•The preferential adsorption of CO2 over CH4 improves with coal rank.•Pore structure of coal affects the thermodynamics of adsorption of CO2 and CH4.•Adsorbed CO2 has a more ordered configuration ...than CH4 on different rank coals.
In this paper, the pore structures of three different rank coals sampled from China (anthracite, bituminous coal and lignite) were characterized by CO2 and N2 adsorption. The isothermal adsorption curves of CO2 and CH4 on three samples were measured by gravimetric method and fitted by Langmuir model. The preferential selectivity (αCO2/CH4) was calculated using the Langmuir parameters of CO2 and CH4, and the Henry’s coefficient (KH) was obtained with the help of virial equation. More importantly, a comparative analysis of adsorption thermodynamics of CO2 and CH4 on three different rank coals, including surface potential (Ω), Gibbs free energy change (ΔG) and entropy loss (ΔS), was presented according to the adsorption data. It is found that the uptakes of CO2 and CH4 on anthracite are the largest, followed by lignite and bituminous coal in sequence. αCO2/CH4 increases with the increase of coal rank. Low temperature helps injected CO2 to displace pre-adsorbed CH4. The KH values on anthracite are the biggest, while KH values on bituminous coal are the smallest. Ω, ΔG and ΔS of CO2 and CH4 all exhibit a U-shaped function with maturity. Anthracite has the highest Ω, ΔG and ΔS, while bituminous coal has the lowest Ω, ΔG and ΔS. The thermodynamics parameters of Ω, ΔG and ΔS are affected by pore size distributions of three coals. Ω, ΔG and ΔS of CH4 are smaller than those of CO2. CO2 adsorption on coal is more favorable and spontaneous, and adsorbed CO2 molecules form a more efficient packing on coal.
Excessive inhalation of mineral dust can cause irreversible damage such as diffuse fibrosis of lung tissue. Water-based dust reduction technology can effectively control the dust concentration. The ...study of the interaction of water-clay mineral dust is helpful to the prevention and treatment of pneumoconiosis by water-based dust removal technology. To better understand the underlying adsorption mechanisms of water molecules on clay mineral dust, the detailed adsorption thermodynamics analysis is necessary. In this paper, we research the thermodynamics of adsorption of water molecules on swelling clay of montmorillonite and non-swelling clay of illite. First, the adsorption isotherms of water molecules on montmorillonite and illite at 293–313 K were measured by gravimetric method. Then, the key thermodynamic variables, including entropy change (ΔS), surface potential (Ω), isosteric heat of adsorption (Qst) and variation of Gibbs free energy (ΔG), were analyzed. Results illustrate that the adsorption amount for water molecules on illite is one order of magnitude smaller than that on montmorillonite, suggesting that swelling clay plays a dominant role in water molecules adsorption process. For water molecules adsorption on montmorillonite, the contribution of secondary adsorption to total adsorption (a2/a) is always less than 30%. For water molecules adsorption on illite, the contribution of primary adsorption to total adsorption (a1/a) is greater than a2/a at the low pressure region, while a2/a can exceed 60% at the high pressure region. The difference in the uptakes of water molecules adsorption on non-swelling and swelling clays is mainly resulted from the difference in primary adsorption on two clays. The Henry’s constant (KAA) for montmorillonite is in the range of 21.37–75.08 mmol/g/kPa, which is evidently larger than the KAA values of 0.34–0.98 mmol/g/kPa for illite. Compared with non-swelling clay, the adsorption spontaneity degree for water molecules on swelling clay is higher, and the interaction of swelling clay-water molecules is stronger. Meanwhile, the movement of adsorbed water molecules in swelling clay is more confined than that in non-swelling clay. These findings can offer meaningful guidelines for the prevention and treatment of pneumoconiosis.
The microstructure, mineral composition, total organic carbon content, etc., of gas shale are crucial parameters for shale reservoirs, which can directly/indirectly affect shale brittleness, ...fracturing effect, adsorption ability and production efficiency. The study proposed a workflow to characterize the physical and mechanical parameters of Lower Silurian Longmaxi shale outcrop samples extracted from the favorable block in Changning, Sichuan, southwest China. This study elaborated on the influence of these physical and mechanical characteristics and proposed a corresponding brittleness index on shale extraction. In addition, it put forward corresponding suggestions for development and risk control. For a better understanding the mechanisms of shale gas storage and production, XRD, XRF, SEM, low temperature Nitrogen adsorption method, nuclear magnetic resonance and other measurements were employed to analyze and study the mineral composition, microstructure, and adsorption performance of shale. The results demonstrated that the pores of shale are mainly slit pores; there are diverse pore types in shale, mainly including intergranular pores, mineral particle dissolution pores, and internal pores of organic matter; The samples with relatively low porosity also noticeably exhibit ultra-low permeability, and the nanopore structure is remarkably significant, with distribution primarily in range of 5–237 nm. Finally, a brittleness index considering the influence of water content and the mechanical properties was proposed, and the coupling interaction of various minerals components and mechanical properties on the brittleness index can more objectively reflect the brittleness characteristics of deep shale formation.
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The combined effects of surface chemistry and pore structure on water vapor adsorption characteristics of coal were studied by evaluating the equilibrium, thermodynamic and kinetic ...properties. Four coal samples of different rank were fully characterized with gas (N2 and CO2) sorption and Fourier Transform infrared spectroscopy (FTIR) methods. Then measurements of water vapor adsorption equilibrium and kinetics were undertaken at elevated temperatures. Equilibrium and kinetic data were fitted by the modified BET model and the unipore model, respectively. The thermodynamic parameters, as well as diffusion activation energy were estimated based on the adsorption data. The results demonstrate that water vapor adsorption depends on the surface chemistry but unrelated to the pore structure, because the pore space is not completely filled by water molecules even at the saturation pressure. The monolayer adsorption capacity decreases with increasing coal rank. Moreover, water vapor needs lower work to attain equilibrium on high-ranked coal. Also, the binding affinity of water molecule with primary sites is stronger than secondary sites. Furthermore, the diffusion coefficient decreases with coal rank, and the diffusion activation energy for high-volatile bituminous is higher than the other coals, as a result of the complexity and poor connectivity of its pore network.
Pneumoconiosis is a systemic disease mainly caused by diffuse fibrosis (scar) of lung tissue due to long-term inhalation of productive mineral dust in occupational activities and retention in the ...lungs. The mineral dust concentrration can be effectively controlled using water-based mineral dust reduction technique. Therefore, the investigation into water-clay mineral dust interaction is useful for pneumoconiosis prevention and treatment. In this study, the adsorption performances for water molecules in kaolinite clay mineral were investigated. The findings reveal that the performances of water molecules adsorption in kaolinite clay mineral can be divided into primary adsorption and secondary adsorption. Under the lower vapour pressure, water molecules are primarily fixed in primary sites. At the higher vapour pressure condition, the secondary adsorption sites become the utilizable main adsorption centers. Primary adsorption is monolayer adsorption, while water molecules adsorption in secondary adsorption centers is occurred by the formation of water clusters.
The physico-chemical effects caused by supercritical CO2 (ScCO2) exposure is one of the leading problems for CO2 storage in deep coal seams as it will significantly alter the flow behaviors of gases. ...The main objective of this study was to investigate the effects of ScCO2 injection on diffusion and adsorption kinetics of CH4, CO2 and water vapor in various rank coals. The powdered coal samples were immersed in ScCO2 for 30 days using a high-pressure sealed reactor. Then, the diffusion and adsorption kinetics of CH4, CO2 and water vapor in the coals both before and after exposure were examined. Results indicate that the diffusivities of CH4 and CO2 are significantly increased due to the combined matrix swelling and solvent effect caused by ScCO2 exposure, which may induce secondary faults and remove some volatile matters that block the pore throats. On the other hand, the diffusivities of water vapor are reduced due to the elimination of surface functional groups with ScCO2 exposure. It is concluded that density of the surface function groups is the controlling factor for water vapor diffusion rather than the pore properties. The unipore model and pseudo-first-order equation can simulate the diffusion and adsorption kinetics of CH4 and CO2 very well, but the unipore model is not capable of well describing water vapor diffusion. The effective diffusivity (De), diffusion coefficient (D) and adsorption rates (k1) of CH4 and CO2 are significantly increased after ScCO2 exposure, while the values of water vapor are decreased notably. Thus, the injection of ScCO2 will efficiently improve the transport properties of CH4 and CO2 but hinder the movement of water molecules in coal seams.
The isothermal sorption experiments of CH4, CO2 and CH4/CO2 mixture gases on shale were performed. The sorption affinities of CH4, CO2 and CH4/CO2 mixture gases were compared using Henry’s ...coefficient (KH). The thermodynamics of sorption of CH4, CO2 and their mixture were discussed based on surface potential (Ω), entropy loss (ΔS) and change of Gibbs free energy (ΔG). It is shown that the uptakes of CO2 is largest, followed by CH4/CO2 mixture gases and CH4 in sequence. The introduction of a small amount of CO2 into mixture gases can significantly enhance the adsorption quantity of mixture gases and suppress the adsorption of CH4. The lower pressure and the smaller CO2 mole fraction in bulk phase (yCO2) improve separation factor and boost the displacement of adsorbed CH4 by CO2. KH, Ω and ΔS for CO2 are higher than those for CH4, and KH, Ω and ΔS for CH4/CO2 mixture gases are between those for CH4 and CO2. Increasing yCO2 increases KH, Ω and ΔS. ΔG of CO2 and CH4/CO2 mixture gases are bigger than that of CH4. CO2 adsorption is more spontaneous and the introduction of CO2 enlarges the spontaneity degree of adsorption of mixture gases.
Investigating the adsorption characteristics of CO2, N2 and CH4 on kaolinite clay is beneficial for enhanced shale gas recovery by gas injection. In this paper, the experiments of CO2, N2 and CH4 ...adsorption at 288 K, 308 K and 328 K on kaolinite clay were conducted, and the thermodynamics analysis of adsorption of three gases was performed. The findings reveal that the order of the uptakes of three gases on kaolinite clay is as follows: N2 < CH4 < CO2. Reducing temperature enlarges the separation coefficients of CO2 over CH4 (αCO2/CH4), CO2 over N2 (αCO2/N2), and CH4 over N2 (αCH4/N2). The value of αCO2/CH4 greater than one validates that CO2 is capable to directly replace the pre-adsorbed CH4. The spontaneity of CO2 adsorption is the highest, while N2 has the lowest adsorption spontaneity. Injecting N2 into gas-bearing reservoir can cause CH4 desorption by lowering the spontaneity of CH4 adsorption. Adsorbed CO2 molecules form a most ordered rearrangement on kaolinite surface. The decrease rate of entropy loss for N2 adsorption is higher than those for CO2 and CH4 adsorption.
Hydraulic fracturing techniques for developing deeply buried coal reservoirs face routine problems related to high initial pressures and limited control over the fracture propagation direction. A ...novel method of directional hydraulic fracturing (DHF) based on hydraulic slotting in a nonuniform pore pressure field is proposed. A mechanical model is used to address crack initiation and propagation in a nonuniform pore pressure field, where cracks tend to rupture and propagate towards zones of high pore pressure for reducing the effective rock stress more. The crack initiation pressure and propagation morphology are analyzed by rock failure process analysis software. The numerical results show that the directional propagation of hydraulic fracturing cracks is possible when the horizontal stress difference coefficient is less than or equal to 0.5 or the slotting deviation angle is less than or equal to 30°. These findings are in good agreement with experimental results, which support the accuracy and reliability of the proposed method and theory.
By constructing the permeability model to distinguish the fracture zone, plastic zone, and elastic zone around the hydraulic fracturing borehole, we study the influence of some important parameters ...on the stress distribution around the borehole and the permeability of the coal rock. The research results show that the greater the ground stress, the greater the radial stress in the fracture zone and plastic zone, and the smaller the radial stress in the elastic zone, while the trend of the stress variation in permeability is the opposite to the radial stress. The greater the gas pressure, the greater the permeability of the coal rock in each stress area. The larger the borehole radius, the smaller the radial stress at the same distance from the borehole center, and the greater the permeability of the coal rock. The greater the fracturing pressure, the greater the radial stress on the coal rock at the same distance from the borehole center, and the smaller the permeability of the coal rock. The research results can provide a theoretical reference for hydraulic fracturing construction in coal mines with different reservoir environments.
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