Low-temperature Liquid Nitrogen Adsorption Research Articles

Reservoir Characteristics of the Main Coal Seams in the Longtan Formation, Guxu Coal Mining Area, Sichuan

To facilitate the efficient exploration and development of coalbed methane of the Longtan Formation in the Guxu coal mining area, Sichuan, it is essential to evaluate the reservoir characteristics of the main production layers. In this study, scanning electron microscopy, low-temperature liquid nitrogen adsorption experiments, high-pressure mercury pressure experiments, and isothermal adsorption experiments were conducted to investigate the reservoir characteristics of favorable coalbed methane reservoirs. The results indicate that the main coal seams in the Longtan Formation are medium-to-high ash content anthracite with a well-developed pore–fracture system. The pore size distribution exhibits both unimodal and bimodal types, while the pore morphology includes impermeable pores closed at one end, open permeable pores, and ink bottle-shaped pores. It shows that the middle part of Longtan Formation acts as an enrichment zone for coalbed methane, which is characterized by a stronger adsorption capacity, high Langmuir volume, high gas content, and high gas saturation distribution. The pH value, mineralization degree, and hydrogen–oxygen isotope of the produced water in the main coal seams indicate that the enrichment zone is typically located in a stagnant flow zone with a reducing environment and favorable storage conditions.

Mechanism of SC-CO2 extraction-induced changes to adsorption heat of tectonic coal

To investigate the change and control mechanism of coal adsorption behavior under supercritical CO2 (SC–CO2), the changes in coal composition, structure and adsorption of coals before and after SC-CO2 action were addressed by means of Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), low temperature liquid nitrogen adsorption instrument and calorimeter. The influence of changes in coal composition and structure on its adsorption was quantitatively analyzed by grey correlation analysis. The results show that the effect of SC-CO2 fluid treatment is proportional to the degree of coal damage. After SC-CO2 treatment, the integral adsorption heat of CO2 changes from −11.10 J/g to 13.744J/g, and the microcrystalline structure parameters and inorganic components in coal have the greatest influence on it. At the same time, it is also affected by temperature and pressure. With the increase of microcrystalline structure parameters d002, La and Lc, the integral adsorption heat of the samples showed a 'V' trend, and the increasing trend was the same as the decreasing trend of ash yield. On the whole, SC-CO2 fluid treatment enhances the adsorption of gas by coal samples to a certain extent.

Influences of Different Acid Solutions on Pore Structures and Fractal Features of Coal

The effect of different acids on the pore structure and fractal characteristics of micropores and mesopores was determined with the help of low-temperature liquid nitrogen adsorption, X-ray diffraction, and the Frenkel–Halsey–Hill (FHH) model by using Yuwu coal as a sample and placing it in acidic environments, such as HF, HCl, HNO3, and CH3COOH. The results show that the acidization effects of HF and CH3COOH are separately dominated by the micropore and mesopore formation effects, while HCl and HNO3 mainly play their roles in expanding mesopores. After acidization, the surface fractal dimensions D1 and D1′ of micropores and mesopores in coal are always negatively correlated with the total specific surface area SBET, specific surface area Smic of micropores, and specific surface area Smes of mesopores. After being acidized by HF, D2 is negatively correlated with the total volume Vtot and the corresponding micropore volume Vmic, while acidization with HCl and HNO3 leads to the opposite result. After being acidized by CH3COOH, D2 has a negative correlation with Vtot and a positive correlation with Vmic. The structural fractal dimensions D2′ of mesopores in samples acidized by HF and CH3COOH are positively correlated with both the volume Vtot and mesopore volume Vmes, while it is the opposite for samples acidized by HNO3. D2′ of coal samples acidized by HCl is negatively correlated with Vtot while positively correlated with Vmes.

Changes in Physicochemical Properties of Coal and Their Mechanism Due to Supercritical CO2–H2O Treatment

The dissolution of supercritical carbon dioxide (ScCO2) in water forms a ScCO2–H2O system, which exerts a transformative influence on the physicochemical characteristics of coal and significantly impacts the CO2-driven enhanced coalbed methane (CO2-ECBM) recovery process. Herein, the effect of ScCO2–H2O treatment on the physicochemical properties of coal was simulated in a high-pressure reactor. The migration of major elements, change in the pore structure, and change in the CH4 adsorption capacity of coal after the ScCO2–H2O treatment were detected using plasma emission spectroscopy, the low-temperature liquid nitrogen adsorption method, and the CH4 adsorption method, respectively. The results show that (1) the ScCO2–H2O treatment led to mineral reactions causing a significant migration of constant elements in the coal. The migration of Ca ions was the most significant, with an increase in their concentration in treated water from 0 to 16–970 mg·L−1, followed by Na, Mg, and K. Al migrated the least, from 0 to 0.004–2.555 mg·L−1. (2) The ScCO2–H2O treatment increased the pore volume and pore-specific surface area (SSA) of the coal via the dissolution and precipitation of minerals in the coal pores. The total pore volume increased from 0.000795–0.011543 to 0.001274–0.014644 cm3·g−1, and the total pore SSA increased from 0.084–3.332 to 0.400–6.061 m2·g−1. (3) Changes in the CH4 adsorption capacity were affected by the combined effects of a mineral reaction and pore structure change. The dissolved precipitates of the minerals in the coal pores after the ScCO2–H2O treatment caused elemental migration, which not only decreased the mineral content in the coal pores but also increased the total pore volume and total pore SSA, thus improving the CH4 adsorption capacity of the coal. This study provides theoretical support for CO2 sequestration and ECBM recovery.

Mechanism of gas-phase passivation inhibition during FeS self-ignition based on pore structure and fractal characteristics

Reactive FeS that spontaneously combusts was prepared under field conditions and subjected to gas-phase passivation using various atmospheres and durations. The goal was to uncover how gas-phase passivation inhibits the spontaneous combustion of reactive FeS. The surface characteristics and elemental composition of FeS were analyzed at different gas-phase passivation levels using scanning electron microscopy (SEM) and X-ray energy-dispersive spectroscopy (EDS). The effect of gas-phase passivation on the evolution of the FeS pore structure was investigated using low-temperature liquid nitrogen adsorption, and the FHH fractal theory was employed to address changes in the fractal properties of the FeS surface. The results highlighted the intricate and densely dispersed pore structure of reactive FeS, along with its pronounced surface roughness. With increased levels of gas-phase passivation, the FeS surface became smoother and more regular, the number of oxidation-expanding pores increased, most sulfur elements were converted into sulfur dioxide gas, and iron trioxide passivation films formed on the FeS surface. Gas-phase passivation significantly reduced the specific surface area and cumulative pore volume of reactive FeS. Specifically, the cumulative pore volume decreased by approximately 20.9%, while the specific surface area decreased by 28.5% as gas-phase passivation deepened. The passivation process transformed more micropores into transition pores, with resulting iron-oxygen compounds adhering to the FeS surface. This reduced the number of reactive adsorption sites and substantially slowed down spontaneous combustion. As gas-phase passivation deepened, the volume proportion of FeS micropores decreased to 16.5%, and the micropore area decreased by around 6%. Gas-phase passivation had a significant impact on reducing the spatial fractal dimension and complexity of FeS, with a greater influence on spatial complexity than surface complexity. The passivation effect was initially enhanced by higher oxygen concentrations within the first 6-hours of passivation, but its effectiveness diminished thereafter.

Effect of Desorption Damage on the Kinetic Characteristics of Coal Particle Gas Desorption.

There is a positive feedback mechanism of desorption, pulverization, and redesorption in the process of coal and gas outburst. The coal pore structure changes to some extent in the process of coal pulverization and has a related impact on the dynamic parameters of coal particle gas desorption. To understand the impact of desorption damage on the dynamic characteristics of coal particle gas desorption, in this paper, a self-developed coal particle gas desorption test device is used to measure the amount of methane desorption of different coal samples repeatedly desorbed under the same adsorption equilibrium pressure. The results show that the methane desorption kinetic curves of the four coal samples based on desorption damage basically have the same trend. Nie's diffusion model can better describe the methane desorption characteristics of coal particles. After desorption, the methane desorption amount, initial desorption velocity, diffusion ability, and ultimate amount of methane desorption of the coal samples are greater than those before desorption. The desorption damage affects the relevant dynamic parameters of the coal particle gas diffusion model, and its impact on the outburst coal is greater than that on raw coal. In addition, the pore size distribution and change characteristics of the coal samples before and after desorption are analyzed quantitatively via a low-temperature liquid nitrogen adsorption test and fractal dimension-related theory. It was found that the pore volume peak area of the coal samples affected by desorption damage within each pore size range was significantly larger than that before desorption. Among them, micropores have the most significant impact on the desorption damage of coal samples, and the peak area of the pore volume of protruding coal is greater than that of raw coal, indicating that the internal pores of coal after desorption damage are more developed than those of raw coal.

Experimental study on the change of coal structure and microbial community structure during supercritical-CO2-H2O-microorganisms-coal interaction process

To study the effects on coal structure and microbial community diversity during CO2 storage in deep unminable water-bearing coal seams, the lignite in the mining area of Inner Mongolia was selected as the research object. The simulation experiments of supercritical-CO2 (ScCO2)-H2O-microbial-coal interaction under different pressure conditions performed by using an autoclave. Then the solid–liquid phases before and after the reaction were analysed by XPS, low-temperature liquid nitrogen adsorption, 16S rRNA and other testing methods. The results showed that CO2 injection caused the pH and OD600 values in the system to decrease, and the pH and OD600 values in the system decreased more obviously with the increase of ScCO2 pressure. Compared with raw coal, the relative content of carbon elements on the surface of the coal after the reaction increased, and the organic carbon (CC/CH) relative content decreased slightly. In contract, the specific surface area, total pore volume, and the pore size of micropores of coal increased, but the average pore size of the mesopores and macropores decreased. At the same time, with the ScCO2 injection under different pressure conditions, some bacterial cells in the system ruptured and inactivated, resulting in the decrease their relative abundance. In contract, some genera, such as Paraclostridium and Methanoculleus, may have the adaptability to the extreme environment and gradually evolve into the dominant genus. In conclusion, the results provide a theoretical basis for studying the changes of ScCO2 on coal structure and microbial community structure during CO2 storage in deep unminable water-bearing coal seams.

Control Mechanism of Microbial Degradation on the Physical Properties of a Coal Reservoir

This study addressed the effect of microbial methane production on the physical properties of a coal reservoir. Two kinds of coal samples before and after anaerobic degradation were tested by a low-temperature liquid nitrogen adsorption test and an isothermal adsorption and diffusion coefficient test. The influence of the characteristics of microbial gas production on the coal physical properties was analyzed. Due to the differences in the physical properties of the coal samples, the effect of microbial production is different. Coal is a macromolecular organic compound, mainly aromatic and lignin derivatives, containing carbon and nitrogen sources that can be used by microorganisms. Microorganisms secrete extracellular enzymes to decompose covalent bonds and functional groups of macromolecules in coal and eventually produce methane, which will change the physical properties of coal. It was found that microbial anaerobic degradation could increase the content of coalbed methane, change the pore structure of coal, reduce the fractal dimension of the coal surface and smooth the coal surface. At the same time, microbial degradation has changed the physical properties of coal reservoirs to some extent, increased the diffusion of coal reservoirs and improved the pore connectivity of coal reservoirs, which provides more of a scientific basis for the development of coalbed methane.

New Insight into Temperature Effects on Low-Rank Coal Flotation Using Diesel as a Collector.

Efficient flotation of low-rank coal is of great significance for the development of green and low-carbon cycles. Temperature is a crucial parameter of flotation, but the mechanism of its effect on flotation lacks understanding. In this paper, the mechanism was studied by kinetic flotation, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy, low-temperature liquid-nitrogen adsorption (LP-N2A), X-ray photoelectron spectroscopy (XPS), and molecular dynamics simulation. The flotation combustible recovery gradually decreases as temperature rises. Compared with 60 °C, the combustible recovery at 5 °C increases by 18.13%. The desorption energy for oil droplets decreases as the temperature rises. As a result, the oil droplets are easier to desorb at high temperatures. The SEM and LP-N2A results demonstrate that the pores and fractures of the coal sample are well developed. Also, the oil-water interfacial tension and viscosity of oil droplets decrease as the temperature rises, while the diffusion ability increases. These increase the volume of oil droplets that penetrate into the pores, resulting in poor spreadability of oil droplets on the coal surface. The average volume of bubbles gradually increases as temperature rises, which renders the flotation foam unstable and worsens the flotation. Therefore, the flotation performance is better at low temperatures.

Microstructure Study of High-Rank Coal in an Alkaline Solution at the Chengzhuang Mine.

To study the adsorption performance of coal bodies after alkaline solution erosion and the microscopic mechanism of alkali erosion on coal bodies, isothermal adsorption experiments at different pH values and with different numbers of soaking days were conducted on high-order coal bodies from the Chengzhuang mine. The results showed that the adsorption capacity of the coal bodies after alkali leaching was improved compared to that of the original coal, all of which was in accordance with the Langmuir equation. The unit adsorption capacity of coal samples increased gradually with an increase in the number of soaking days and solution pH, reaching the maximum at pH 13 and eight soaking days. The adsorption constant a of the coal sample was positively correlated with the pH, and the number of soaking days was a power exponential function; the adsorption constant b increased gradually with an increase in the pH of the solution and increased first and then decreased with an increase in the number of soaking days. The change in the adsorption of coal samples occurs because the alkaline solution reacts with the minerals in the coal as well as the mineral ions, and the resulting complex gels and precipitates block the pore channels of the coal body, which in turn inhibits the adsorption of gases. The presence of Na, Mg, Al, Si, Ca, Fe, and other elemental compounds detected in the generated sediments verified the mechanism of alkaline solution erosion. The changes in the microscopic pore structure of the coal body were quantified by low-temperature liquid nitrogen adsorption experiments. The small and medium pore volumes of the coal samples reached the maximum values at pH 13 and with eight soaking days, which is in agreement with the conclusion of optimal alkali modification.

The effects of supercritical CO2 on the seepage characteristics and microstructure of water-bearing bituminous coal at in-situ stress conditions

CO2 geological storage (CGS) is considered to be an important technology for achieving carbon peak and carbon neutralization goals. Injecting CO2 into deep unminable coal seams can achieve both CGS and enhance coalbed methane (ECBM) production. Therefore, the deep unminable coal seams are considered as promising geological reservoirs. CO2 exists in a supercritical CO2 (ScCO2) when it was injected into deep unminable coal seams. The injection of ScCO2 can induce changes in the seepage characteristics and microstructure of deep water-bearing coal seams. In this study, typical bituminous coal from Shenmu, Shanxi Province was used to investigate the effects of ScCO2 on seepage characteristics, pore characteristics, and mineral composition through experiments such as seepage tests, low-temperature liquid nitrogen adsorption, and X-ray diffraction (XRD). The results indicate that ScCO2 treatment of dry and saturated coal samples caused a significant increase in clay mineral content due to the dissolution of carbonates, leading to the conversion of adsorption pores to seepage pores and an improvement in seepage pore connectivity. Therefore, the Brunauer-Emmett-Teller (BET) specific surface area and pore volume of the two coal samples both decreased after ScCO2 treatment. Moreover, the permeability of dry and saturated coal samples increased by 191.53% and 231.71% at 10 MPa effective stress respectively. In semi-saturated coal samples, a large amount of dolomite dissolved, leading to the precipitation of Ca2+ and CO32- to form calcite. This caused pore throats to clog and macropores to divide. The results show that the pore volume and average pore size of coal samples decrease, while the specific surface area increases after ScCO2 treatment, providing more space for gas adsorption. However, the pore changes also reduced the permeability of the coal samples by 32.21% and 7.72% at effective stresses of 3 MPa and 10 MPa, respectively. The results enhance our understanding of carbon sequestration through ScCO2 injection into water-bearing bituminous coal seams.

Effect of new modified materials on the microscopic pore structure and hydration characteristics of sealing materials in coal seam boreholes

To improve the efficiency of coal mine gas extraction, it is necessary to optimize the performance of sealing materials. In this paper, fly ash, nano-silica, and triethanolamine was selected to optimize the pore structure of the sealing materials. The qualitative and quantitative analysis of the seal material was performed by low-temperature liquid nitrogen adsorption experiments, scanning electron microscopy, and combined with fractal theory. The results show that the pores of common sealing materials are mainly mesoporous, accounting for 52.19%, and the volume of micropores is the smallest, accounting for 9.88%. The single doping of fly ash made the pore volume of the material decrease by 9.5%, and the percentage of mesopores increased by 1.3%, but the percentage of micropores decreased by 0.9%. Co-doping with nano-silica and fly ash increased the microporous percentage of the material by 0.77% and the peak stress by 17.43%. The synergistic effect of fly ash, nano-silica, and triethanolamine was most effective in optimizing the pore structure of the sealing material. The pore volume was reduced to 0.08198 cm3/g, and the fractal dimensions D1 and D2 were increased by 2.88% and 2.48%, respectively. The microscopic morphology of the material was also flatter and denser, and the hydration of fly ash was also more complete.

Experimental study on the influence of acid fracturing fluid on coal wettability

With the continuous improvement of mechanized mining intensity, dust has seriously threatened the safety of mine production and the health of underground workers. Coal seam water injection is one of the effective means to moisten the primary coal dust in the coal body. Through testing by the surface tension tester, contact angle meter, FTIR, low-temperature liquid nitrogen adsorption, and SEM, the impact of acid fracturing fluid which can be used for coal seam water injection on the wettability and pore structure of coal was studyed. The results showed that HCl and CTAB had good synergistic acidification effect, which can effectively reduce the surface tension of CTAB by 10 ∼ 30 mN/m. Also the contact angle between the solution and the coal sample is significantly affected by the acidity of the solution. However, the impact of the acid fracturing fluid varies for coal with differing levels of metamorphism and mineral content. For low rank bituminous coal, the contact angle decreased by 39.55° after contact with the sample surface for 1 s, and the solution was completely immersed in the coal body after 5 s. The absorbance of hydrophilic functional groups of coal becomes stronger after treatment with acid fracturing fluid, which is conducive to upgrading the wettability of coal. Meanwhile, the acid fracturing fluid increases coal’s connectivity and average pore diameter, which in turn influences the wettability of coal. Exploring the influence of acid-based fracturing fluid on coal can provide guidance for improving the wettability of coal, reducing dust concentration and optimizing the underground working environment.

Pore Structure Multifractal Characteristics of Coal Reservoirs in the Central and Eastern Qinshui Basin and Influencing Factors

The heterogeneity of the pore structure of coal reservoirs affects the desorption and diffusion characteristics of coalbed methane, and determining its distribution law is conducive to improving the theory of coalbed methane development. The central and eastern parts of the Qinshui Basin are rich in coalbed methane resources, but the heterogeneity characteristics of the pore structure of coal reservoirs are not clear. NMR has the advantages of being fast, non-destructive and full-scale, and multifractal can describe the self-similarity of NMR T2 curve at different scales so as to analyze the complexity of pore distribution. Based on this, 15 samples with different coal ranks were collected from the central and eastern Qinshui Basin (Ro,max between 1.54 and 2.78%), and quantitative pore characterization experiments such as low-field nuclear magnetic resonance (LF-NMR) and low-temperature liquid nitrogen adsorption (LTN2A) were conducted. Based on multifractal theory, the heterogeneity law of pore structure was quantitatively evaluated, and its influencing factors were elucidated. The results showed that the BJH pore volume of coal samples in the study area ranged from 0.0005–0.0028 cm3/g, with an average of 0.0014 cm3/g, and the BET specific surface area was 0.07–2.52 m2/g, with an average of 0.41 m2/g. The NMR T2 spectrum peaked at 0.1–1, 10–100 and 100–1000 ms, and the spectrum was mostly bimodal or trimodal, indicating that pores of different pore sizes were developed. There were great differences in the pore structure of different coal ranks; high-rank coal was dominated by micropores, and the proportion of mesopores and macropores of medium-rank coal was higher. The pore structure of coal samples showed obvious multifractal characteristics, and the fractal characteristics of the sparse region (low-value information) were more significant; they dominated the pore distribution and had a stronger influence on the distribution of pore space. Pore structure heterogeneity is closely related to the degree of coalification, and with the increase in coalification, it is closely related to coal lithotype and quality, and high mineral and inertinite contents lead to the enhancement of pore structure heterogeneity in coal reservoirs, while Ro,max, Mad and vitrinite group contents have opposite effects. The research results provide theoretical guidance for the subsequent exploration and development of coalbed methane in the region.

Fractal characteristics and theirs influence on methane adsorption in high-rank coals with NMR

To further understand the pore structure characteristics and their effect on CH4 adsorption capacity for high-rank coals. Based on 11 fresh coal samples from the Zhina coalfield of South China. We analyzed the pore structure characteristics of coal samples by low-temperature liquid-nitrogen adsorption (LP-N2A) measurements. On the basis of nuclear magnetic resonance (NMR), we obtained the fractal dimensions of different types of pores by the new model, studied the relationship between the fractal dimensions, and the characteristic parameters of coals (composition and pore characteristics) and discussed the influence of the fractal dimensions on CH4 adsorption. The results show that according to LP-N2A isotherms, all coals can be classified into three types. The micropores provide the largest proportion of the specific surface area (SSA) of coals. Two fractal dimensions, Da (adsorption pore) and Ds (seepage pore), ranged from 2.471 to 2.805 and from 2.812 to 2.976, which were acquired in the saturated water condition by NMR. Furthermore, Da and Ds have different correlations with ash yield, carbon contents, moisture, SSA and irreducible fluid porosity. The coal composition and pore parameters have much greater control over fractal dimensions. Moreover, the different fractal dimensions have different influences on methane adsorption. With the increase of Da, the methane adsorption capacity is enhanced, but it is weakened with the increase of Ds. The high-rank coals have more SSA with higher Da and provide more adsorption sites for CH4. Langmuir pressure PL has different correlations with fractal dimensions. Da decreases with the increase of PL. The adsorption velocity is faster with higher Da. Thus, the fractal dimensions are the comprehensive reflection of differences among the physical properties of coal and are able to show the effect of coal properties on methane adsorption fully.

Characteristics of Gas Seepage and Pore Structure Response in CO2-ECBM Process of Low Permeability Coal Seam

CO2-ECBM is a method of enhanced coalbed methane extraction followed by cutting greenhouse gas emissions and new energy development. In order to reveal the characteristics of gas flow in porous media and the pore structure response characteristics of coal rocks, the experiments were carried out to simulate the process of CO2 displacement of N2 at a buried depth of 900 m, including monitoring the changes in gas permeability and strain of coal samples along with a comparison of the pore structure of low-temperature liquid nitrogen adsorption on coal samples both before and after displacement were both done. The findings of the experiment are listed below. The N2 permeability of the LiuZhuang sample ranges from 0.0008mD to 0.0014mD, whereas the permeability of QiDong is around 0.0003mD. With an increase in gas injection duration and an expansion of the coal matrix for N2 adsorption, the permeability steadily decreases. The efficient stress compression of the coal pore fracture structure during sample preparation and testing avoids the visible fracture region, which results in poor permeability. The displacement stages of CO2 can be divided into three phases. Free nitrogen flows from the end of the position and the permeability diminishes during the phase of free nitrogen. When CO2 is introduced into the penetration stage, the permeability tends to rise, however when there is no penetration, the permeability test values are frequently low. During the CO2 steady displacement phase, gas permeability gradually declines. Axial and radial strains are progressively raised during the initial stage of the CO2 injection whereas they are gradually reduced during the initial stage of the N2 injection. While CO2 is continuously supplied through the coal body stage, there are modest axial and radial strain changes. The axial and radial stresses are stabilized by the CO2 displacement. The overall pore volume of the coal significantly rises following the displacement. The increase part of the pore volume is primarily focused on the pore of absorption and filling (aperture < 10nm), whereas the decreased part is mainly concentrated in the diffusion pore of the Fick type and the permeability part (aperture > 50nm). The increased in pore volume ratio surface area is centered mostly in the fill pore region (aperture 10 nm) and is four times greater than it was before the displacement. The CO2 injection exerts an expansion impact on the adsorptionfilled and diffusion pores during the CO2-ECBM process, whereas the compression effect on the percolation pores results in a reduction in permeability.

Effects of chemical solvents on coal pore structural and fractal characteristics: An experimental investigation

Chemical solvent treatment is one of the most effective means to improve pore structure of low-permeability coal and increase production of coalbed methane. Low-temperature liquid nitrogen adsorption method, X-ray diffraction (XRD) mineral analysis and fractal theory were adopted to study variations in coal pore structural parameters and fractal characteristics affected by hydrochloric acid (HCl), tetrahydrofuran (THF) and carbon disulfide (CS2). Liquid nitrogen adsorption results of coal samples were analyzed using density functional theory (DFT), Brunauer-Emmett-Teller (BET) equation and Barrett-Johner-Halenda (BJH) model. Meanwhile, changes in fractal dimensions of pore surface (D1) and pore structure (D2) were calculated based on Frenkel-Halsey-Hill (FHH) model. Relationship between pore volume, average pore diameter, specific surface area and two fractal dimensions were established. Results show that: (1) Hysteresis lines exist in all isothermal adsorption-desorption processes of coal. Pores of raw coal are mainly semi-open pores with poor connectivity. The types of pores do not change significantly after THF and CS2 treatment, while the number of pores rise. In contrast, affected by HCl treatment, ink-bottle pores and open pores increase and pore fracture network connectivity becomes better. (2) Pore sizes of treated coal mainly distribute in range of 0–10 nm. After HCl treatment, many closed pores are opened, with growth in proportion of micropores. Meanwhile, specific surface area (SSA) of pores increases by 55.21% and 31.36%, respectively. THF extraction results in an increase in pore volume and SSA. After CS2 extraction, dissolution of organic material increases pore size and further improve volume of transition pores and mesopores. (3) HCl treatment causes decrease in D1 value of pores. However, in organic-solvent-treatment cases, solvent residues retain on coal pores, resulting in growth in D1 value and pore surface becoming rougher. Proportion of total pore volume of transition pores and mesopores of acidized coal decreases and D2 value rises. Pore distribution is more uneven. Relevant results could provide theoretical references for revealing mechanism of chemical solvents effects on gas flow in coal.

Experimental Study of the Source of CO Anomalies in Mines Based on Microscopic Changes

The phenomenon of abnormal CO emergence occurred in a working face of Tangshan mine, and the CO source was analyzed from the two perspectives of CO detection method optimization and microstructure changes in the low-temperature environment of the coal body. Then, the critical index system was optimized. The CO identification tube test results and gas chromatograph test results are combined to derive a fitting formula, and the CO identification tube test results are used as the independent variable to obtain the gas chromatograph test results, which can effectively eliminate the error of small CO identification tube test results. The analysis of raw coal and coal samples heated by water bath at 30 °C, 40 °C, and 50 °C was carried out using low temperature liquid nitrogen adsorption and thermogravimetric and infrared spectroscopy experiments. It was found that the pore structure of the coal body developed as the temperature increased; the oxidation reaction occurred in the low-temperature state when heat was absorbed to produce CO. The thermal decomposition of carbonyl group was found to be the main source of CO. Finally, the index of spontaneous combustion of coal is optimized according to the temperature, and the index systems represented by O2/(CO2+CO), CH4 and CO2/CO were determined from 30~80 °C, 90~180 °C and 18~240 °C, respectively.

High Rank Coal Pore Fracture Structure and its Impact on Reservoir Characteristics in the Southern Qinshui Basin

Although the southern Qinshui Basin is the most successful area for coalbed methane (CBM) development in China, the production of CBM wells in different blocks in the area is significantly different. One of the key reasons is the difference in pore structure in various-ranked coal. In this study, No. 3 coal seam of Sihe and Zhaozhuang blocks in southern Qinshui Basin was selected as the research object to investigate the high rank coal pore fracture structure and its impact on reservoir characteristics. Mercury intrusion porosimetry (MIP), low-temperature liquid nitrogen adsorption (LTN2A), scanning electron microscopy (SEM), and isothermal adsorption tests were conducted. The results show that the Sihe No.3 coal seam was mainly composed of open cylindrical and flat pores with a high proportion of transition pores (10–100 nm), large specific surface area, good connectivity, strong adsorption capacity, high gas content, and reservoir energy. Zhaozhuang No.3 coal had high proportion of mesopores (100–1,000 nm), small specific surface area, poor pores connectivity, weak adsorption capacity, poor gas content, low reservoir energy, and critical desorption pressure. The proportion of cylindrical pores, parallel plate pores, and wedge-shaped pores closed at one end was high. The anomalies in pore morphology and pore structure characteristics of coal reservoir were the main factors that caused variation in gas production of No.3 coal seam in Sihe and Zhaozhuang blocks.

Effect of Nanoparticle Adsorption on the Pore Structure of a Coalbed Methane Reservoir: A Laboratory Experimental Study.

Coalbed methane (CBM) is an important unconventional energy resource, and its micropore structure has a vital impact on its exploitation. Based on the nuclear magnetic resonance (NMR) experiment, the low-temperature liquid nitrogen adsorption experiment, and the contact angle experiment, in this paper, we investigated the influence of nanofluids on the micropore structure of a CBM reservoir from many aspects. The influence of different adsorption mechanisms of TiO2 nanoparticles on the surface wettability of rock samples was analyzed. The influence of nanoparticle adsorption on the drainage and distribution of liquid in the rock sample was discussed in depth. In addition, the effects of nanofluid treatment on the micropore structure were investigated by comparing the data of low-temperature liquid nitrogen adsorption experiments, including the pore diameter, pore volume, and specific surface area (SSA). The experimental results show that the treatment of nanofluids helps to open the micropores and greatly increases the SSA, pore diameter, and pore volume of the sample. The maximum increase percentages of SSA, pore volume, and pore diameter are 228.12, 80.65, and 18.89%, respectively. It is found that the adsorption of particles is conducive to enhancing the water wettability of the pore throat surface and reducing the damage to water locks.