Coal Pore Research Articles

Study on the water transport characteristics during spontaneous imbibition and its impact on gas desorption

During the exploitation of deep coal resources, the gas hazard becomes increasingly significant, severely threatening the high-quality development of the coal industry and the occupational safety and health of miners. Coal seam water injection is underground coal mines' most commonly used dust control measure. Additionally, it is an effective method for preventing localized coal and gas outburst hazards. After injecting water into the coal seam, spontaneous imbibition causes the retained water in the fractures to migrate under capillary action and widely distribute the water within the coal seam, which enhances the wetting degree of the coal by water and the effectiveness of gas prevention. To reveal the migration characteristics of water in the coal during spontaneous imbibition and its impact on gas control, a self-developed gas-bearing coal spontaneous imbibition system was used to conduct isobaric water injection imbibition and gas desorption experiments on gas-bearing coal samples. The results showed that after the water was injected into the coal sample, it migrated from bottom to top under capillary action, resulting in three stages of abrupt, rapid, and gradual changes in the electrical resistivity of different layers of the coal sample over time. The migration distance of water increased with the imbibition time, but the rate of increase gradually decreased. The compaction load increase reduced the coal sample's average pore size, resulting in faster water migration speed. After imbibition, the water content decreased gradually from bottom to top along the axial direction and showed a non-uniform distribution in the radial direction. During imbibition, the water displaced gas within the coal sample, and the amount of gas displaced increased with the imbibition time. Increasing the amount of water injected and the coal sample compacted load can promote water migration within the coal, expanding the wetted area and displacing more gas. After imbibition, due to the displaced gas being released and the water-lock effect, the atmospheric gas desorption amount of the coal sample decreased with an increase in the amount of water injected, following a modified power function relationship. When the amount of water injected into the coal sample was low, increasing the water-injected amount could significantly reduce the gas desorption expansion energy of the coal sample. However, this trend became less apparent as the amount of water injected continued to increase. To improve the effectiveness of coal seam water injection in preventing gas outburst hazards, it is recommended to carry out water injection operations as early as possible to expand the wetted area of the coal seam and to add gas drainage holes to prevent increased coal body outburst tendencies due to local gas pressure rise in the coal seam.

Pore evolution and reaction mechanism of coal under the combination of chemical activation and autothermal oxidation

Aiming at the limitation problems such as limited scope, high cost and energy consumption of coal bed penetration enhancement technology by traditional physical and chemical methods, a coal seam methane penetration enhancement method based on the combined action of chemical activation and auto thermal oxidation is proposed from the idea of utilizing the in-situ energy of the coal body. In this paper, N2 and CO2 adsorption, XPS and FTIR methods are used to study the pore structure evolution law and the change characteristics of the reactive structure of coal matrix under the effect of chemically activated autothermal oxidation. The synergistic microporous pore volume of chemical activation and auto thermal oxidation was 48.91 cm3/g−1 × 10-3, which was 28.96 % and 22.7 % higher than chemical activation and auto thermal oxidation respectively; The microporous specific surface area was 162.7 m2/g, which was 38 % and 26.6 % higher than chemical activation and auto thermal oxidation respectively. Activated coals expand and merge holes to a deeper degree after oxidative warming, and the complexity of the pore structure in the coal decreases; Highly reactive radicals in coal pores substantially promote the oxidation of coal structure, enhance the aromatic polymerization reaction of activated coal, and improve the maturity and aromaticity of activated coal, and the oxygen-containing functional groups in activated coal are negatively correlated with the oxidation temperature. The results of this study reveal the potential of the auto thermal oxidation-driven penetration enhancement method under chemical activation for coal seam gas penetration enhancement and extraction.

Effects of inorganic salt ions on the wettability of deep coal seams: Insights from experiments and molecular simulations

Coal wettability plays a significant role in the efficiency of coalbed methane (CBM) extraction since it determines fluid transport and distribution. Deep coal seam aqueous fluids have a high salinity. Consequently, coal wettability experiments utilizing fresh water are not representative for those coal seams. Therefore, the effect of aqueous fluids with inorganic salt ions on coal wettability needs to be investigated. In this study, both experiments and molecular simulations are conducted to unravel the wetting behaviour and mechanisms of saline fluids on coal surfaces, investigating different fluid salt concentrations, temperatures, valence states, and coal pore sizes. The results reveal that a more saline fluid displays a more hydrophobic behaviour on the coal surface. Microscopically, water molecules have a tendency to assemble around cations and anions as hydration shells and gradually detach from the coal surface, leading to a gradual transition from hydrophilicity to hydrophobicity. A change in wettability with fluid salinity can be caused by two mechanisms. On the one hand, an increase in ion concentration constrains the diffusion of water molecules through hydration, weakening the mobility of water molecules. On the other hand, a higher ion concentration weakens the interaction between water molecules and oxygen-containing functional groups, reducing the affinity of the water molecules with the pore wall. Moreover, at higher temperatures, ion valence states, and for coal with smaller pores, the impact of inorganic salt ions in fluids on the wetting behaviour of coal surfaces becomes more significant. The coal wettability influences the capillary pressure of coal pores. This study provides new insights on the effect of saline fluids on coal wettability which may help address challenges of fracturing fluid loss and water-blocking in CBM production activities.

Study on full-scale pores characterization and heterogeneity of coal based on low-temperature nitrogen adsorption and low-field nuclear magnetic resonance experiments

The characteristics and heterogeneity of coal pores are crucial for understanding the production mechanism of coalbed methane (CBM). In this study, coal samples with varying degrees of metamorphism (0.58% ≤ RO, max ≤ 3.44%) were collected. The characteristics of pore development and the heterogeneous properties of pores were revealed through low-temperature nitrogen adsorption (LTNA) and low-field nuclear magnetic resonance (NMR) experiments. The results indicate that pores with varying diameters exhibit favorable development in low-rank coals, along with favorable pores connectivity. The micropores composition of middle-rank coals was found to be 73.56%, however, the connectivity among transitional, meso, and macropores was observed to be poor. In high-rank coals, the proportion of micropores was 92.74%, with numerous micropores being closed or semi-closed. This resulted in inferior connectivity between micropores and transitional pores. As coal metamorphism progressed, the DL1 (characterizing the roughness of adsorption pores (AP) surface, ranging from 2.13 to 2.45) and DL2 (characterizing the complexity of AP structure, ranging from 2.56 to 2.77) initially decreased and then increased, whereas the DN (characterizing the heterogeneity of seepage pores (SP), ranging from 2.92 to 2.95) consistently improved. Furthermore, the roughness of pore surface and the complexity of pore structure in AP increased as the specific surface area and volume of pores increased. On the contrary, as the SP content increased, the uniformity of the pore structure improved. When the volume of SP remained constant, the complexity of the pore structure decreased due to increased pore connectivity.

Pore Size Distribution and Fractal Characteristics of Deep Coal in the Daning-Jixian Block on the Eastern Margin of the Ordos Basin.

Important breakthroughs have recently been achieved in deep coalbed methane (CBM) exploration and development in regions such as the eastern margin of the Ordos Basin, China. Investigating the development characteristics of various-scale pores in deep coalbeds is of great significance for resource assessment and selection of favorable zones for CBM exploration. Herein, six deep coal samples were selected from the Shanxi and Taiyuan Formations in the Daning-Jixian block on the eastern margin of the Ordos Basin. Low-pressure CO2/N2 adsorption (LP-CO2/N2GA) and high-pressure mercury intrusion (HPMI) methods were employed to analyze pore volume, specific surface area, and pore size distribution, thereby evaluating the full-scale pore characteristics. Furthermore, the fractal dimension characteristics of deep coal rock pores were elucidated, revealing the influence of pore structure, burial depth, and coal composition. The results indicate that micropores in deep coal rocks have the highest volume and specific surface area proportions, while mesopores have the smallest volume proportion, and macropores make the least contribution to the total specific surface area. The V-S, Frenkel-Halsey-Hill, and Sierpinski models were suitable for calculating the fractal dimensions of micropores, mesopores, and macropores with LP-CO2GA, LP-N2GA, and HPMI experimental data, respectively. Other than the relatively smaller mesopore fractal dimension of samples 20-8 and 20-10, the micropore, mesopore, and macropore fractal dimensions successively increased in the other four samples. The comprehensive fractal dimension, which exhibited a decreasing trend with increasing pore volume and specific surface area, was negatively correlated with burial depth, mineral and moisture contents, and ash and volatile component yields, while it was positively correlated with vitrinite and fixed carbon contents.

Effect of Coal Rank and Coal Facies on Nanopore-Fracture Structure Heterogeneity in Middle-Rank Coal Reservoirs.

Considerable variations in microscopic and industrial components (ash, moisture, volatile matter, etc.) have been reported within identical coal seams. These disparities in coal quality and pore structure within the same coal seam profoundly affect the drainage of deep coalbed methane (DCBM). This study focuses on 22 coal samples collected from two wells in the Benxi Formation of the central and eastern parts of the Ordos Basin. First, the coal facies were determined for all samples using submicroscopic components, and then, the adsorption pore and seepage pore structures were studied through CO2/N2 adsorption and mercury intrusive tests. Subsequently, the study delves into the correlation between coal rank, coal facies, and the distribution of the pore structures across various pore sizes, elucidating the primary controlling factors influenced by coal rank and coal phase. The results are as follows: (1) For a given coal seam, R o,max exhibits minimal variation among the samples, which suggests R o,max is not the primary factor affecting pore structure. Conversely, the ash content occupies the pore space, thereby revealing a negative correlation between the ash content and adsorption pore volume (PV). (2) On the basis of the texture preservation index (TPI) and gelatification index (GI), coal facies were classified into moist forest swamp facies (type A), moist herbaceous swamp facies (type B), and water-covered herbaceous swamp facies (type C). Type A is characterized by higher TPI, lower GI, and ash content, whereas type C exhibits lower TPI, higher GI, and ash content. (3) Type A samples, with the lowest ash content, display larger PV and specific surface area (SSA) compared with type B, while type C has the lowest values. Type C, with the highest vitrinite content, predominantly consists of semibright and bright coal, prone to microcracks, which results in a higher seepage PV compared with types A and B. (4) The coal facies represent variations in ash content and microscopic components, which significantly impacts both adsorption and seepage pores. Moist forest swamp facies samples are characterized by micropore development and the highest content of adsorbed gas. Herbaceous swamp facies samples display macropore development and the highest content of free gas.

Effects of supercritical CO2 fluids on pore structure and fractal characteristics of bituminous coal

Enhanced coalbed methane recovery with CO2 coal seam storage (CO2-ECBM) technology is an important way to achieve China's strategic goals of carbon peak and carbon neutrality. Presently, to date there has been rarely research conducted on the effect of coal sample scale on pore structure under supercritical CO2 (ScCO2) fluids. In this study, a high-pressure geological environment simulation system was adopted to analyze coal samples of different scales for ScCO2 saturation. Subsequently, low-pressure nitrogen gas adsorption (LP-N2GA), mercury intrusion porosimetry (MIP), and low-field nuclear magnetic resonance (LF-NMR) were used to analyze the pore structure and fractal dimension changes in saturated coal samples at different scales. The experimental results show that the mesopore ratios of cylindrical and granular coal decrease by an average of 1.68% and 2.30%, respectively, after the saturation of ScCO2. The proportion of macropores in cylindrical coal increased by an average of 5.50% after ScCO2 saturation, while the proportion of macropores in granular coal changed by 176.86% compared to cylindrical coal. The fractal dimension of the ScCO2 saturated coal samples obtained with LP-N2GA, MIP, and LF-NMR all show a decreasing trend, again confirming the modification of the coal pore surface by ScCO2. Finally, a conceptual model is presented to analyze the mechanism of the effect of coal sample scale on the pore structure under ScCO2. The difference in the transport paths of ScCO2 molecules at different coal scales is the main reason for the difference in the evolution of the pore structure. In addition, the impact of the amount of adsorption obtained in the laboratory using coal samples of different scales on the assessment of the CO2 storage capacity was discussed. Therefore, the results of this study are expected to provide a reference for the CO2 storage capacity assessment of the CO2-ECBM project.

Scanning Electron Microscope Micromorphology Characterization of Coal Samples from No.3 Coal Seam in Chengzhuang Mine Area

Matrix pores in coal reservoirs are important storage sites for adsorbed and free state CBM, and their adsorption amount is closely related to the development of pores and pore structure characteristics in coal. In this paper, coal samples from No. 3 coal seam of Chengzhuang coal mine were taken as the research object, and the microscopic morphology of coal matrix pore fissures was studied by scanning electron microscope. The results found that: in the coal samples of the study area, the pneumatic pores and dissolution pores are especially developed, and more fissures are developed at the same time due to the influence of tectonic action on the coal seam. The coal samples of Chengzhuang No.3 coal seam have many kinds of pore morphologies, mainly including round, wedge-shaped, triangular, slit-shaped and some irregular pores. The morphology, size and distribution of these pores have an important influence on the physical and chemical properties of the coal seam, which provides an important experimental basis for further research on the physical and chemical properties of coal seams and their roles in CBM adsorption, transportation and mining processes.

A Fractal Adsorption Model on Methane in Coal with Temperature Effect Dependence

The traditional Langmuir equation displays drawback in accurately characterizing the methane adsorption behavior in coal, due to it assuming the uniform surface of coal pores. Additionally, the decay law of gas adsorption capacity with an increasing coal reservoir temperature remains unknown. In this study, the fractal adsorption model is proposed based on the fractal dimension (Df) of coal pores and the attenuation coefficient (n) of the adsorption capacity. The principles and methods of this fractal adsorption model are deduced and summarized in detail. The results show that the pore structures of the two coal samples exhibit obvious fractal characteristics, with the values of fractal dimensions (Df) being 2.6279 and 2.93. The values of adsorption capacity attenuation coefficients (n) are estimated as −0.006 and −0.004 by the adsorption experiments with different temperatures. The proposed fractal adsorption model presents a greater theoretical significance and higher accuracy than that of the Langmuir equation. The accuracy of the fractal adsorption model with temperature effect dependence is verified, establishing a prediction method for methane adsorption capacity in deep coal reservoirs. This study can serve as a theoretical foundation for coalbed methane exploration and development, as well as provide valuable insights for unconventional natural gas exploitation.

Pore Size Distribution and Pore Connectivity Response Characteristics of Anthracite Injected with Carbon Dioxide in Different Phases

The adsorption of CO2 changes the pore content in coal. After CO2 adsorption, pore content in coal shows a trend of first increasing, then decreasing and then increasing. CO2 adsorption pressure, CO2 adsorption time and CO2-H2O slightly change the variation range of pore size, but the trend remains unchanged. For example, in the case of CO2 adsorption for 72 hours, the pore content of 2~10nm and >700nm is generally increased, and the pore content of 10~700nm is significantly reduced. In the case of 144 hours of CO2 adsorption, the pore content of 2~20nm and >700nm generally increased, and the pore content of 20~700nm significantly decreased. When water +CO2 was adsorbed for 144 hours, the pore content of 2~12nm and >700nm generally increased, and the pore content of 12~700nm generally decreased. The coal samples show a certain lag of mercury removal before and after CO2, which indicates that there are some ineffective pores in the coal. The mercury removal efficiency of the coal sample after CO2 adsorption is generally lower than that of the dry sample, indicating that a large number of inefficiently connected pores are generated after CO2 adsorption. That is, CO2 does increase a large number of pores in coal, but the pores are mainly non-connected pores, that is, CO2 has a weak improvement in the connectivity of the pores in coal

Application and Research of Multi-Scale Dynamic Diffusion Permeability Model in Different Coal Ranks

The spatial scale of coal mining and rock layer control is distributed between 10-10 and 107, spanning 17 orders of magnitude. There are multi-scale pore cracks from millimeter to micro and nanometers in the coal, and the pore size can reach one million times, which makes the coal permeability also show a multi-scale characteristics of millions and time. For the permeability of the coal seam, it is necessary to test and analyze the distribution of micro and nano pores and cracks in the coal [1]. CBM mining is accompanied by the dynamic change of adsorption pressure in coal and the complex deformation of multi-scale heterogeneous structure of coal, and the multi-scale deformation feature of pore fissure structure plays an important role in the transport of methane [2]. However, existing studies focus on treating coal as a homogeneous isotropic medium, without considering the influence of coal multiscale pore structure on permeability. The variation of heterogeneous degree of coal structure with structural scale and its influence on dynamic diffusion and permeability are rarely reported. Li Zhiqiang et al. [3] conducted an experimental-model-mechanism study of CBM gas micro-nano series multi-scale dynamic diffusion permeability. On this basis, the dynamic diffusion and permeability of different coal scales.

New insight into CH4 adsorption mechanism in coal based on modeling analysis of different adsorption theories

Based on the principle of Gibbs excess adsorption measurement, high-pressure isothermal adsorption and desorption experiments of CH4 were performed on fractured coal samples, and CH4 adsorption phase density was obtained by the intercept method according to the experimental data. The Langmuir model, BET model, DA model, and their corrected model, the supercritical adsorption models were used for the fitting analysis of CH4 excess adsorption capacity. A hybrid model of different adsorption theories was developed based on CH4 adsorption phase density and theoretical adsorption capacity predicted by the intercept method. The transition in the occurrence pattern and control mechanism of CH4 in coal were investigated. The fitting of the Langmuir model, BET model, DA model, and their corrected models, the SBET model, the SDA model, and the SDA-L model to the CH4 excess adsorption capacity in coal shows that all multilayer adsorption theory model is not suitable for fitting the CH4 excess adsorption capacity in coal. The uncorrected Langmuir model and DA model are only suitable for fitting the CH4 excess adsorption capacity in coal in the low-pressure stage (pressure below 10 MPa). The corrected Langmuir model, the corrected DA model, the SDA model, and the SDA-L model can be used to fit the CH4 excess adsorption capacity in coal, but they show some bias in predicting the theoretical CH4 adsorption capacity and the CH4 adsorption phase density. The fitting of the three main adsorption theory models suggests that there may be simultaneous monolayer adsorption and microporous filling adsorption. The fitting results of the hybrid model based on monolayer adsorption and microporous filling adsorption theory depend on showing that with the increase of temperature, the occurrence of CH4 in coal gradually changes from mainly monolayer adsorption to mainly microporous filling adsorption, and the transformation of occurrence mode is related to temperature and the development of pores in coal.

The Response Characteristics of Pore Volume and Specific Surface Area of Different Phases of Carbon Dioxide Injection into Anthracite Coal Pores

After the coal sample reacts with CO2, the porosity and pore volume change. Compared with dry raw coal sample, the porosity of coal after CO2 adsorption is increased. With the increase of CO2 injection pressure, the porosity of coal will gradually increase, especially when CO2 enters the supercritical phase. In terms of pore capacity of coal samples, CO2 adsorption time, CO2 adsorption pressure and CO2-H2O treatment do not show obvious regularity in the changes of pore capacity of coal samples, but in general, the increase of CO2 adsorption pressure and CO2-H2O treatment have a positive effect on the increase of pore capacity of coal samples, especially ScCO2-H2O. The mesoporous pore capacity increased by 137.1%. In terms of macro pore volume of coal sample, the change of macro pore volume after CO2 adsorption is more regular. Extending CO2 adsorption time, increasing CO2 adsorption pressure and CO2-H2O treatment all have positive effects on the increase of macro pore volume of coal sample. Under 8MPaScCO2-H2O, macro pore volume reaches a maximum of 0.016462ml/g. The increase also reached 130.4 percent. On the mesoporous specific surface area of coal sample, increasing CO2 adsorption pressure, prolongating CO2 adsorption time and CO2-H2O treatment can increase the mesoporous specific surface area of the sample. The mesoporous specific surface area of the sample at 8 MPa ScCO2-H2O is 0.631 m2/g, which reaches the maximum in this experiment, with an increase of 334.27%.

Influence of IDS and SDS compound ratio on pore structure and wettability of bituminous coal and anthracite: Experimental and simulation discussion

The pore and wetting characteristics of bituminous coal and anthracite treated with different proportions of tetrasodium iminodisuccinate (IDS) and sodium dodecyl sulfate (SDS) were measured. The pore structure characteristics of the coal were studied via low-temperature nitrogen adsorption analysis and the Frenkel-Halsey-Hill fractal theory. Zeta potential measurements were conducted to quantitatively analyze the degree of aggregation or dispersion of coal particles dissolved in solution. Changes in the coal surface morphology were observed and analyzed by scanning electron microscopy (SEM). The results show that when the ratio of IDS:SDS is 1:3, the specific surface area (SSA) values of bituminous coal and anthracite are the lowest. The composite reagent has a greater effect on improving the pore structure of anthracite than bituminous coal. Compared with bituminous coal, anthracite has better pore connectivity and a wider pore network. The absolute zeta potentials of bituminous coal and anthracite reach the maximum at an IDS:SDS ratio of 1:3, with values of 91.08 mV and 91.35 mV, respectively. The electrostatic potential distributions of the molecules show that the advantages of the electrostatic potential difference with water molecules enable IDS and SDS molecules to effectively attract water molecules and form hydrogen bonds, thus revealing the optimization mechanism of coal wettability.

A Comprehensive Study on Methane Adsorption Capacities and Pore Characteristics of Coal Seams: Implications for Efficient Coalbed Methane Development in the Soma Basin, Türkiye

This study represents a comprehensive assessment of methane adsorption capacity and pore characteristics for the coal seams of the Soma Basin in Western Türkiye, with a focus on their implications for coalbed methane potential. Twenty-one exploration wells were utilized to obtain coal samples from the kP1 and kM2 coal seams in the Kınık coalfield of the Soma Basin. High-pressure methane adsorption experiments using the indirect gravimetric method were conducted to quantify the storage capacities of these coal seams. Results revealed a wide range of methane adsorption capacities, ranging from 10.5 to 28.3 m3/t (air-dry basis), indicating significant methane storage potential for the kP1 and kM2 coal seams. The gas contents, ranging from 1.1 to 4.3 m3/t (as-received basis), suggested that the coal seams were undersaturated. Low-pressure N2/CO2 adsorption tests, along with standard proximate and gross calorific value analyses, were performed to investigate the influence of coal quality and pore characteristics on methane adsorption capacities. The findings demonstrated correlations between coal quality parameters and adsorption capacity, with ash yield showing a moderately negative correlation and fixed carbon content and gross calorific values exhibiting moderately positive correlations. Microporosity was identified as the critical factor governing methane adsorption, with a strong positive correlation observed between micropore surface areas and volumes and adsorption capacity. These results highlight the significant methane storage capacities of the coal seams in the Soma Basin and underscore the importance of micropores in determining methane adsorption capacity. The findings provide valuable insights for optimizing methane extraction and utilization in the region and offer important considerations for reservoir characterization and development strategies in similar low-rank coal deposits.HighlightsExtensive evaluation of methane adsorption and pore characteristics in Soma Basin coals, uncovering substantial potential for coalbed methane.The study reveals a diverse range of methane adsorption capacities, indicating highly promising methane storage capabilities.Correlation between coal quality parameters and methane adsorption, offering valuable insights into gas storage influenced by coal composition.Emphasis on the crucial role of micropores in methane storage, underscoring their significance as primary adsorption sites.Practical implications for optimizing methane extraction and utilization, guiding reservoir development in low-rank coal deposits like Soma Basin.

Quantitative Characterization of Coal Shale Pores and Fractures Based on Combined High-Pressure Mercury Pressure and Low-Temperature N2/CO2 Adsorption Methods.

Coal shale gas is an important type of shale gas. The microscopic pore size distribution and pore structure characteristics of coal shale determine the macroscopic storage and transportation of coal shale gas. In order to quantitatively characterize the microscopic pore size distribution and pore structure properties of coal shale from a multiscale perspective, the pore size distribution and pore structure of coal shale specimens with different grain sizes were quantitatively characterized using low-temperature N2/CO2 adsorption and high-pressure mercuric pressure methods, taking the coal shale of high-gas mines of the Dongbaowei Mine in the Shuangyashan Basin as the object of study. The pore size distribution fractal pattern and pore structure characteristics of coal shale were quantitatively analyzed by the joint characterization method of the coal shale pore fractal theory and pore size distribution. Relevant experimental studies found the following: (1) The specific surface area and volume of coal shale pores and fractures decrease gradually with the increase in coal shale grain size. (2) The pore size distribution of coal shale has obvious fractal characteristics at the stages of high and low mercury pressures, and the demarcation point of medium and large pores is 55 nm. (3) The difference in pore and fracture structural parameters between coal shale with different grain sizes and the original rock specimens is relatively small, and it is feasible to study the general rules of gas adsorption, desorption, diffusion, and seepage in the pores and fractures of coal shale by using coal shale shapes instead of the original rock specimens.

Full-Scale Pore and Microfracture Characterization of Deep Coal Reservoirs: A Case Study of the Benxi Formation Coal in the Daning–Jixian Block, China

The pore-fracture structure of deep coal reservoirs is highly important for evaluating, exploring, and developing coalbed methane (CBM) resources. This study considers three coal samples from the DJ57 well in the Benxi Formation in the Daning–Jixian block on the eastern margin of the Ordos Basin as the research object. Based on the coal quality parameters of the coal samples, field emission scanning electron microscopy (FE-SEM), gas adsorption experiments, high-pressure mercury intrusion porosimetry (MIP), and microcomputed tomography (micro-CT) scanning were used to quantitatively characterize the nanoscale pores and microscale fractures in deep coal reservoirs and to evaluate the pore-fracture structure at different scales. The results reveal that the pore types in the Benxi Formation coal samples are diverse and include mainly organic matter (OM) pores, inorganic pores (intraparticle and interparticle pores), and microfractures. The organic pores are diverse in shape and mainly exhibit round, oval, and wedge shapes, while the microfractures exhibit slender stripes or serrated curves. The multiscale quantitative characterization of deep coal reservoir pores and fractures is based on a variety of pore characterization methods, and the pore and fracture volume distributions are mainly U-shaped, revealing the coexistence of micropores and microfractures. The volumes of micropores (0.3–2 nm), mesopores (2–50 nm), macropores (50 nm to 10 μm), and microfractures (>10 μm) account for 78.00%, 6.78%, 2.08%, and 13.14%, respectively, of the total pore volume (PV). Based on a full-scale pore-fracture splicing calculation, the total permeability of the Benxi Formation coal samples ranges from 5.77 to 28.22 mD. The observation results indicate that the microfractures are connected to each other, forming a network structure with strong connectivity. The microfractures are mainly associated with pore diameters>100 μm, accounting for approximately 95% of the total permeability. Moreover, micropores in deep coal reservoirs provide a large space for CBM adsorption, and microfractures enhance the seepage capacity of CBM.

Physicochemical Effect on Coal Pores of Hydrocarbon Chain Length and Mixing of Viscoelastic Surfactants in Clean Fracturing Fluids.

Clean fracturing fluids are environmentally friendly and could have broad applications in permeability enhancement of coal seams. The hydrophobic chain length of the viscoelastic surfactant (VES) and the mixing of VESs with different ionic types have marked effects on the performance of clean fracturing fluids. This paper analyzes the effects of the hydrocarbon chain length of VES and mixing of VESs with different ion types on the pores of coal and discusses the mechanisms controlling the pore changes from a physical and chemical perspective. We found that the coal samples treated with clean fracturing fluid B had the largest porosity change. Adding two methylene groups to the hydrocarbon chain of the cationic VES will increase clay swelling in coal treated with fracturing fluids. Adding 0.1 wt % cocoamidopropyl betaine (zwitterionic VES) to cationic VES fracturing fluids can reduce the extent of clay expansion induced by fracturing fluids. VES with a long hydrocarbon chain has a strong ability to remove kaolinite in hard coal, and the addition of zwitterion VES increases the ability of a clean fracturing fluid to remove kaolinite. These results provide theoretical guidance for the synthesis of new VES molecules and the design of new fracturing fluid formulations.

Construction of a microscopic molecular model of Hongqingliang long-flame coal for potential application in adsorption simulation

The present study aims to develop a tailored microscopic molecular model for analyzing the adsorption properties of Hongqingliang (HQL) coal. The chemical structure of the coal was thoroughly analyzed through the utilization of various techniques, including elemental analysis, Carbon-13 Nuclear Magnetic Resonance spectroscopy (13C NMR), X-ray Photoelectron Spectroscopy (XPS), and Fourier Transform Infrared Spectroscopy (FTIR). Subsequently, a three-dimensional periodic microscopic molecular model for HQL coal was constructed, followed by the development of a 4 nm fracture pore model. To validate the modeling approach, a comprehensive comparative analysis was conducted utilizing nitrogen adsorption isotherms at 77.35 K. The results indicate that the amorphous cell model constructed in Material Studio solely comprises micropores narrower than 1 nm, making it unsuitable for simulating coal's pore characteristics. Conversely, the nitrogen adsorption isotherm of the 4 nm fracture pore model matches actual results. Therefore, to analyze the pore adsorption characteristics of coal, it is necessary to utilize the amorphous cell model to construct a characteristic pore model for coal.

Study on the Change Law of Pore Structure and Permeability of Coal Under Chemical Permeability Enhancement Technology

In order to deeply explore the permeability enhancement effect and permeability enhancement mechanism of the three chemical permeability enhancement technologies, the pore structure change and permeability enhancement mechanism of anthracite coal were studied from the perspective of coal pore structure. The following conclusions are drawn: the permeation effect of the four solutions from large to small is 99% CS2, 10% NaClO, 8% HF, 8% The total pore volume of HCl and HCl solution decreases after acidification, the pore volume of HF solution increases after acidification, the number of pores in the coal sample after oxidation of NaClO solution increases, and the micropores are transformed into mesopores, and the pore volume of CS2 solution increases after extraction, and the pore expansion effect is better than that of the other three solutions, and the pore volume with a pore diameter greater than 2 nm affects the flow of gas between pores, and the larger the pore volume, the higher the permeability. The pores with a pore diameter of less than 2 nm have little correlation with the permeability of coal and rock, and the micropores do not affect the gas flow