Adsorption Swelling Research Articles

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

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

Characterization of Methane Extraction during ScCO2-ECBM: Consideration of Competitive Adsorption of ScCO2 and CH4.

The supercritical CO2 enhanced coalbed methane (ScCO2-ECBM) technology is still in the development stage, and many simulation experiments and theoretical studies related to ScCO2-ECBM are being improved. Previous research works have conducted many studies on the competitive adsorption of CO2 and CH4 in coal, but there is less research on the competitive adsorption of ScCO2 and CH4 and its impact on methane extraction characteristics. In this study, a permeability model considering the competitive effects of effective stress and adsorption swelling on permeability was established. Based on the assumed conditions and permeability evolution model, different injected pressure and initial methane pressure conditions were set to obtain quantitative results of the competitive adsorption of ScCO2 and CH4, permeability changes, and CH4 production. By obtaining the competitive adsorption relationship between ScCO2 and CH4, we analyzed the evolution law of permeability and its impact on CH4 production. It was found that ScCO2 has a stronger competitive adsorption capacity, and the competitive adsorption capacity of ScCO2 and CH4 is more sensitive to injected pressure. Under two different conditions, it was found that the higher the injected pressure or injected differential pressure, the higher the initial permeability. However, due to the greater sensitivity of the competitive adsorption capacity of ScCO2 and CH4 to injected pressure, the greater the injected pressure in the later stage, the greater the decrease in permeability, resulting in a situation where the permeability at an injected pressure of 10 MPa is lower than that at an injected pressure of 8 MPa. A simple comparison was made between gaseous CO2 and ScCO2, and it was found that although injecting ScCO2 has a stronger adsorption swelling capacity that affects permeability changes, its stronger adsorption capacity can effectively displace methane and higher injected pressure, injected temperature, and advantages such as fracturing and extraction that are not yet reflected in the model. This study provides some guidance for numerical simulation of the ScCO2-ECBM process and the enhancement of coalbed methane extraction.

Changes in the three-dimensional molecular structure of coal during methane adsorption induced swelling

Methane (CH4) adsorption-induced swelling is one of the critical factors controlling the permeability of coalbed methane (CBM). CH4 adsorption alters the molecular structure of coal so as to induce coal swelling, and many uncertainties still exist in the process. In this study, the change in the molecular structures of different chemical structures by CH4 adsorption was investigated using the Grand Canonical Monte Carlo method to simulate the alteration of bond lengths and bond angles during swelling. The results demonstrate that the alteration of chemical structure is more extensive than a chemical bond, which is the critical factor causing the swelling behavior. Owing to the complex molecular structure of coal, among the different types of chemical structures, the C-O-C (-O-) chemical structure showed the most significant change in bond angle, with the largest degree of change is 12.89%. Compared with other chemical structures, the C-C-C (aromatic -C-) chemical structures are more stable and the largest degree of change is 0.65%. For the different types of chemical bonds, the C-C chemical bonds showed the most significant change in bond lengths, with the largest degree of change is 2.94%. And the O-H chemical bond showed the smallest change, with the largest degree of change is 0.79%. Considering the structure evolution of coal, the C-O-C (-O-) chemical structure decreases with increasing maturity and changes to the greatest after the adsorption of methane. The aromatic structure increases and the degree of deformation decreases, which is consistent with the previous experimental values for swelling. These results reveal the details of different types of chemical group deformation, providing a molecular-level insight into adsorption swelling and permeability changes.

Revaluating coal permeability-gas pressure relation under various gas pressure differential conditions

Identifying changes in coal permeability with gas pressure and accurately codifying mean effective stresses in laboratory samples are crucial in predicting gas-flow behavior in coal reservoirs. Traditionally, coal permeability to gas is assessed using the steady-state method, where the equivalent gas pressure in the coal is indexed to the average of upstream and downstream pressures of the coal, while ignoring the nonlinear gas pressure gradient along the gas flow path. For the flow of a compressible gas, the traditional method consistently underestimates the length/volume-averaged pressure and overestimates mean effective stress. The higher the pressure differential within the sample, the greater the error between the true mean pressure for a compressible fluid and that assumed as the average between upstream and downstream pressures under typical reservoir conditions. A correction coefficient for the compressible fluid pressure asymptotes to approximately 1.3%, representing that the error in mean pressure and effective stress can be on the order of approximately 30%, particularly for highly pressure-sensitive permeabilities and compressibilities, further amplifying errors in evaluated reservoir properties. We utilized this volume-averaged pressure and effective stress to correct permeability and compressibility data reported in the literature. Both the corrected initial permeability and the corrected pore compressibility were found to be smaller than the uncorrected values, due to the underestimation of the true mean fluid pressure, resulting in an overestimation of reservoir permeability if not corrected. The correction coefficient for the initial permeability ranges from 0.6 to 0.1 (reservoir values are only approximately 40% to 90% of laboratory values), while the correction coefficient for pore compressibility remains at approximately 0.75 (reservoir values are only approximately 25% of laboratory value). Errors between the uncorrected and corrected parameters are quantified under various factors, such as confining pressure, gas sorption, and temperature. By analyzing the evolutions of the initial permeability and pore compressibility, the coupling mechanisms of mechanical compression, adsorption swelling, and thermal expansion on the pore structure of the coal can be interpreted. These findings can provide insights that are useful for assessing the sensitivity of coal permeability to gas pressure as truly representative of reservoir conditions.

Alterations in coal mechanical properties and permeability influenced by liquid CO2 phase change fracturing

Liquid CO2 phase change fracturing (LCPCF) is a promising approach for stimulating coal reservoirs and enhancing gas drainage efficiency of low-permeability coal seams. In this work, low pressure CO2/N2 adsorption, CH4 permeability measurements and UCS tests are combined to quantitatively analyze the changes in coal mechanical properties and permeability before and after LCPCF treatment. The results show that the axial peak strain is increased and ductile failure takes place in treated specimens after 1.26 L of liquid CO2 was used to conduct physical blasting. The UCS and elastic modulus after LCPCF are reduced by 10.82–25.78% and 12.92–28.57%, respectively. Muti-scale pore structures within coal are altered to varying degrees. After LCPCF treatment, mesopore volume and macropore volume are increased by 11.33–41.57% and 33.82–67.5%, respectively, while micropores are slightly affected. Due to the adsorption swelling and Klinkenberg effect, CH4 permeability of all the specimens is reduced with the increase of injection pressure. Comparative analysis of gas permeability has been made between the raw coal and treated specimens. Coal permeability is enhanced by 14.48-29.43% and 10.54-24.82% after LCPCF treatment at the injection pressure of 0.5 MPa and 1.4 MPa, respectively. During the LCPCF process, shock waves and stress waves trigger tensile damage to coal mass, and a large number of new macropores/fractures are generated within coal. This provides more accessible paths for gas flow and coal permeability are greatly promoted. Crack propagation is accelerated by the released high-pressure CO2, and both coal porosity and pore connectivity are further enhanced to facilitate gas seepage in coal seams.

Anisotropic strain of anthracite induced by different phase CO2 injection and its effect on permeability

To study the swelling/shrinkage strain effects of different phases of CO2 when injected into anthracite and the influence that has on the dynamic evolution of coal permeability, an experimental study of coal anisotropic adsorption of CO2 was carried out by using a triaxial compression test device, and the relationship between the swelling/shrinkage strain and permeability of the coal was discussed. The results show that the adsorption strain and time of gas exposure throughout different phase states of the CO2 injected show three stages of evolutionary behavior. The adsorption swelling of the coal sample increases with rising gas pressure. The adsorption strain of coals under the action of supercritical CO2 increased by 16.28% and 17.36% against the action of subcritical CO2, respectively. The coal portrayed anisotropic characteristics during the process of adsorption-caused swelling deformation and desorption-caused contraction, and the deformation in the vertical layer direction was greater than that in the parallel layer direction, and the anisotropy gradually weakened with increasing pressure. The permeability of the coal and the temperature is a quadratic function relationship under different effective stress conditions, and there is a zone where the changes occur. The study provides a theoretical basis CO2- Enhanced coalbed methane recovery.

Impact of Adsorption Swelling and Slippage Effects on Changes in Coal’s Permeability at Different Temperatures

As a critical parameter, coal’s permeability influences the coalbed’s methane (CBM) production efficiency, which, in turn, is affected by a slippage effect, adsorption swelling, and effective stress in the CBM exploitation process. In this paper, an isothermal adsorption experiment under various temperatures, including seepage tests comprising CH4 and He in coal at different temperatures under constant effective stress, was carried out separately. We established a modified Shi&Duruca (S&D) model that considered constant effective stress and a modified adsorption model, in which the effect of excess adsorption capacity and temperature was considered. The results revealed that swelling deformation occurred when coal adsorbed CH4, which would result in the permeability of CH4 being lower than that of He under similar conditions. With an increase in pore pressure, coal’s permeability decreased gradually under the combined effects of slippage and adsorption swelling. Temperature is an important factor that affects the movement and storage of gas. Any change in temperature and pore pressure would result in coal’s permeability being comprehensively affected by thermal expansion, thermal cracking, and adsorption swelling, including the effects of slippage. Thermal expansion and thermal cracking increase coal’s permeability under low pressure and decrease coal’s permeability under high pressure. The modified adsorption model and permeability model could better characterize the adsorption and permeability properties of coal. The calculation results illustrated that the slippage effect and adsorption swelling are two important factors affecting permeability, with its permeability increment sharing a similar change trend with changes in permeability. Under the condition of low pressure, the adsorption swelling effect was dominant, and the contributary rate of the slippage effect on permeability would increase, obviously, with an increase in temperature.

Expansion deformation and permeability characteristics of bituminous coal with different moisture content after CO2 adsorption

Injecting CO2 into deep unexploitable coal seams is a means with dual benefits of enhanced coalbed methane exploitation (CO2-ECBM) and CO2 geological storage (CGS). The permeability change of the coal seam after CO2 injection is an important factor for the CO2-ECBM and CGS. However, the impact of the interaction of CO2–H2O-coal in the coal seam on the adsorption swelling and permeability characteristics is poorly understood. In this study, we quantitatively analyzed the adsorption characteristics, permeability characteristics, and microstructure changes of coal samples under CO2–H2O-coal interaction by Geotechnical Consulting & Testing Systems (GCTS), nuclear magnetic resonance (NMR), and X-ray diffraction (XRD). The results show that the increase of moisture content inhibits the adsorption of CO2 due to the clusters formed by water molecules occupy the position of adsorption pores, reducing the expansion rate of coal samples. The reaction rate content of carbonate minerals in coal samples increases with the increase of moisture content after subcritical CO2 treatment, leading to an increase in the relative content of clay minerals, which can enlarge the pore volume. However, the subcritical CO2 treatment cannot completely eliminate the negative impact of loading and unloading processes on the mesopore and macropore, and therefore show a decrease of the pore volume and permeability. The dissolution of carbonate mineral in coal samples is more severe after supercritical CO2 (ScCO2), which can effectively enlarge seepage pores and thus improves the permeability. The clay mineral content increases slightly at 5.6% moisture content, indicating that it also reacts significantly after ScCO2 treatment. For instance, the permeability growth rate of 2.8% moisture content coal sample reaches 421.74% at 8 MPa effective stress, which is the best moisture content for permeability enhancement. In addition, the coal samples with 5.6% moisture content show the transformation from mesopores and macropores to micropores and small pores after subcritical CO2 and ScCO2 treatment, which provides more adsorption sites for CGS.

The High‐Strength, Flexible Organic Solvent Driven Reversible “Deformation‐Recovery‐Reverse Deformation” Responsive g‐PLA/PPC/PVA Film for Contactless Actuation

AbstractThe non‐contact deformation response driven by organic solvent has potential applications in the fields of intelligent devices, soft robots, sensing detection, and biomedical micro‐devices. The modified polyactic acid (g‐PLA)/polypropyl carbonate (PPC)/polyvinyl alcohol (PVA) sandwich with deformation response film is prepared by the asymmetric swelling response mechanism. Specially, the flexible sandwich film achieves the non‐contact driving response of “deformation‐recovery‐reverse deformation”, showing excellent deformation performance. The curvature of sandwich film reaches 13.79 cm−1 within 15 s, and the film recovers to its initial state within 3 s, then the curvature of the reverse coil reaches 18.18 cm−1 within 15 s and finally achieves a stable state. This special phenomenon is caused by the different adsorption swelling and desorption rates of volatile organic solvents in g‐PLA, PPC, and PVA layers. Additionally, the flexible film has high strength, reversible circulation, and stable actuation performance. The application scenarios of the bionic flower actuator, artificial octopus, intelligent protective cover, and transportable objects are successfully implemented. This work will significantly provide substantial guidance for the development of intelligent driving technology in the future.

Permeability evolution and gas flow in wet coal under non-equilibrium state: Considering both water swelling and process-based gas swelling

Accurate knowledge of gas flow within the reservoir and related controlling factors will be important for enhancing the production of coal bed methane. At present, most studies focused on the permeability evolution of dry coal under gas adsorption equilibrium, gas flow and gas diffusion within wet coal under the generally non-equilibrium state are often ignored in the process of gas recovery. In this study, an improved apparent permeability model is proposed which accommodates the water and gas adsorption, stress dependence, water film thickness and gas flow regimes. In the process of modeling, the water adsorption is only affected by water content while the gas adsorption is time and water content dependent; based on poroelastic mechanics, the effective fracture aperture and effective pore radius are derived; and then the variation in water film thickness for different pore types under the effect of water content, stress and adsorption swelling are modeled; the flow regimes are considered based on Beskok’s model. Further, after validation with experimental data, the proposed model was applied to numerical simulations to investigate the evolution of permeability-related factors under the effect of different water contents. The gas flow in wet coal under the non-equilibrium state is explicitly revealed.

Analysis of the Change Feature and Action Mechanism of Coal Pore Structure under the Action of Supercritical CO2

In this study, supercritical CO2- (ScCO2) induced permeability enhancement tests were performed on two different-rank coal samples (i.e., coking coal and anthracite) by a self-developed ScCO2-induced permeability enhancement test device for the purpose of exploring the change feature and action mechanism of ScCO2 on coal pore structure during the improvement of coalbed methane extraction. The following results were obtained: after the injection of ScCO2, the original pore structure of coal changes. Specifically, the connectivity between macropores and fractures increases, that between mesopores and small pores decreases and that between small pores and micropores increases. Besides, the total pore volumes of the two types of coal samples both grow primarily due to the increase in macropores and mesopores, and the sample of a higher rank corresponds to a higher growth rate. The growth in the number of effective pores conduces to dredging and enlarging pores in some ways, promoting the surface roughness of macropores and mesopores and reducing that of small pores and micropores. With respect to the action mechanism, ScCO2-induced changes in the coal pore structure are jointly induced by pore adsorption swelling, dissolution-migration, and dissolution-precipitation. Among them, the internal cause of pore adsorption swelling is the surface-free energy of fine pores. The sample of a higher rank contains more fine pores and greater surface-free energy; resultantly, it experiences stronger adsorption swelling and thus greater changes in its pore structure. The carbonic acid generated by ScCO2 and the strong acid minerals existing in the coal matrix and pore space dissolute and migrate, which stands to dredge and expand pore space and meanwhile promote pore connectivity and volume. Moreover, dissolution-precipitation leads to the blockage of pore space and pore channels, hence reducing pore connectivity and pore volume.

Deformation mechanism and displacement ability during CO2 displacing CH4 in coal seam under different temperatures

Liquid CO2 has the synergistic effect of low-temperature damage and displacing CH4 after injecting into the coal seam, which can effectively improve coalbed permeability and promptly promote the adsorbed CH4 to desorption state for preventing the coal and gas outburst disasters. The injected CO2 gets adsorbed at the surface of the coal pores, which causes the coal swelling. In this work, we developed a triaxial experimental platform to explore the features of CO2 displacing CH4 under different temperatures. The variation of coal swelling and segmentation features of the displacing concentration was elucidated. The seepage ability and mechanism of gas mitigation in the coal seam during LCO2-ECBM were revealed. The results showed that the coal samples displayed swelling deformation in the CH4 adsorption and CH4 displacement stages, and the strain curves can be divided into rapid and slow deformation phases. The strain in the CO2 displacing CH4 stage is notably larger than that in the CH4 adsorption stage. Three dominant results were obtained during the CH4 displacement: Free CH4 driving by CO2 injection, CH4 self-desorption, and CO2–CH4 competitive adsorption. The displacing flow rate increased swiftly in the initial stage, and then decreased to a stable tendency. The cumulative displacement volume of CH4 in different stages exhibited distinct functional relationships. The integrated contribution of the coal matrix shrinkage and thermal stress damage to gas seepage improvement was more prominent than that of the adsorption swelling to seepage inhibition in the coal seam.

Permeability Response Characteristics of Primary Undeformed Coal and Tectonically Deformed Coal under Loading-Unloading Conditions in Huainan Coalfield, China.

Reservoir pressure relief is a practical method to enhance permeability for coalbed methane (CBM) extraction in tectonically deformed coal (TDC) reservoirs. To explore the coal permeability response to stress changes, the primary undeformed coal (PUC) and TDC from the same coal seam were sampled for the pore–fissure structure analysis, mechanical property test, and permeability experiments under different stress loading–unloading methods in this study. The experimental results demonstrated that the coal permeability is more sensitive to the changes in confining pressure (perpendicular to airflow) than axial stress (parallel to airflow). Coal permeability decreases negatively exponentially as the confining pressure increases, and its change process with increased axial pressure can be divided into five stages in this study. The pore structures and mechanical properties of coal samples affected their permeability response to stress changes. Under the stress loading condition, the coal matrix and fractures of PUC samples were compressed simultaneously, and the permeability was regulated by the pore–fissure structures in the coal matrix. Due to the deformation and displacement of coal particles, the permeability of the TDC sample is predominantly dependent on changes in intergranular pores. At the initial stress unloading stage, the fissure recovery and expansion lead to a rapid increase in permeability, but the permeability cannot rereach the original value when the stress is fully released. Furthermore, the influencing factors of coal permeability in response to stress loading–unloading also include confining pressure conditions and coal matrix adsorption swelling. Research on the permeability response characteristics of the stress loading–unloading process can provide some clarifications for the reservoir depressurization and permeability enhancement of CBM extraction in the TDC reservoir.

Effects of CO2 intrusion on pore structure characteristics of mineral-bearing coal: Implication for CO2 injection pressure

CO2 intrusion has a crucial effect on the pore structure of mineral-bearing coal. In this study, we selected long flame coal, lean coal, and anthracite after CO2 adsorption at different pressures and tested the coal samples using X-ray diffraction, mercury intrusion porosimetry, and N2 (77 K) adsorption methods. The tests were conducted to determine the variations in mineral content, pore structure, and fractal characteristics. The results showed that supercritical CO2 had a greater ability to dissolve minerals in coal than that of subcritical CO2. Although the total pore volume and BET specific surface area gradually increased with the increase in CO2 intrusion pressure in coal, the transformation of different pores and partial new pores caused by the dissolution of minerals and the adsorption swelling of coal matrix caused the micro-macropores in the three coal samples to exhibit different trends. The pore surface roughness and pore structure complexity of seepage pore in the long-flame coal after CO2 adsorption increased while those of the lean coal and anthracite decreased. Meanwhile, CO2 intrusion caused the surface of the adsorption pore in coal to become smooth, and the pore structure was more regular, except for the lean coal. A conceptual model of the mineral-bearing coal was developed to describe the relationship between the mineral composition and pore structure induced by CO2 intrusion. These findings help to understand the transformation effect of CO2 on coal seams. Thus, a higher CO2 injection pressure should be used to obtain a larger injection volume and shorter injection time during CO2 storage implementation.

Quantification of the asynchronous gas diffusivity in macro-/micropores using a Nelder-Mead simplex algorithm and its application on predicting desorption-based indexes

Accurate and intelligent calculation of diffusion coefficient not only helps reduce greenhouse gas emissions, but also conduces to CO2 geological storage and CH4 displacement in coal seams. In this study, a kind of object-oriented software for conveniently and quickly obtaining bidisperse (i.e., macro-pore and micro-pore) diffusion coefficients and grasping main controlling factors of macroscopic desorption curves was designed based on the Nelder-Mead simplex algorithm. Furthermore, the gas desorption characteristics under different pressures were analyzed by performing conventional desorption experiments. The results show that the apparent diffusion coefficient obtained by the bulk technique displays three different trends as the pressure rises, namely a rising, falling and constant diffusion coefficient respectively. The three variation trends mainly result from the difference in deformation caused by adsorption swelling and mechanical compression. As desorption continues, the gas diffusion coefficient values in the macro-pores and micro-pores fluctuate for a short time, but increase in the long term. This is because the early stage corresponds to intense mechanical expansion and a flow boundary effect, while the later stage corresponds to an increasingly intense effect of desorption-induced shrinkage. Moreover, the time-dependent contribution law of macro-pore diffusion and micro-pore diffusion affects the reliability of outburst risk index, making △h2 more stable than K1. The conclusions provide theoretical guidance for the selection of rational gas drainage methods for diffusion-controlled coal seams.

Why coal permeability changes under unconstrained displacement boundary conditions: Considering damage effects

Permeability is an important indicator of coalbed methane production. Previous classical theoretical permeability models under unconstrained boundary conditions have difficulty matching experimental results. Consideration of the actual cleat geometry can explain the non-equilibrium evolution of permeability to some extent, but the matrix swelling strain ignores the impact of damage effects. In this study, cleat damage is considered and the damage ratio is calculated. Damage as a function of strain can be solved iteratively by post-processing calculations using COMSOL Livelink with MATLAB software for correcting the permeability equation. Unlike conventional permeability models, this modified permeability model incorporates damage effects. Damage effects and matrix adsorption swelling are interactive and coupled processes. The main parameters controlling the matrix swelling strain are Young's modulus, Heterogeneous coefficient, gas injection pressure, Langmuir strain constant, and Poisson's ratio. Based on these parameters, eight working conditions are designed to compare the models without and with damage effects, and the numerical solutions of the two models are also compared with experimental data. The results show that the cleat damage model is valid better in coal permeability prediction than conventional models without damage effect. This is because the cleat damage inhibits fracture aperture rebound, which has a crucial influence on the evolution of permeability. Through this study, it is shown that the experimentally tested permeability results can indeed be explained by the impact of damage deformation on the cleat aperture.

Evolution features of in-situ permeability of low-maturity shale with the increasing temperature, Cretaceous Nenjiang Formation, northern Songliao Basin, NE China

Temperature-triaxial pressure permeability testing at the axial pressure of 8 MPa and confining pressure of 10 MPa, closed shale system pyrolysis experiment by electrical heating and scanning electron microscopy analysis are used to study the evolution mechanism of in-situ permeability in the direction parallel to bedding of low-maturity shale from Member 2 (K2n2) of Cretaceous Nenjiang Formation in northern Songliao Basin with mainly Type I kerogen under the effect of temperature. With the increasing temperature, the in-situ permeability presents a peak-valley-peak tendency. The lowest value of in-situ permeability occurs at 375 °C. Under the same temperature, the in-situ permeability decreases with the increase of pore pressure. The in-situ permeability evolution of low-maturity shale can be divided into 5 stages: (1) From 25 °C to 300 °C, thermal cracking and dehydration of clay minerals improve the permeability. However, the value of permeability is less than 0.01×10−3 μm2. (2) From 300 °C to 350 °C, organic matter pyrolysis and hydrocarbon expulsion result in mineral intergranular pores and micron pore-fractures, these pores and fractures form an interconnected pore network at limited scale, improving the permeability. But the liquid hydrocarbon, with high content of viscous asphaltene, is more difficult to move under stress and more likely to retain in pores, causing slow rise of the permeability. (3) From 350 °C to 375 °C, pores are formed by organic matter pyrolysis, but the adsorption swelling of liquid hydrocarbon and additional expansion thermal stress constrained by surrounding stress compress the pore-fracture space, making liquid hydrocarbon difficult to expel and permeability reduce rapidly. (4) From 375 °C to 450°C, the interconnected pore network between different mineral particles after organic matter conversion, enlarged pores and transformation of clay minerals promote the permeability to increase constantly even under stress constraints. (5) From 450°C to 500 °C, the stable pore system and crossed fracture system in different bedding directions significantly enhance the permeability. The organic matter pyrolysis, pore-fracture structure and surrounding stress in the different stages are the key factors affecting the evolution of in-situ permeability.

Contribution of thermal expansion on gas adsorption to coal sorption-induced swelling

Adsorption of gases such as methane and carbon dioxide in coal is an exothermic process. Under isentropic conditions (fully insulated condition), the ejected heat results in an additive transient thermal expansion in addition to the sorption-induced swelling. The magnitude of thermal expansion and its feedback on methane adsorption remains ill-defined. We explore this response in high volatile bituminous coal by measuring the exothermal release of heat (integral heat of adsorption) via temperature change during methane adsorption and use these calorimetric data to define the thermodynamic response. These data link thermal expansion directly to adsorption and adsorption swelling. The results show that methane adsorption can lead to the elevation of coal temperature by more than 10 °C. The resulting thermal expansion is consistent with adsorption swelling and related to adsorption capacity and pressure. The decline of surface potential energy resulting from adsorption is usually considered to be the reason for adsorption swelling. However, the results of this study show that thermal expansion due to the heat of adsorption may account for up to 35% of the total adsorption strain under ideal isentropic conditions. In non-isentropic laboratory experiments of adsorption swelling (approaching isothermal conditions), thermal expansion during adsorption cannot be measured accurately due to the rapid heat loss. However, in-situ within coalbed reservoirs, reduced heat dissipation may retain thermal deformation for longer and impact short-term permeability evolution.

CO2 and N2 adsorption/desorption effects and thermodynamic characteristics in confined coal

Gas injection into coal seams is gaining increasing attention for improving coalbed methane (CBM) production. This technical development requires detailed and reliable information on gas adsorption/desorption-induced deformation and gas occurrence mechanisms. Hence, in this paper, high volatile bituminous coal was taken as the experiment samples from northern Shannxi Province, China, and adsorption-induced swelling experiments with CO 2 and N 2 were carried out under the conditions of one-time injection and step-by-step injection. Then, based on adsorption-induced swelling characteristics, a thermodynamic model was further established to disclose the adsorption/desorption-induced deformation mechanism of confined coals. Results showed that the swelling/shrinkage strain curves of CO 2 and N 2 adsorption/desorption in the coal matrix were generally divided into four stages, including pressurization-deformation, adsorption-induced swelling, depressurization-deformation, and finally desorption-induced shrinkage. It can be seen that the coal swelling effect caused by N 2 adsorption was far less than that caused by CO 2 adsorption. However, both CO 2 and N 2 adsorption displayed that the apparent volumetric swelling weakened with increasing strain when compared with the same adsorption pressure under different strain conditions. In contrast, the absolute volumetric swelling of the confined coal was a coupling result from adsorption-induced swelling and pore pressure effects. Further, based on the thermodynamic model, it was verified that the adsorbate type, the chemical potential of the adsorbate molecule and stress conditions are key factors affecting adsorption-induced swelling of coal under a certain temperature and pressure. In addition, a desorption lag phenomenon during the desorption-induced shrinkage stage was observed, due to incomplete gas desorption and gas retention in the coal macromolecular network. It was also found that there was a shrinkage phenomenon after the peak strain during gas adsorption. That is to say, when fixed adsorption sites on the coal pore surface reached the adsorption saturation state at the peak strain, gas molecules would be stored in an absorption state in the coal macromolecular network under continuous adsorption. These phenomena further explained coal swelling and gas occurrence mechanisms. • Swelling/shrinkage strain curve can be generally divided into four stages. • Absolute volumetric swelling is controlled by adsorption swelling and pore pressure. • Thermodynamic model reflects Gibbs free energy of gas adsorption in coal decreases. • Adsorption-induced swelling effect tends to be dropped with increasing stress. • Shrinkage effect after peak strain and desorption lag are occurred in whole process.

Supercritical CO2-Shale interaction induced natural fracture closure: Implications for scCO2 hydraulic fracturing in shales

Multi-stage hydraulic fracturing has been identified as a must to develop shale gas reservoirs by increasing the stimulated reservoir volume (SRV). Supercritical CO2 (scCO2) has been studied as an alternating fracturing fluid due to its tendency to solve numerous problems associated with conventional aqueous based hydraulic fracturing such as formation damage, clay swelling, water scarcity and ground water contamination. However, its consequences to the host rock are not well understood. It has been recognized that scCO2-shale interaction alters the petrophysical properties during the long-term exposure of shale into scCO2, far little attention has been paid to understand the impact of this process for the short term. Thus, laboratory fracturing experiments using scCO2 on cubic shale samples (50 × 50 × 50 mm) in true triaxial stress cell (TTSC) were conducted. X-ray computed tomography (CT) imaging and low-pressure N2 adsorption were also performed to gain a deeper understanding of the fluid-rock interactions on the studied shales at a short-time process. Post-fracturing x-ray CT scans revealed a significant reduction, in the range of 14% to 46%, in the aperture of the natural fractures, indicating towards a possible scCO2 induced swelling. Mechanical compression test on the sample results in around 12% reduction in the fracture aperture, ruling out the possibility of confining stress being the key factor behind the fracture closure observed during fracturing. scCO2 soaking and N2 adsorption experiments showed the narrowing down of the macropores after scCO2 treatment implying the adsorption swelling as one of the controlling factors for the reduction of fracture aperture. Taken together, our results suggest that scCO2-shale interactions during the short term process of hydraulic fracturing can contribute to decreasing the conductivity of pathways between matrix and hydraulic fractures and hence adversely affecting the post-fracturing productivity of the rock.