Investigation on methane adsorption behavior and swelling in coal: Thermodynamic characteristics and modeling

The research of methane adsorption on tectonic coal is an important content to gas disaster prevention and coalbed methane (CBM) exploration in outburst coal seams. Many projects of methane adsorption capacity and behavior of tectonic coal, such as adsorption difference between tectonic coal and its untectonic coal, adsorption evaluation of tectonic coal, factors for adsorption capacity of tectonic coal, gas-solid coupling feature in tectonic coal and supercritical adsorption phenomena of tectonic coal, were carried out by scientists. Combined with a long-term study on organic matter structure and methane adsorption of tectonic coal, the author summarized recent-years’ researches on adsorption capacity and behavior of tectonic coal-methane at home and abroad from the dispute of adsorption ability determination, the thermodynamic characterization of methane adsorption capacity, and the methane adsorption behavior of quantum chemical calculation of the adsorption and the gas content calculation based on loss compensation, respectively. It is believed that the coal structure controls the methane adsorption capacity and behavior characteristics of different tectonic coals, and from the perspective of thermodynamics, the adsorption capacity of different types of coals can be better distinguished. In the future, a more scientific and complete quantum chemical calculation of methane adsorption by tectonic coal and a compensation method based on instantaneous emission loss should be established, so as to better reveal the methane adsorption behavior of tectonic coal and the mechanism of coal and gas outburst. The research has a reference to fine research of coal adsorption and CBM exploration practices.

To better understand the methane adsorption behavior after microwave exposure, the importance of quantitatively characterizing the effect of cyclical microwave exposure on the molecular structures of coals cannot be overemphasized, with implications for enhancing coalbed methane (CBM) extraction. Thus, cyclical microwave exposure experiments of three different metamorphic coals were conducted, and the methane adsorption capacity before and after each microwave exposure (10 in total) for 120 s was evaluated. Fourier transform infrared spectroscopy analysis and peak fitting technology were applied to quantitatively characterize the changes in the structural parameters of coal molecules. The results showed that after modification, the structural parameters like aromatic carbon fraction (fa–F), aromaticity (I1 and I2), degree of condensation (DOC1 and DOC2), and the maturity of organic matter (“C”) gradually increased with increasing exposure times, while the length of the aliphatic chain or its branching degree (CH2/CH3) and the hydrocarbon generating capacity (“A”) showed a decreasing trend. The Langmuir volume (VL) of three different rank coal samples decreased from 29.2, 32.8, and 40.4 mL/g to 25.7, 29.3, and 35.7 mL/g, respectively; the Langmuir pressure (PL) increased from 0.588, 0.844, and 0.942 MPa to 0.626, 1.007, and 1.139 MPa, respectively. The modification mechanism was investigated by analyzing the relationship between the methane adsorption behaviors and molecular structures in coals. The release of alkane side chains and the oxidation of oxygen-containing functional groups caused by microwave exposure decreased the number of methane adsorption sites. As a result, the methane adsorption capability decreased. In addition, the decomposition of minerals affects methane adsorption behaviors in coals. This work provides a basis for microwave modification of coal as well as in situ enhancement of CBM extraction using microwave exposure.

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

Methane adsorption behavior on shale matrix at in-situ pressure and temperature conditions: Measurement and modeling

Understanding methane adsorption behavior on deep shales is crucial for estimating the original gas in place and enhancing gas recovery in deep shale gas formations. In this study, the methane adsorption on deep shales within the lower Silurian Longmaxi formation from the Sichuan Basin, South China was conducted at pressures up to 50 MPa. The effects of total organic carbon (TOC), temperatures, clay minerals, and moisture content on the adsorption capacity were discussed. The results indicated that the methane excess adsorption on deep shales increased, then reached its peak, and finally decreased with the pressure. The excess adsorption data were fitted using the adsorption models, and it was found that the Dubinin–Radushkevich (D–R) model was superior to other models in predicting the methane adsorption behavior. The methane adsorption capacities exhibited strong positive correlations with the TOC content and negative relationships with clay minerals. The methane excess adsorption decreased with the temperature, while the opposite trend would occur once it exceeded some pressure. The presence of the moisture content on deep shales sharply decreased the methane adsorption capacities, and the reduction of the adsorption capacity decreased with the pressure. The moisture would occupy the adsorption sites in the shale pores, which could result in the methane adsorption capacity that decreased.

Combined Monte Carlo and molecular dynamics simulation of methane adsorption on dry and moist coal

Effects of pore structure on methane adsorption behavior of ductile tectonically deformed coals: An inspiration to coalbed methane exploitation in structurally complex area

Effect and mechanism of ultrasonic mechanical vibration on methane adsorption

Insights into shale gas adsorption and an improved method for characterizing adsorption isotherm from molecular perspectives

To produce natural gas (methane) and simultaneously sequester CO2 in unconventional geologic reservoirs such as gas shale, coalbeds and so on, it is necessary to understand the adsorption behavior of methane and CO2 in these reservoir formations. In this article, adsorption behavior of methane and CO2 on shale samples from Gondwana Basin and KG Basin of India are studied. Adsorption experiments are conducted on as‐received shale samples from these basins at a temperature of 313 K to a maximum equilibrium pressure of approximately 9 MPa for methane and 6 MPa for CO2. The methane and CO2 adsorption data are applied to test the applicability of Langmuir, Dubinin‐Polanyi, BET, and Ono‐Kondo models. A comparison of these models is performed using linear and nonlinear Chi‐squared methods. It was observed that Dubinin‐Astakhov equation was the most accurate adsorption isotherm model for adsorption of methane and CO2 on tested shales. Further, the better fitting by Dubinin‐Polanyi equation over BET, Langmuir, and Ono‐Kondo models suggest that the mechanism of volume filling may be applicable during the adsorption of methane and CO2 on shales. The preferential adsorption of methane and carbon dioxide on Pakur, KG, and Salanpur shales were investigated. The Pakur shale had CO2:CH4 adsorption ratio ~1. © 2019 American Institute of Chemical Engineers Environ Prog, 38: 13222, 2019

Fast prediction of methane adsorption in shale nanopores using kinetic theory and machine learning algorithm

Examination of adsorption behaviors of carbon dioxide and methane in oxidized coal seams

ABSTRACTThe adsorption behavior of methane in simulated coal-bed gas on several micropore zeolites is studied. Alkali and steaming treatments of ZSM-5 are carried out in order to adjust the pore structure and to further elucidate the effect of pore structures on methane adsorption behavior. Further, the effect of adsorbed water on ZSM-5 zeolite is also considered. Textural properties of material are characterized by nitrogen adsorption-desorption at 77 K. The results indicate that pore structures of zeolite adsorbent have a more important effect on the adsorption performance, compared with the specific surface area. The presence of adsorbed water on micropore zeolite is unfavorable to the methane adsorption.

Pore structure characterization and its effect on methane adsorption on shale kerogen are crucial to understanding the fundamental mechanisms of gas storage, transport, and reserves evaluation. In this study, we use 3D scanning confocal microscopy, scanning electron microscopy (SEM), X-ray nano-computed tomography (nano-CT), and low-pressure N2 adsorption analysis to analyze the pore structures of the shale. Additionally, the adsorption behavior of methane on shales with different pore structures is investigated by molecular simulations. The results show that the SEM image of the shale sample obviously displays four different pore shapes, including slit pore, square pore, triangle pore, and circle pore. The average coordination number is 4.21 and the distribution of coordination numbers demonstrates that pores in the shale have high connectivity. Compared with the adsorption capacity of methane on triangle pores, the adsorption capacity on slit pore, square pore, and circle pore are reduced by 9.86%, 8.55%, and 6.12%, respectively. With increasing pressure, these acute wedges fill in a manner different from the right or obtuse angles in the other pores. This study offers a quantitative understanding of the effect of pore structure on methane adsorption in the shale and provides better insight into the evaluation of gas storage in geologic shale reservoirs.

Coal is characterizedby a complex pore-fracture network and functionalgroups, which are derived from various geological origins and whichfurther affect methane adsorption. To explore the relationship betweenthe geological origins of pore-fractures and methane adsorption behaviors,we conducted pore structure tests and adsorption isotherms on sixQinshui high-rank coals. The pores and fractures were observed usingan optical microscope (OM), a field emission scanning electron microscope(FESEM), and a high-resolution transmission electron microscope (HRTEM),and the pore structure parameters were determined using mercury intrusionand low-pressure N2 and CO2 adsorption. High-pressureCH4 adsorption isotherms were obtained at 30 °C usingthe manometric method. Results show that the Qinshui high-rank coalsdevelop five stages of pore size distribution, consisting of the smallermicropore stage (0.3–1 nm), the larger micropore and smallermesopore stage (1–10 nm), the mesopore and smaller macroporestage (10–110 nm), the microfracture stage (0.11–40μm), and the larger macropore stage (>40 μm). The microporesdominate the total pore volume (PV) and specific surface area (SSA).Pores and fractures of various morphologies and sizes have differentgeological origins, which are related to coalification and stressfield evolution. Methane adsorption on coals mainly occurs in themicropores as a form of volume filling. The maximum pore size forcomplete gas filling (MPSCGF) ranges from 0.60 to 0.88 nm in Qinshuihigh-rank coals. The coal-forming geological processes, such as coalificationand stress field evolution, contribute to various pores and fractures,which show different pore sizes and functional groups. The geologicalorigins of pores and fractures control the methane adsorption behaviorsin coals by way of the pore size and functional groups. Surface coverage-relatedmethane adsorption behavior occurs in fractures, primary pores, andlarge-scale secondary pores, while micropore filling is the methaneadsorption behavior in macromolecular pores and small-scale secondarypores. The aim of this study is to provide a new insight into themethane adsorption on coals from the geological process of the formationand modification of pores and fractures.