied samples, the low values of transport parameters, measured initially, were also measured after alteration, showing a non-catastrophic alteration of the material.     

Surface area and swellability of coal

Sorption data have been obtained for a range of vapours on a Kentucky No.12 coal. The data at high relative vapour pressures (P/P₀ > 0.3) are interpreted in terms of coal swelling. The sorption data at low relative vapour pressures (P/P₀ < 0.3) are used to obtain information on surface area. The results indicate that surface areas obtained in this way are influenced by vapour-induced swelling effects.     

Changes in char reactivity and structure during the gasification of a Victorian brown coal: Comparison between gasification in O2 and CO2

Char reactivity is an important factor influencing the efficiency of a gasification process. As a low-rank fuel, Victorian brown coal with high gasification reactivity is especially suitable for use with gasification-based technologies. In this study, a Victorian brown coal was gasified at 800 °C in a fluidised-bed/fixed-bed reactor. Two different gasifying agents were used, which were 4000 ppm O₂ balanced with argon and pure CO₂. The chars produced at different gasification conversion levels were further analysed with a thermogravimetric analyser (TGA) at 400 °C in air for their reactivities. The structural features of these chars were also characterised with FT-Raman/IR spectroscopy. The contents of alkali and alkaline earth metallic species in these chars were quantified. The reactivities of the chars prepared from the gasification in pure CO₂ at 800 °C were of a much higher magnitude than those obtained for the chars prepared from the gasification in 4000 ppm O₂ also at 800 °C. Even though both atmospheres (i.e. 4000 ppm O₂ and pure CO₂) are oxidising conditions, the results indicate that the reaction mechanisms for the gasification of brown coal char at 800 °C in these two gasifying atmospheres are different. FT-Raman/IR results showed that the char structure has been changed drastically during the gasification process.     

Kinetic studies on Victorian brown coal hydroliquefaction

Reaction kinetics of the liquefaction of Victorian brown coal in a process development unit (PDU) having three reactors in series have been studied at temperatures of 430–470°C, and pressures of 15–25 MPa. It is shown that the rate of hydrogen consumption can be expressed as a function of the concentrations of coal and catalyst, hydrogen partial pressure, reaction temperature and residence time, and is controlled by the rates of hydrogenation of polynuclear aromatic components, and the rates of formation and stabilization of radicals. The relative contribution of these reactions, at any temperature, determine the influence of the hydrogen partial pressure on the rate of the hydrogen consumption. The kinetics of the decomposition reactions of brown coal to preasphaltene, asphaltene and to oil also have been studied. The apparent activation energies determined are 25 kJ mol^?1 for the brown coal to preasphaltene, 50 kJ mol^?1 for preasphaltene to asphaltene, 76 kJ mol^?1 for asphaltene to oil, and 184 kJ mole^?1 for oil to gases.     

Selective transport of CO2, CH4, and N2 in coals: insights from modeling of experimental gas adsorption data

Selective adsorption and transport of gases in coal are important for natural gas recovery and carbon sequestration in depleted coal seams for environmental remediation. Gases are stored in coal mainly in the adsorbed state. In this study, the interaction energies of adsorbates (CO₂, CH₄, and N₂) and micropores with various widths are investigated using a slit-shape pore model. The experimental adsorption rate data of the three gases conducted on the same coal sample are numerically simulated using a bidisperse model and apparent diffusivities of each adsorbate in the macropore and micropore are determined. The results indicate that the relative adsorbate molecule size and pore structure play an important role in selective gas adsorption and diffusion in micropores. Generally, the microporous coals diffusion is activated and the apparent micropore diffusivities of gases in coal decrease strongly with increase in gas kinetic diameters. Apparent micropore diffusivity of CO₂ is generally one or two order of magnitude higher than those of CH₄ and N₂ because their kinetic diameters have the relation: CO₂ (0.33 nm)<N₂ (0.36 nm)<CH₄ (0.38 nm). In contrast to theoretical values, apparent macropore diffusivity of CO₂ is also larger than those of CH₄ and N₂, suggesting that coal has an interconnected pore network but highly constricted by ultra micropores with width <∼0.6 nm. It is also found that the apparent diffusivity strongly decreases with an increase in gas pressure, which may be attributed to coal matrix swelling caused by gas adsorption. Therefore, rigorous modeling of gas recovery and production requires consideration of specific interaction of gas and coal matrix.     

深部煤层封存CO2过程中的煤基质溶胀效应

强化煤层气甲烷(CH₄)采收率的深部煤层封存二氧化碳(CO₂)技术能够将主要人为温室气体(CO₂)进行有效的地质存储。考虑到CO₂流体和煤体的自身特性, 封存过程中的CO₂流体将会诱导煤基质发生溶胀效应。溶胀效应将会对煤层封存CO₂技术构成潜在的影响。为此, 本文结合国内外相关研究工作, 归纳了流体诱导煤基质溶胀的规律, 指出了煤基质溶胀对煤层封存CO₂过程的影响, 介绍了流体诱导煤基质溶胀的分析手段, 提出了封存过程中煤基质溶胀的研究趋势。分析表明:①煤基质溶胀程度与流体种类、压力、温度和煤的变质程度有关;②CO₂诱导煤基质溶胀效应将会降低煤层的渗透性能, 进而影响CO₂等流体在煤层内部的有效运移;③溶胀效应会影响煤层的CO₂封存性能, 建立涉及溶胀效应的煤吸附理论模型是溶胀效应研究的重要课题;④煤基质溶胀机理及其可逆性能研究目前存在争议, 研究人员需要从煤体理化结构的研究出发以明确上述问题。     

NOx generation from the combustion of Australian brown and subbituminous coals

The formation of NO_x during the combustion of pulverized brown and subbituminous coals from Victoria and Queensland respectively was investigated in an entrainment reactor. As no NO₂ was detected, all the NO_x was present in the form of NO. The brown coals exhibited a significantly greater potential for NO emission under fuel-lean conditions than did the subbituminous coal, even though the latter coal had a higher nitrogen content. However, under fuel-rich conditions the conversion of coal nitrogen to NO for the subbituminous coal was higher than for the brown coals. The differences in conversion efficiency may have been related in part to the nature and reactivity of the volatile nitrogen species. Reactivity differences between the chars produced from the brown and subbituminous coals may also have accounted for different extents of removal of NO. There was a significant reduction in the amount of NO emitted when brown coal was added to a combustion gas stream containing an appreciable quantity of NO before coal injection.     

Microstructure relaxation process of polyhexafluoropropylene after swelling in supercritical carbon dioxide

This paper studies the process of relaxation of a polymer after swelling in supercritical carbon dioxide. Polyhexafluoropropylene (PHFP) was chosen as the object for investigation. The relaxation process was monitored by a change of the permeability coefficients for a number of gases. Thin polymeric films of PHFP were modified by different treatments: drying to a constant weight, annealing at a temperature slightly higher than the glass‐transition temperature, and swelling in supercritical carbon dioxide. The permeability coefficients of six gases, He, H₂, O₂ N_2, CO₂, and CH₄, were measured after each stage of the treatment. It was shown that the permeability coefficients in the films were increased by 2.4 times for He, 3.6 for H₂, 5.9 for O₂, 8.1 for N₂, 6.7 for CO₂, and 10.9 for methane. The permeability coefficients of the same gases were measured 50 days later after swelling in supercritical carbon dioxide. A decrease in the permeability coefficient demonstrated that the relaxation process had taken place. Nevertheless, the values exceeded the initial ones for annealed samples by 2.0 times for He, 2.4 for H₂, 1.8 for O₂, 1.7 for N₂, 1.7 for CO₂, and 1.3 for methane. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43105.