Coal Pore Research Articles

Coal pore structure changes in upper protective seam after mining: Pingdingshan Shenma Group, Henan Province, China

Protective layer mining is a safe and effective method of gas control in coal mining; we should pay more attention to changes in the pore structure of coal in protected layers, but there is less research in this aspect. We analyzed the characteristics of different-size pores in coal samples affected and unaffected by a protective layer at a distance of 900 m from the open-off cut of the protected layer. The pore morphology, type, size, and distribution in the coal seams affected and unaffected by the protective layer were qualitatively studied via field-emission scanning electron microscopy. Mercury intrusion capillary pressure testing was applied to analyze differences in pores of the same size between coal seams affected and unaffected by the protective layer. Protective layer mining increases the pore sizes of irregular pores. The protective layer has a large effect on various pores at different distances from the open-off cut. Within 100–700 m of the open-off cut of the protected layer, the upper protective layer mining promotes the conversion of 1–10 μm pores to 8–200 μm pores. When the distance from the open-off cut is less than 100 m, the upper protective layer mining facilitates the transformation of 10–100-nm pores into 100 nm to 10 μm pores. At distances of 100–900 m from the open-off cut, the porosity of pores with sizes from 10 nm to 200 μm increases markedly (with a maximum increase of 14%). The results of this study provide a theoretical foundation for designing a protective layer, evaluating its effect, and the research of gas seepage law and gas emission.

Manufacture of porous carbon thin films for energy storage

In the present work, a technique to generate pores in Vulcan VCX-72 and VCX-72R commercial coal through the Hummers method and the elaboration of membranes is shown. The product obtained through the process was cleaned and dried, then SEM microscopy in which the generation of pores in the coal was observed. Membranes were elaborated using the material and were placed in an electrochemical cell in a set up of three electrodes immersed in an acid solution of H2SO4. Autolab® potentiostat and the Nova® software were used to carry out electrochemical tests that showed capacitive properties in the Hummers method’s coal.

NMRI online observation of coal fracture and pore structure evolution under confining pressure and axial compressive loads: A novel approach

Quantitative characterization of the spatial distribution, content and heterogeneity of fracture and pore structure (FPS) in coal reservoirs under confining pressures and axial compressive loads is significant for the engineering of coal bed methane. A novel online observation approach that combines nuclear magnetic resonance imaging with triaxial loading techniques is employed to achieve the visualization and full-scale quantitative characterization of the evolution of FPS in coals in the laboratory. The relationship between the stress states and FPS evolution was formulated. The results show that the spatial distribution of the FPS evolution process of coal samples can be divided into four stages: initial pore and fracture compaction closure, pore and fracture stable growth, pore and fracture unstable growth, and failure stages. As the deviatoric stress increases, the content of the adsorption pores, the heterogeneity of the adsorption space, and the gas adsorption capacity of coal samples gradually increase. In contrast, the seepage pore and fracture content as well as the permeability of coal samples decrease first and then increase. The heterogeneity of the seepage space of coal samples initially increases and then decreases. The maximum compression of seepage space and increase of adsorption space are 4.742% and 14.743%, respectively.

Study on the difference of pore structure of anthracite under different particle sizes using low-temperature nitrogen adsorption method.

The low-temperature nitrogen adsorption test was used to study anthracite from Jiulishan coal mine with different particle size ranges of 60-80 mesh, 150-200 mesh, and > 200 mesh. The adsorption isotherm, adsorption capacity, pore volume, pore specific surface area, and average pore diameter of coal samples were analyzed by BET and DFT models in order to study the influence of particle size on the pore structure of anthracite and determine the optimal range of particle size for low-temperature nitrogen adsorption test. The results indicate that the particle size plays a significant effect on the pore structure of anthracite and the adsorption capacity of soft coal is less affected by particle size, while hard coal is substantially affected by particle size. The adsorption capacity of hard coal with particle size of > 200 mesh is increased by 7 times when compared with the particle size of 60-80 mesh, indicating that the gas molecular mobility hindrance decline and pore connectivity improves with the decrease of particle size. The average pore diameter of hard coal decreases continuously from 3.1424 to 2.854nm, while that of soft coal expands from 2.8947 to 3.2515nm and then to 3.0362nm with the decrease of particle size. The effects of particle size on the pore surface area of soft and hard coal are concentrated within the < 10nm pore aperture. Effect of particle size on hard coal pore volume is mainly focused in the pore size < 10nm, whereas that of soft coal is primarily concentrated in the pore with aperture ranges of 2-100nm. When the particle sizes varies from 60-80 mesh to 150-200 mesh, the collapse of large pore of hard coal appears better than that of closed pore. When the particle size of hard coal reaches > 200 mesh, the collapse of closed pores and the damage to small pores are stronger than the collapse of large pores. The fractal dimensions with relative pressure of 0-0.20 and 0.20-0.995 are defined as D1 and D2, respectively, and when the fractal dimension D1 increases, the surface roughness and structural complexity of coal samples increase with the decrease of anthracite particle size, while the fractal dimension D2 shows the opposite trend, which indicates that anthracite of smaller particle size possess higher adsorption capacity. Therefore, 150-200 mesh is recommended as the preferred anthracite particle size in low-temperature nitrogen adsorption test.

Revealing the Effects of Water Imbibition on Gas Production in a Coalbed Matrix Using Affected Pore Pressure and Permeability

The effect of water imbibition on characteristics of coalbed methane reservoirs, such as permeability, gas occurrence state, and gas production, is controversial. According to the mechanism of imbibition, gas and water distribution in blind pores is reconfigured during the fracturing process. Therefore, a new comprehensive model of pore pressure and permeability, based on the perfect gas equation and the weighted superposition of viscous flow and Knudsen diffusion, was established for micro- and nanoscale blind pores during water drainage. Using the numerical simulation module in the Harmony software, the effects of imbibition on coal pore pressure, permeability, and gas production were analyzed. The results showed that (1) water imbibition can increase pore pressure and reduce permeability, and (2) water imbibition is not always deleterious to gas production and estimated ultimate reserve (EUR), when the imbibition is constant, the thicker water film is deleterious to coalbed methane wells; when the thickness of water film is constant, more imbibition is beneficial to gas production and EUR. This research is beneficial to optimize the operation of well shut-ins after fracturing and provides methods for optimizing key parameters of gas reservoirs and insights into understanding the production mechanism of coalbed methane wells.

Influence of sub-supercritical CO2 on pore structure and fractal characteristics of anthracite: An experimental study

The geological storage of CO2 in coal seams has become one of the most effective means to alleviate the greenhouse effect, and the choice of CO2 injection and storage pressure is very critical. In this study, the effects of sub-supercritical CO2 intrusion on coal pore structures were investigated by nuclear magnetic resonance (NMR) and low-pressure N2 adsorption method (LP-N2GA), and the causes were analyzed by X-ray diffraction (XRD), Fourier transform infrared (FTIR) and Raman spectroscopy. The results showed that total porosity increased by 1.25%, 4.53%, and 5.82%, respectively, and micro-minipore volume increased by 10.1%, 42.2%, and 42.7%, respectively, after anthracite was treated with 4, 8, and 12 MPa CO2. The reasons for the change of coal pore structure caused by CO2 intrusion mainly include mineral dissolution, extraction of functional groups, destruction of aromatic layers in the microcrystalline structure, and rearrangement of macromolecular structure. Additionally, supercritical CO2 possesses more of these capabilities than subcritical CO2, resulting in greater changes in coal pore structure. These findings guide us that the injection pressure of CO2 should be increased as much as possible within the scope of cost and technology and the storage pressure of CO2 near the critical pressure point should be selected.

Multiscale Fractal Characterization of Pore Structure for Coal in Different Rank Using Scanning Electron Microscopy and Mercury Intrusion Porosimetry

Multiscale fractal analysis of the pore system for coal is necessary to obtain more inner information. The techniques of Scanning Electron Microscopy (SEM) and Mercury Intrusion Porosimetry (MIP) are combined to characterize the pore structure of natural coal. A total of eight coal samples, of a different rank and coalification degree, are prepared for experiments. Methods of SEM image processing, piecewise curve-fitting and correction of intrusion data are adopted to obtain more useful results. According to the pore size range of the MIP probe, pores in coal are classified as seepage pore (pore size ≥ 1000 nm), transition pore (pore size ≥ 50 nm and &lt;1000 nm) and mesopore (pore size &lt; 50 nm). Variations of multi-scale fractal dimensions are studied from the perspective of coalification degree or coal rank. Fractal dimension from SEM data (D1) and fractal dimensions of seepage pore, transition pore and mesopore (D2, D′2 and D″2) from MIP data are calculated by fitting curves, and consequently correlations of those with volatile matter (Vdaf), pore volume and pore size are analyzed and discussed. The U-shape relationships between fractal dimensions (D1, D2 and D′2) and Vdaf are observed. Macropores are presented as the isolated clusters embedding in the network of smaller pores, and the difference of the order of magnitude of the pores’ size affects the connectivity between pores. Both the pore size and volume have a direct influence on multiscale fractal dimensions. Overall, multiscale fractal analysis is beneficial to explore the structure of natural coal.

Enhanced coalbed methane recovery by the modification of coal reservoir under the supercritical CO2 extraction and anaerobic digestion

Coal reservoir is considered as the ideal site for coalbed methane (CBM) occurrence, biomethanation and CO2 sequestration. Four coal samples with different ranks were selected to conduct supercritical CO2 (Sc-CO2) extraction and anaerobic digestion (AD). X-ray photoelectron spectroscopy, mercury intrusion, and isothermal adsorption experiments were performed on the raw coal samples and the remaining coal samples after Sc-CO2 and Sc–CO2–AD to analyze the mechanism of enhanced CBM recovery by reservoir modification. The results showed that coal pore volume and porosity are greatly increased after Sc-CO2 extraction. Meanwhile, the reduction of the oxygen-containing functional groups and the micropore specific surface area determined the weakening of the adsorption capacity of coal. After Sc–CO2–AD, the macropore volume and porosity of coal continued to increase, and the permeability of coal was effectively improved. And the oxygen-containing functional groups and micropore specific surface area continued to decrease, which determined that the adsorption capacity of coal for methane continued to decrease. Sc-CO2 and AD can mutually promote the modification of coal reservoirs. These effects are quite favorable for gas desorption, migration and production during CBM exploitation, which provide powerful supports for CO2 emission reduction, CO2 geological sequestration and CBM production enhancement.

Investigation on the influences of CO2 adsorption on the mechanical properties of anthracite by Brazilian splitting test

To understand CO2 sequestration in coal seam, the Brazilian splitting test is adopted to examine the effect of CO2 saturation pressure on anthracite mechanical properties, and the variations in the internal structure of coal are analyzed using X-ray diffraction and low-pressure N2 adsorption methods. The results show that as the CO2 adsorption pressure increases, the Brazilian splitting strength, Brazilian splitting modulus, absorbed energy, and Brittleness index of anthracite decrease by 11.5%–49.2%, 11.0%–44.2%, 17%–42.3%, and 6.7%–49.3%, respectively, while the BET-SSA, DFT and BJH pore volumes increase by 60.3%–225.1%, 11.6%–61.3% and 10.1%–42.7%, respectively. Moreover, the influence of supercritical CO2 on coal pore structure and mechanical property is more prominent than that of subcritical CO2. The mineral dissolution, organic matter extraction and molecular structure rearrangement caused by CO2 intrusion develop the pore structure of anthracite, which resultes in the degradation of coal mechanical properties. In addition, the swelling, plasticity, and ‘Rehbinder effect’ caused by CO2 adsorption contribute to the mechanical damage of anthracite. Due to the remarkable impact of high-pressure CO2 on the pore structure and mechanical properties of coal, the critical point pressure of CO2 should be used in the storage process to ensure enhanced CO2 storage capacity while reducing storage risks.

Experimental investigation of erosion effect on microstructure and oxidation characteristics of long-flame coal

To study the effect of erosion on coal microstructure and its oxidation characteristics, eroded coals were prepared by eroding coal samples in an acidic aqueous solution for 30 days, 90 days and 180 days, respectively. The microcrystalline structure, pore structure and active groups distribution of coal samples were studied using X-ray diffractometer, low-temperature N2 adsorption and infrared spectra. Temperature-programmed experiment was used to analyze the oxidation characteristics of raw coal and eroded coals. The results reflected that the number of aromatic layers and coalification degree of eroded coal are reduced compared with raw coal. The average pore diameter increases, which promotes oxygen transportation in coal pores. Under the erosion effects, the content of active groups such as hydroxyl (–OH), methylene (–CH2), and methyl (–CH3) in coal increases. Eroded coals, especially the coal eroded for 180-day demonstrate more oxygen consumption and CO production than raw coal during the oxidation process, and their apparent activation energies decrease. Besides, the enhancement mechanism of eroded coal's oxidation activity is expounded from the aspects of the increase of active sites and the improvement of pore structure. In brief, erosion effect increases the spontaneous combustion tendency of coal.

Effect of Fragmentation on Pore Structures and Fractal Characteristics of Intact Coal and Deformed Coal Based on Experiments

Coal mining has to develop to deeper strata with the increasing demand for coal, which leads to frequent coal and gas outburst disasters. Especially, when there is a deformed coal in the coal seam, the outburst disasters are easier and more intense. Coal and gas outburst disasters can be promoted by the fragmentation. In order to clearly explain the reasons for the impact of fragmentation on deformed coal and gas outburst, mercury intrusion porosimetry, low-temperature liquid N2 adsorption test, and scanning electron microscope were carried out in this paper. The differences in pore characteristics of intact coal and deformed coal under different particle size conditions were analyzed from pore shapes, pore size distributions, and fractal dimensions. The damage paths of fragmentation on various kinds of pores in intact coal are as follows: macropores, mesopores, minipores, and micropores, and those on various kinds of pores in deformed coal are as follows: macropores, micropores, mesopores, and minipores. According to the fractal dimension theory and experimental results, the calculative results of fractal dimensions show that the fragmentation and tectonism reduce the surface roughness and structural complexity of macropores and mesopores and increase those of minipores and micropores in deformed coal. Finally, combined with the coupling effect of fragmentation and tectonism on pores, the reason of deformed coal with high crushing degree in fields of outburst was explained. The research provides the basis for the prevention of coal and gas outburst.

Research on the evolution of pore and fracture structures during spontaneous combustion of coal based on CT 3D reconstruction

To study the evolution law of the pore and fracture structures during the spontaneous combustion of coal, a self-built high-temperature tube furnace was used in the experiment to heat the coal samples at different temperatures and gas atmospheres. The coal samples were scanned by Xray-CT technology, and the three-dimensional (3D) pore structure and equivalent pore network model of coal samples were extracted by CT 3D reconstruction technology. In the process of heating at 25–200 °C, the porosity and fractal dimension were more significantly developed for coal samples heat-treated in an air atmosphere than those for the coal samples heat-treated in nitrogen atmosphere. The connectivity and permeability of coal samples were investigated, and it was found that the connectivity of pore and fracture structures of coal samples heated in the air atmosphere was better. However, the coal samples heated after nitrogen injection had weak connectivity and permeability. At this temperature stage, the nitrogen injection into the goaf will have a stronger inhibitory ability to develop coal sample pore and fracture structures and a more obvious weakening of the permeability. Therefore, oxygen circulation inside the coal sample may be suppressed, which is more conducive to preventing and controlling coal spontaneous combustion.

Research on the Effect of Freeze-Thaw Cycles at Different Temperatures on the Pore Structure of Water-Saturated Coal Samples.

To study the pore structure transformation of coal at different temperatures, freeze–thaw cycle experiments at different temperature intervals (20 to −20 °C, 20 to −40 °C, 20 to −196 °C) were carried out. The low-field nuclear magnetic resonance equipment was used to characterize the peak area, pore size distribution, and pore number of each group of coal samples. The pore transformation effect of coal samples at different temperature intervals was compared, and the change characteristics of the pore structure of coal samples under the freeze–thaw action were explored. The research shows that the freeze–thaw cycles at different freezing temperature intervals have obvious differences in the effect of coal pore transformation. The area of each peak spectrum in the T2 distribution curve of coal samples increased significantly under the action of freeze–thaw cycles in different freezing temperature intervals. The increased value of the number of mesopores and macropores shows the phenomenon of “first increase and then decrease” with the increase of the temperature difference. There is a quadratic function relationship between the temperature difference in the freezing temperature interval and the proportion change rate of the adsorption pore or seepage pore. The continuous increase of the temperature difference has a certain marginal effect on the proportion change rate of seepage pores and adsorption pores in coal.

Characterization of Coal Pore Structure and Matrix Compressibility by Water Vapor Injection

Characterization of Coal Pore Structure and Matrix Compressibility by Water Vapor Injection

Coupled Seepage Mechanics Model of Coal Containing Methane Based on Pore Structure Fractal Features

The paper applies fractal theory to the structure of fractal coal pores and calculates the fractal dimension and integrated fractal dimension for each pore section &gt;100 nm, 100 nm &gt; d &gt; 5.25 nm, and &lt;2 nm. In the experiment, we performed the full stress–strain-seepage experiment of methane-bearing coal, revealed the deformation–seepage characteristics of methane-bearing coal under load, and deduced the dynamic prediction mechanical model of methane-bearing coal permeability based on pore heterogeneity, followed by practical verification. The results show that the permeability change in methane-bearing coal is an external manifestation of coal pore deformation, and the two are closely related and affected by changes in the effective stress coefficient. The derived fractal-deformation-coupled methane permeability mechanics model based on coal pore heterogeneity has high accuracy, a general expression for the stress–strain-permeability model based on coal heterogeneity is given, and the fractal Langmuir model is verified to be highly accurate (&gt;0.9) and can be used for coal reservoir permeability prediction.

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.

Study on the effect of coal microscopic pore structure to its spontaneous combustion tendency

Coal is a porous medium. Due to the large number of pores in coal and the pore size on its surface, usually ranging from millimeter to nanometer, it is difficult to measure and analyze the microscopic pore structure of coal. In order to investigate the effect of the microscopic pore structure of coal on its spontaneous combustion tendency, coal samples from different coal mines of the Kailuan Group were selected as the research objects, and the data of the microscopic pore distribution of three different coal samples were measured by using mercury injection apparatus. The regression analysis of microscopic pore data of coal samples obtained in the mercury injection experiment shows that the correlation coefficients of the regression curves are all greater than 0.94 and the fitting degree is good, indicating that there is a good correlation between the pressure, mercury intake and pore size of the coal samples, indicating that the fractal dimension of pore distribution is very effective. The fractal dimension is generally between 2 and 3, indicating that the microscopic pores of coal samples have good fractal characteristics and meet the fractal theory to describe the distribution characteristics of microscopic pores in porous media. Through the simulation system of natural combustion of coal, the simulation experiment of temperature rise oxidation of different coal samples (gas coal, fat coal, and coke coal) was carried out, and the curve of the concentration of gas products CO and CO2 in the process of temperature rise and oxidation of coal samples was drawn in the experiment. The experimental results show the relationship between the distribution structure of coal pores and its spontaneous combustion tendency, and the coal with a good distribution dimension has a stronger combustion tendency

Characteristics and Evolution of Low-Rank Coal Pore Structure Around the First Coalification Jump: Case Study in Southeastern Junggar Basin

Characteristics and Evolution of Low-Rank Coal Pore Structure Around the First Coalification Jump: Case Study in Southeastern Junggar Basin

Effect of high-temperature environment of mine goaf on pore and fracture of coal

To study the evolution law of pores and fractures in the process of injecting nitrogen to prevent and extinguish fire in the goaf of coal mines. With the help of a self-built high-temperature tube furnace, the coal samples were processed under different temperatures and gas atmospheres. Through comprehensive use of electron microscope scanning, low temperature nitrogen adsorption, and 13C NMR, the pores and fractures development rules of coal before and after heat treatment were researched. The coal samples heated in nitrogen atmosphere produced many strip-shaped fractures and coal debris, while the coal samples heated in air atmosphere produced many pores and an evident melting phenomenon. In addition, the proportion of carboxyl groups in coal samples heated in nitrogen atmosphere was always greater than that of the coal heated in air atmosphere. The pore specific surface area and volume of coal samples heated under air atmosphere increased significantly, while the degree of development was significantly reduced in nitrogen atmosphere. Thereby weakening the coal-oxygen reaction to a certain extent and inhibiting the generation of harmful gases (CO, CH4, etc.), which is conducive to preventing coal spontaneous combustion disasters.

Exploration of pore structure evolution and damage mechanism of coal under liquid nitrogen freeze-thaw cycles

Liquid nitrogen (LN2) freeze-thaw cycle, a new waterless fracturing method, has a significant potential in coalbed methane (CBM) production. The ultra-low temperature of LN2 causes continuous damage to coal's pore structure and mechanical properties during the freeze-thaw cycles. A series of related experiments were conducted to investigate the damage mechanism of freeze-thaw cycles. The same coal samples were tested by ultrasonic tester, a nuclear magnetic resonance (NMR) instrument, and a comprehensive mechanics test system after freeze-thaw cycles. The experimental results show that the fracture gradually expanded with an increased width under the action of freeze-thaw cycles. Some secondary fractures were formed along the original fracture and connected to form a fracture network. As the number of freeze-thaw cycles increased, the micropores transformed to mesopores and macropores. The proportion of micropores gradually decreased, while mesopores and macropores continuously increased. The porosity of coal samples was positively correlated, while the wave velocity was negatively correlated with the number of freeze-thaw cycles.The uniaxial compressive strength and elastic modulus of coal samples showed a slight and then rapid decrease with increasing freeze-thaw cycles. Based on rock damage mechanics, wave velocity, elastic modulus, and two-parameter coupling models were chosen to define the damage of coal samples by freeze-thaw cycles. It was found that the predicted values of the coupled model were closest to the experimental values. The two-parameter coupling model has a good prediction of the uniaxial compressive strength of coal samples in freeze-thaw cycles without destroying the coal samples. The results would guide LN2 fracturing technology.