Pore Structure Of Coal Research Articles

Nanopore Structure of Different Rank Coals and Its Quantitative Characterization.

Based on gas adsorption theory, high-pressure mercury intrusion (HPMI), low-temperature liquid nitrogen gas adsorption (LT-N₂GA), CO₂ adsorption, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and small-angle X-ray scattering (SAXS) techniques were used to analyze the pore structures of six coal samples with different metamorphisms in terms of pore volume, specific surface area (SSA), pore size distribution (PSD) and pore shape. Combined with the gas adsorption constant a, the influence and mechanism of the pore structure of different coal ranks on gas adsorption capacity were analyzed. The results show that there are obvious differences in the pore structure of coals with different ranks, which leads to different adsorption capacities. To a large extent, the pore shapes observed by SEM are consistent with the LT-N₂GA isotherm analysis. The pore morphology of coal samples with different ranks is very different, indicating the heterogeneity among the coal surfaces. Adsorption analysis revealed that mesopore size distributions are multimodal and that the pore volume is mainly composed of mesopores of 2-15 nm. The adsorption capacity of the coal body micropores depends on the 0.6-0.9 nm and 1.5-2.0 nm aperture sections. The influence of coal rank on gas desorption and diffusion is mainly related to the difference in pore structure. The medium metamorphic coal sample spectra show that the number of peaks in the high-wavenumber segment is small and that it is greater in the high metamorphic coal. The absorption intensity of the C-H stretching vibration peak of naphthenic or aliphatic hydrocarbons varies significantly among the coal samples. Over a small range of angles, as the scattering angle increases, the scattering intensity of each coal sample gradually decreases, and as the degree of metamorphism increases, the scattering intensity gradually increases. That is, the degree of metamorphism of coal samples is directly proportional to the scattering intensity. The influence of coal rank on gas adsorption capacity is mainly related to the difference in pore structure. The gas adsorption capacity shows an asymmetric U-shaped relationship with coal rank. For higher rank coals (Vdaf < 15%), the gas adsorption consistently decreases significantly with increasing Vdaf. In the middle and low rank coal stages (Vdaf > 15%), it increases slowly with the increase of Vdaf. We believe that the results of this study will provide a theoretical basis and practical reference value for effectively evaluating coal-rock gas storage capacity, revealing the law of CBM enrichment and the development and utilization of CBM resources.

Nano/Micro Pore Structure and Fractal Characteristics of Baliancheng Coalfield in Hunchun Basin.

The pore structure characteristic is an important index to measure and evaluate the storage capacity and fracturing coal reservoir. The coal of Baliancheng coalfield in Hunchun Basin was selected for experiments including low temperature nitrogen adsorption method, Argon Ion milling Scanning Electron Microscopy (Ar-SEM), Nuclear Magnetic Resonance (NMR), X-ray diffraction method, quantitative mineral clay analysis method. The pore structure of coal was quantitatively characterized by means of fractal theory. Meanwhile, the influences of pores fractal dimension were discussed with experiment data. The results show that the organic pores in Baliancheng coalfield are mainly plant tissue pores, interparticle pores and gas pores, and the mineral pores are corrosion pores and clay mineral pores. There are mainly slit pore and wedge-shaped pore in curve I of Low temperature nitrogen adsorption. There are ink pores in curve II with characteristics of a large specific surface area and average pore diameter. The two peaks of NMR T₂ spectrum indicate that the adsorption pores are relatively developed and their connectivity is poor. The three peaks show the seepage pores and cracks well developed, which are beneficial to improve the porosity and permeability of coal reservoir. When the pore diameter is 2-100 nm, the fractal dimensions D1 and D₂ obtained by nitrogen adsorption experiment. there are positive correlations between water content and specific surface area and surface fractal dimension D1, The fractal dimension D₂ was positively and negatively correlated with ash content and average pore diameters respectively. The fractal dimensions DN1 and DN2 were obtained by using the NMR in the range of 0.1 μm˜10 μm. DN1 are positively correlated with specific surface area of adsorption pores. DN2 are positively correlated volume of seepage pores. The fractal dimension DM and dissolution hole fractal dimension Dc were calculated by SEM image method, respectively controlled by clay mineral and feldspar content. There is a remarkable positive correlation between D1 and DN1 and Langmuir volume of coal, so fractal dimension can effectively quantify the adsorption capacity of coal.

Investigation on the Structure and Fractal Characteristics of Nanopores in High-Rank Coal: Implications for the Methane Adsorption Capacity.

The structure and fractal characteristics of nanopores of high-rank coal were investigated using an approach that integrates N₂ adsorption and field emission scanning electron microscopy (FE-SEM). The results indicated that the high-rank coal of the Shanxi Formation has a complex pore-fracture network composed of organic matter pores, mineral-related pores, and microfractures. The pore type of high-rank coal tends to be complicated, and the main pore types are inkbottle pores and open pores, which are more conducive to methane enrichment. The Ro,max has a negative relationship with the total pore volume. In addition, the ash and inertinite contents show a positive correlation with the average pore size (APS), while the fixed carbon content exhibits a negative relationship with the APS. The pore structure of high-rank coal is controlled not only by the degree of metamorphism but also by coal composition, which leads to the variation in pore structure becoming more complicated. With the increase in coal metamorphism, high-rank coal with high amounts of fixed carbon content generally possesses a higher irregularity in pore structure. No obvious relationship was observed between D₂ and the coal components, which indicates that the pore structure, ash content, moisture content and other factors controlled by the metamorphism of coal have different effects on D₂ that lead to this inapparent relationship. A negative relationship exists between adsorption volume and D1, which indicates that the high irregularity of the pore structure is not conducive to methane absorption and that no obvious correlation exists between the adsorption volume and D₂. In the high-rank coal, the high D1 value represents the complexity and heterogeneity of the pore structure and represents a low adsorption affinity for methane molecules; in addition, D₂ has no effect on the methane adsorption capacity.

Experimental Study of Long-Term Gas Adsorption Effect on Pore Structures and Mechanical Strength Characteristics of Coal

Experimental Study of Long-Term Gas Adsorption Effect on Pore Structures and Mechanical Strength Characteristics of Coal

Small Angle X-ray Scattering Test of Hami Coal Sample Nanostructure Parameters with Gas Adsorbed Under Different Pressures.

The complexity and multiscale structure of coal pores significantly influence the gas diffusion and seepage characteristics of coal. To apply small angle X-ray scattering (SAXS) to study the coal pore structure parameters within the scale of 1-100 nm in the methane adsorption process, the X-ray window was optimized and a gas adsorption chamber was designed to interface with the small angle X-ray scattering platform. The fractal dimension and porosity of Hami coal samples under different methane pressures were studied using the small angle X-ray scattering platform and adsorption chamber. The surface and nanopore fractal information of the nanopores in coal were distinguished. The variation trends of the pores and surface fractal dimension with time under the same methane pressure were compared. The results indicate that the surface dimension changes from 2.56 to 2.75, and the extremum point may indicate that the primary nanopore structure is crushed by the adsorbed gas after approximately 15 minutes. This work clarifies that the fractal dimension can characterize the changes in nanopores in the process of gas adsorption by using SAXS. According to the fractal characteristics, the adsorption of gas in coal nanopores is summarized as four steps: expansion from adsorbance, deformation, crushing and recombination. The minimum porosity is 0.95% and the extreme value point is 1.47%. This work also shows that decrease in surface energy affect the porosity changes in nano-size pores. This work is of some significance to coalbed methane permeability improvement and gas extraction.

Influence of High-Pressure Water Injection on the Pore Structure of Anthracite Coal in Xinjing Coal Mine.

High-pressure water injection, as an important measure for coal and gas outburst prevention, is still under-researched, especially its mechanism on the coal pore structure. The anthracite samples taken from no. 3 coal seam in Xinjing coal mine were dried and injected with high-pressure water, after which their pore characteristics were studied by using mercury porosimetry (MP) and low-pressure N2 gas adsorption (LP-N2GA). The results of MP showed that after the water was injected into the coal samples, the pore volume and the pore size of samples increased, but the specific surface area (SSA) remained almost unchanged. It could be concluded from LP-N2GA experiments that after the high-pressure water injection, the SSA of coal samples reduced greatly, but their pore size increased significantly. Through detailed analysis, the mechanism of high-pressure water injection on the coal pore structure is described as follows: the pores within the samples fracture after high-pressure water injection and the diameter of pores becomes bigger, resulting in increases in both the pore volume and the pore size. In addition, water molecules injected will stay at the end of micropores, so there is almost no change in the SSA, as indicated by MP testing results. However, the SSA of coal samples decreased significantly in the LP-N2GA testing. This is because it is really difficult to evaporate water molecules staying in the micropores by heating because of the strong interaction between water and coal. This study is helpful to further understand the mechanism of high-pressure water injection on preventing coal and gas outburst at the microlevel.

Influence of tectonic evolution on pore structure and fractal characteristics of coal by low pressure gas adsorption

In the process of tectonic evolution, a comprehensive understanding of pore structure and fractal characteristics of coals with different tectonic degrees is critical. For this paper, the tectonic evolution process of coal is divided into two stages (Stage 1 and Stage 2) and N2 (77 K)/CO2 (273 K) adsorption methods were conducted. The results show that the original coal-bearing strata underwent multiple periods of stress action, the original stratified strip-band structure of the coal was destroyed gradually and formed pulverized coal in the Stage 1. In Stage 2, the original structure of pulverized coal disappeared, fractures in the coal body developed intensively and the internal structure changed greatly, forming tectonic coal with low strength and weak cohesion. The mesopore, macropore volumes and BET specific surface area of coal increase gradually during the tectonic evolution due to the closed and open pores that are connected by constricted pore openings may be destroyed in the Stage 1, and some chemical changes may have occurred in Stage 2, resulting in relatively developed pore structure. However, the micropore structure characteristics of coals first suffered from the closed pores being opened and then collapsed in the Stage 1, and the changes of micropore were not significant in Stage 2. Moreover, the fractal dimension D1 of coal samples shows a general trend of increasing (from 2.075 to 2.359) and D2 decreases from 2.683 to 2.469 during the Stage 1 of tectonic evolution, illustrating the pore surface becomes rougher and the irregularity of pore structure gradually decreases. The Stage 2 of tectonic evolution shows the same results. The variations of pore structure characteristics of coal are of great significance for understanding desorption and diffusion characteristics of coal in the process of tectonic evolution.

Study for the Effect of Temperature on Methane Desorption Based on Thermodynamics and Kinetics.

Desorption hysteresis is important for primary gas production. Temperature may cause serious changes in the methane adsorption/desorption behaviors. In order to study the mechanism of methane desorption and desorption hysteresis, three sets of samples of long-flame coal, coking coal, and anthracite were collected, and experiments such as microscopic composition determination, liquid nitrogen adsorption, and isothermal adsorption/desorption were performed. From the perspectives of desorption kinetics, desorption thermodynamics, and methane occurrence state, the differences in methane and methane desorption characteristics and the desorption hysteresis mechanism are discussed. The results show that at the same temperature, anthracite (SH3#) has the largest saturated adsorption capacity and residual adsorption capacity, followed by coking coal (SGZ11#), and long-flame coal (DFS4#) has the smallest. As the temperature increases, the theoretical desorption rate and residual adsorption capacity of anthracite (SH3#) and coking coal (SGZ11#) will increase first and then decrease. Temperature and methane desorption do have positive effects, but temperature may have a threshold for promoting methane desorption. It is necessary to comprehensively consider the influence of temperature on the activation of gas molecules and the pore structure of coal. Under the premise of a certain temperature, as the pressure increases, the desorption hysteresis rate changes in a logarithmic downward trend, the methane desorption hysteresis rate in the low-pressure stage (P < 4 MPa) is large, and the methane desorption hysteresis rate in the high-pressure stage (P > 4 MPa) is lower; during the isobaric adsorption process, the adsorption capacity of anthracite (SH3#) increases the fastest, followed by SGZ11#, and that of DFS4# is the smallest. In the low-pressure stage (P < 4 MPa), the adsorption capacity increases significantly with the increase in pressure, but in the high-pressure stage (P > 4 MPa), the adsorption capacity does not change significantly with pressure, instead gradually stabilizes. Under the same pressure, the molecular free path of methane increases with temperature. Under the premise of constant temperature, in the low-pressure stage (0 < P < 4 MPa), when the pressure continues to decrease, the free path of methane molecules increases significantly, resulting in a decrease in diffusion capacity. In the high-pressure stage (4 < P < 8 MPa), when the pressure continues to decrease, the free path of methane molecules does not change significantly; the sample desorption process of three sets of samples DFS4#, SGZ11#, and SH3# occurs, and the intermediate adsorption heat is greater than the isometric adsorption heat during the adsorption process, indicating that the desorption process needs to continuously absorb heat from outside the system. The energy difference produced in the process of adsorption and desorption causes the desorption hysteresis effect. The greater the difference in the isometric heat value of adsorption, the more significant the hysteresis is.

The dynamic change of pore structure for low-rank coal under refined upgrading pretreatment temperatures

Pore structure characteristics are significant factor in the evaluation of the physical characteristics of low-rank coal. In this study, three low-rank coal samples were collected from the Xishanyao Formation, Santanghu Basin, and low-temperature liquid-nitrogen adsorption (LP-N2A) measurements were taken under various pretreatment temperatures. Owing to the continuous loss of water and volatile matter in low-rank coal, the total pore volume assumes a three-step profile with knee temperatures of 150 °C and 240 °C. However, the ash in the coal can protect the coal skeleton. Pore collapse mainly occurs for mesopores with aperture smaller than 20 nm. Mesopores with apertures smaller than 5 nm exhibit a continuous decrease in pore volume, whereas the pore volume of mesopores with apertures ranging from 5 to 10 nm increases at lower pretreatment temperatures (<150 °C) followed by a faint decrease. As for mesopores with apertures larger than 10 nm, the pore volume increases significantly when the pretreatment temperature reaches 300 °C. The pore structure of low-rank coal features a significant heating effect, the pretreatment temperature should not exceed 150 °C when the LP-N2A is used to evaluate the pore structure of low-rank coal to effectively evaluate the reservoir characteristics of low-rank coal.

Study on Microstructure Differences of Coal Samples before and after Loading

The microscopic pore structure of coal affects the content of adsorbed gas. The microstructure of coal sample before and after loading is different, which will affect the adsorption and permeability of coal seam gas. In order to study this difference, the authors carried out mercury intrusion experiments on coal containing different coal samples and used nondestructive nuclear magnetic resonance (NMR) techniques, scanning electron microscopy, and transmission electron microscopy, to study the microstructure of coal samples before and after loading. The experimental results show that the pores of coal samples are mainly micropores and small pores, and the mesopores and macropores are relatively few. The T2 spectrum area of the coal sample is significantly increased after loading, and the parallel-layer coal samples’ T2 spectrum area is 46735, which is 9112 more than the vertical layer coal samples. The T2 spectrum of the vertical coalbed of saturated water samples shows a three-peak shape, the peak of the T2 spectrum is 12692, and the parallel bedding shows a bimodal morphology. The peak area of the T2 spectrum is 11277. The permeability of the parallel bedding coal sample is good, and the coal sample exhibits anisotropic properties. The pores and cracks of the coal samples increased after loading, and the localized area of the coal sample collapsed and formed a fracture zone, which was not conducive to the occurrence of coal seam gas. Further explanation of the changes in the permeability of the coal sample before and after loading will affect the gas storage and transportation.

Mineralogical and pore structure of organic-rich deltaic shales and sub-bituminous coals from early Maastrichtian Mamu Formation, Anambra Basin, Nigeria

The Mamu Formation source rocks in the Anambra Basin are targets for shale gas reservoir. However, fundamental data on the critical pore structures for assessing the shale reservoir capabilities are lacking. In this study, the source rocks were investigated for their mineralogical compositions and pore structure characteristics by X-ray diffraction, accelerated surface area and porosimetry and field emission scanning electron microscopy (FE-SEM). The bulk mineralogy of the samples is dominated by kaolinite and quartz with average percentage composition of 52.0 and 41.5, respectively, suggesting that the rocks are potential candidates for fracking. The total average pore volume and specific surface area in the samples are 0.0421 cm3/g and 22.43 m2/g, respectively. The shale samples show higher amounts of total pore volumes relative to the sub-bituminous coals. No obvious organic pore is observed in the shale samples, but some inherited primary organic matter pores of plant origin exist in the sub-bituminous coals. Therefore, inorganic minerals pores are the major contributors to the pore development in the immature shales. The FE-SEM results reveal that the pore network in the shales is dominated by intraparticle pores and fractures. The observed well-developed fractures within the Mamu shales that may have resulted from tectonic events are capable of positively influencing the petrophysical properties of shales in the basin and by extension assist in shale gas exploration and development in the entire Lower Benue Trough.

Microscopic Pore Structure Characteristics and Methane Adsorption of Vitrain and Durain

During reservoir evaluation, the microscopic pore structure of low-rank coal is mainly characterized in order to study the coalbed methane diffuse and migration mechanisms and control. The low-rank coals are very different in pore type and size, so it is necessary to use various techniques to describe their pore structure. For vitrain and durain of the Coal Member of the Yan’an Formation from Huanglong Coalfield, their chemical composition and microscopic pore structure characteristics were studied, and the factors of influencing the pore size distribution (PSD) were explored. Obviously, vitrain and durain are different in chemical composition. Vitrain has higher moisture content, volatile yield, and vitrinite group content than durain. Vitrain and durain mainly contain vitrinite and inertinite, respectively. The pore structure characteristics (e.g., pore types and PSD) of vitrain and durain were systematically by mercury intrusion porosimetry (MIP), low-temperature nitrogen adsorption, and carbon dioxide (CO2) adsorption. The vitrain and durain samples with a micropore size of &lt;2 nm were mainly tested on their specific surface area (SSA) and pore volume (PV). The results show that microporous vitrain has larger SSA and PV than microporous durain, while mesoporous and macroporous vitrain has smaller SSA and PV than mesoporous and macroporous durain. SSA is very positively correlated with PV. The ash content is negatively correlated with SSA and PV. The ash content influences microporous vitrain more greatly than microporous durain, but mesoporous and macroporous durain more greatly than mesoporous and macroporous vitrain. SSA is positively correlated with the vitrinite content of durain and negatively correlated with the inertinite and exinite contents of durain. However, SSA is negatively correlated with the vitrinite and exinite contents of vitrain and positively correlated with the inertinite content of vitrain. Vitrain has higher methane adsorption capacity, desorption rate, and recovery ratio than durain. There are parameters that are obviously affected by the micropore characteristics.

Evaluation of liquid CO2 phase change fracturing effect on coal using fractal theory

As an alternative and safe method for coal reservoir stimulation, liquid CO2 phase change fracturing (LCPCF) technology has large potential for enhancing coalbed methane drainage. In order to gain clearer insights into the changing laws of pore structure and gas permeability for samples before and after LCPCF treatment, various methods including mercury intrusion porosimetry (MIP) test, gas emission index experiments, gas permeability analysis were employed in this study. Fractal theory was also adopted to evaluate the fracturing effect. Based on the results, compared with the raw coal, the average pore diameter of the treated coal is greatly enlarged and the total pore volumes are increased from 15.75% to 41.31%. The fractal dimension (D) varies in the range of 2.6414 to 2.8701 and 2.2899 to 2.4971 for the raw coals and treated samples, respectively. The reduced D values of treated samples indicate that LCPCF treatment enables the coal pore structure to become more regular and homogeneous. Both the gas content and gas emission index exhibit an inverse relationship with fractal dimension, but coal permeability demonstrates an exponential increasing trend with the decline of fractal dimension. New pore generation, pore transformation and mineral dissolution account for this enhancement of gas permeability. The change of fractal dimension not only reflects the pore alteration, but also represents the methane seepage capacity in coal. Fractal theory bridges the gap between pore structure and methane flow, and it is expected to be widely used in evaluating fracturing effect of coal seams.

Studies on the influence of moisture on the sorption and structural properties of hard coals

In the conducted research, the effect of moisture on CO2 sorption and transport processes and changes occurring in the coal pore structure were investigated in detail. The measurements were carried out under isothermal (313 K) and isobaric conditions (0.1−0.8 MPa) on the original device - a manometric sorptomat. Coal sorption capacities, Langmuir sorption isotherms, effective diffusion coefficients and structural parameters were analysed. The obtained sorption capacities of the tested samples relative to CO2 for pressures 0.1, 0.3 and 0.8 MPa confirmed that moisture significantly (even up to 6 times) reduces the sorption capacity of coal relative to CO2, and the largest reduction in sorption capacities was noted on coal with the lowest vitrinite reflectance. It was observed that the increase of the sorption equilibrium pressure decreases the effect of the reduction of the sorption capacity of coal under the influence of moisture, and above 1.0 MPa this effect disappears completely. The critical moisture content was estimated, above which there is no further reduction of the coal sorption capacity due to the increase in moisture content. It was observed that the influence of moisture on CO2 diffusion in its pore space is not clear. Structural analysis were carried out by low-pressure CO2 adsorption at 273 K and 0−0.1 MPa. After increasing the coal moisture content, the surface area decreased by 50 % and a decrease in the volume of available pores with diameters in the range of 0.5−0.6 nm was noted.

Study on the Wettability of a Composite Solution Based on Surface Structures of Coal.

In order to research the influence of composite surfactant solutions on the wetting ability of coal, the wettability parameters, pore structure parameters, and surface microscopic morphology were determined in this study. The results show that the wettability of the surfactant can be improved by adding NaCl. As the concentration of NaCl increased, the spreading coefficient of the NaCl–SDS composite solution increases; the spreading coefficient increased from −10.838 to −3.7624 mN/m. The surface free energy decreased from −2.031 to 3.670 J/mol × 103, and the adhesion work decreased from 113.802 to 55.058 mJ/m2. In terms of the pore structure and surface morphology of coal, the pore size will affect the wettability of coal. The contact angle reduces with pore size increase (R2 = 0.955). The surface pores of coal treated with the NaCl–SDS composite solution increase and the fracture connectivity is improved, which facilitated the solution to penetrate coal for wetting. The results of this paper have great practical significance for mining at the face to reduce coal dust pollution.

Quantification of pore modification in coals due to pulverization using synchrotron small angle X-ray scattering

Small angle X-ray scattering (SAXS) has been widely employed to characterize the pore structure of coal and other geomaterials. However, in the many previous experimental investigations by SAXS, the probed samples were not standardized in terms of shape and particle size, even for the same geomaterial. The influence of pulverization on the pore structure of coal is generally ignored by the researchers using SAXS for structure characterization. In this study, SAXS is used to investigate the effect of pulverizing on pore modification of virgin coal. Two kinds of coal with different ranks were selected and pulverized for SAXS measurements. Both powder samples (five particle size fractions) obtained by grinding and slice samples were investigated. The results show that the nano-pore structure of coal has been significantly modified due to pulverization. The sample pore volume per unit mass increases with the decrease of sample particle size. Pulverizing the slice sample into the minimum particle sample particle size of 0.075–0.125 mm leads to an increase in the pore volume per unit mass of coal by 39.53% and 18.29% for tested medium volatile bituminous coal and anthracite, respectively. Pulverization can reduce the volume of smaller pores (<20 nm) and increase the volume of larger pores (>20 nm). After grinding the original sample into the minimum particle size of 0.075–0.125 mm, the fractal dimension of the tested medium volatile bituminous coal and anthracite decreased by 0.266 and 0.179, respectively. Surface fractal dimension of pore decreases with the reduction in coal particle size. In the case of the powder coal sample, the increase in pore volume (especially for pore greater than 20 nm) and the reduction of fractal dimension are beneficial to the diffusion of gas in coal and promoting the desorption of residual gas. This study may provide a quantitative insight into the effect of pulverization on pore structure of coal sample. Moreover, it is of great significance for understanding the mechanism of coalbed methane storage and migration and is helpful for the selection of the sample size in SAXS experiments.

Structure feature and evolution mechanism of pores in different metamorphism and deformation coals

Coal pore structure plays a crucial role in the adsorption, migration and production of coalbed gas, and its feature and evolution are mainly controlled by metamorphism and deformation. In this study, the original and tectonically deformed structures of middle- and high-rank coal samples (Ro,max = 1.14–3.44%) were obtained through low-temperature nitrogen adsorption (LTNA), small-angle X-ray scattering (SAXS) and scanning electron microscopy (SEM) to analyze the pore structure of both open pores and closed pores. Moreover, we investigate the evolution features and mechanism of pore structure with different metamorphism and deformation degrees by comparing the pore structures of these samples with those of low-rank tectonically deformed coals (Ro,max = 0.88–0.96%). The differentiation and inheritance are summarized as the distinctive evolution feature of pore structure of coal with different metamorphism and deformation degree. The pore size of tectonically deformed coals decreases with increase in the degree of deformation under the action of the tectonic stresses. This phenomenon is in line with the metamorphism evolution phenomenon of the original structure that the macro-pores decrease and the micro-pores develop as the coal metamorphism degree increases. Tectonic stress has a dual effect on the transformation between closed pores and open pores of coal in the process of deformation. Closed pores increase is controlled by local uneven shrinkage of the matrix, reduction of pore throat caused by tectonic stress, and partial dislocation of open and semi-open pores. Larger closed pores may open more easily due to deformation caused by shear stress.

Multifractal analysis of coal pore structure based on NMR experiment: A new method for predicting T2 cutoff value

Nuclear magnetic resonance (NMR) is a commonly used method to quantitatively characterize the pore structure of coal. T2 cutoff value is closely related to the microscopic pore structure of coal reservoirs, which directly affects the calculation of porosity, permeability, movable and irreducible fluid saturation. In this study, the pore structure of four types of coal samples were studied based on NMR experiments and the multifractal characteristics of saturated coal samples were analyzed with multifractal theory. Furthermore, the correlation between T2 cutoff values and multifractal characteristic parameters were discussed. The results show that the pore structure characteristics of CZ, GD and GHS coal samples are basically the same, all of which are mainly micropores, with relatively less content of mesopores, macropores and microcracks, while PDS coal samples have relatively more content of mesopores, macropores and microcracks. Different types of coal samples have similar multifractal characteristics and all coal samples are dominated by micropores. There is an obvious linear relationship between T2 cutoff values and the multifractal characteristic parameters. Five models of T2 cutoff values changing with the multifractal characteristic parameters are obtained by linear fitting. The error range between T2 cutoff values calculated by the five models and the experimental values are within 0.1 ms, among which T2 cutoff values calculated by model 5 are highly consistent with the experimental values, indicating that model 5 can better predict T2 cutoff values. The results can provide theoretical reference for the characterization of coal pore structure and the prediction of T2 cutoff values.

Experimental Study on the Fractal Features and Permeability Characteristics of Low Metamorphic Coal Pore Structure under Thermal Damage

Underground coal gasification and exploitation of geothermal mine resources can effectively improve coal conversion and utilization efficiency, and the basic theory of the above technologies generally relies on the change law of the coal pore structure under thermal damage. Therefore, the influence mechanism of the development of the coal pore structure under thermal damage is analyzed by the nuclear magnetic resonance experiment, and the temperature-permeability fractal model is created. The results show that compared with microtransitional pores, the volume of meso-macropores in the coal body is more susceptible to an increase in temperature, which was most obvious at 200-300°C. During the heating process, the measured fractal dimension based on the T2 spectral distribution is between 2 and 3, indicating that the fractal characteristics did not disappear upon a change in external temperature. The temperature has a certain negative correlation with DmNMR, DMNMR, and DNMR, indicating that the complexity of the pore structure of the coal body decreased gradually with the increase of the temperature. Compared with the permeability calculated based on the theoretical permeability fractal model, the permeability obtained from the temperature-permeability fractal model has a similar increasing trend as the permeability measured by the NMR experiment when the temperature increases. The experimental study on pore structure and permeability characteristics of the low metamorphic coal under thermal damage provides a scientific theory for underground coal gasification and geothermal exploitation.

Effect of faults on the pore structure of coal and its resultant change on gas emission

Faults are one of the important factors influencing coal and gas outburst in outburst coal seams. In this paper, 3# and 8# coal seams in Xinjing Coal Mine were taken as the research objects in the hope of revealing the mechanism of how faults affect coal seam. The pore structure and its resultant change on gas emission of coal samples at different distances from the faults were studied by means of scanning electron microscope, nuclear magnetic resonance and adsorption/desorption experiments. The results show that the fault significantly influences the pore structure. The coal sample closer to the fault has a greater number of well-developed macropores and fractures as well as better connectivity between pores and fractures. Besides, compared with 8# coal seam, 3# coal seam which is more notably impacted by the fault possesses a more developed pore structure at the micron scale. As the distance from the fault increases, the gas adsorption volume declines while the desorption hysteresis coefficient grows on the whole. The coal sample closer to the fault adsorbs a larger volume of gas and meanwhile desorbs gas more easily, so it is more prone to coal and gas outburst. The initial velocity of gas emission falls gradually as the distance from the fault increases. The desorption hysteresis coefficient generally drops with the rise of the initial velocity of gas emission. Judged by the influence scope of the fault, 3# coal seam possesses a much higher outburst risk than 8# coal seam. The research results reveal the mechanism of how faults affect coal pore structure and gas sorption characteristics, which can provide a basis for the prevention and control of gas dynamic disasters in fault-containing coal seams.