Coal Pore Research Articles

An Experimental Investigation of the Gas Permeability of Tectonic Coal Mineral under Triaxial Loading Conditions

Underground coal mining of CH4 gas-rich tectonic coal seams often induces methane outburst disasters. Investigating gas permeability evolution in pores of the tectonic coal is vital to understanding the mechanism of gas outburst disasters. In this study, the triaxial loading–unloading stresses induced gas permeability evolutions in the briquette tectonic coal samples, which were studied by employing the triaxial-loading–gas-seepage test system. Specifically, effects of loading paths and initial gas pressures on the gas permeability of coal samples were analyzed. The results showed the following: (1) The gas permeability evolution of coal samples was correlated with the volumetric strain change during triaxial compression scenarios. In the initial compaction and elastic deformation stages, pores and cracks in the coal were compacted, resulting in a reduction in gas permeability in the coal body. However, after the yield stage, the gas permeability could be enhanced due to sample failure. (2) The gas permeability of the tectonic coal decreased as a negative exponential function with the increase in initial gas pressure, in which the permeability was decreased by 67.32% as the initial gas pressure increased from 0.3 MPa to 1.5 MPa. (3) Coal samples underwent a period of strain development before they began to fail during confining pressure releasing. After the stress releasing-induced yield stage, the coal sample was deformed and cracked, resulting in a quickly increase in gas permeability. With a further releasing process, failure of the sample occurred, and thus induced rapidly increasing gas permeability. These obtained results could provide foundations for gas outburst prevention in mining gas-rich tectonic coal seams.

Study on the Evolution Mechanism and Compression Effect of Coal Pores Subjected to Water Injection

Study on the Evolution Mechanism and Compression Effect of Coal Pores Subjected to Water Injection

Pore structure characterization and its significance for gas adsorption in coals: A comprehensive review

The coal reservoir has multifaceted and hierarchical characteristics influenced by heterogeneity of the pore and fracture networks. The coal pore including micropore, mesopores and macropores determines the critical migrate processes of gas molecules, especially for adsorption. Normally, coal pore structure refers to the pore shape, size distribution and interconnection of variable pores. Together with pore volume, pore size distribution, specific surface area and porosity. For coalbed methane, pore structure is the storage place, which largely controls the methane adsorption capacity of coal. Fluid injection, microscope observation and X-ray and spectroscopic analysis can be used to explore coal pore structure. Understanding the pore structure in varied rank coals is very challenging mostly due to the complex pore system leading to ambiguous petrophysical properties and gas performance. Among several important factors controlling the performance of coalbed methane, adsorption is the most crucial one, which affected by the pore aspects including pore volume, specific surface area, pore size distribution, pore shape and porosity. Therefore, accurate characterization of pore structure is coupled with advanced technologies, which resulted in the development of a wide range of methods. In this work, all the necessary technologies from imaging, segmentation, and modeling of the pores to various methods of gas adsorption evaluation are reviewed. This paper presents a critical review of all of the existing relevant and significant techniques and compares their performances together with their limitation with special reference to pore structure characterization. And the impacts of pore structure are thus reviewed. Finally, this review work proposes the current challenges and possible future research. • The coal pore classification scheme is systematically reviewed. • The pore characterizing techniques are summarized. • The influence of coalification on pore structure and its impact on CH 4 adsorption is analyzed. • The challenges and prospects on coal pore research are proposed.

Study on Adsorption Characteristics of Coal Reservoirs in Yangyi Mine

The method for studying the adsorption characteristics of the coal seam has the density method, the mercury-pressing method and the adsorption method. In this experiment, a low temperature nitrogen adsorption method was used to determine the relative data of the coal-like porosity by using the surface area meter TriStarI3020 to determine the arrangement of the nitrogen molecules on the surface of the coal-like samples. Before the beginning of the experiment, the samples of Yang dong coal reservoir were prepared, weighed, the devaporation time was set at 2h, and the temperature was kept at 105℃.The low temperature nitrogen adsorption experiment was carried out by TriStarII3020.The data of adsorption of nitrogen molecules by coal samples at low temperature are obtained by experiments. The gap characteristics of coal samples are studied by analyzing the characteristic folding points and relative relations of the generated linear diagram. Get coal samples under the condition of low temperature changing with pressure adsorption nitrogen molecular data changes, through the analysis of the linear graph of generated features that are characteristic of fold point and relative relationship to study the space characteristics of the coal sample. Through the analysis can determine the area mainly for both ends of the open pores in coal mine, the porosity of the thin neck bottle type, this research has great significance to the adsorption of gas in the coal reservoir, facilitate monitoring during the coal mining operations have an impact on safety production of the gas such as gas. Can get the conclusion: in the low relative pressure p/p°< 0.1 the surface of the coal samples are distributed in nitrogen molecular monolayer, with the increase of pressure surface of coal samples began to multilayer adsorption intensified will appear the characteristics of the molecular cohesion, condensation of a pressure value after the single nitrogen molecules adsorption quantity also presents the geometric growth tend to be infinite.The pore of coal is mainly based on open type and fine neck bottle, and there is a small number of open pores.

A small furnace for in-situ SAXS/WAXS study on medium temperature carbonisation of coal

The medium temperature carbonisation of coal refers to the process of decomposition of coal by heating to 700°C in absence of air. It is one of the important ways of coal utilisation. Small/wide angle X-ray scattering (SAXS/WAXS) can be used to study the pore and carbon matrix structure of coal. Recently, a small furnace for in-situ study of medium temperature carbonisation of coal by simultaneous or respective SAXS and WAXS was developed and integrated into the SAXS instrument at the 1W2A station of Beijing Synchrotron Radiation Facility (BSRF). A test on a kind of non-caking coal shows that the furnace works well, and the experimental result is reasonable. [Received: January 5, 2021; Accepted: March 2, 2021]

Microscopic pore structure changes in coal induced by a CO2-H2O reaction system

Geological carbon dioxide (CO 2 ) sequestration in deep coal beds is a potential technique for enhancing coal bed methane recovery. Since porosity and connectivity between pores or fractures are the most important parameters affecting mineral dissolution and percolation of coal bed methane, it is crucial to understand the pore structure changes in coal induced by CO 2 injection. To investigate the role that mineral reactions play in coal pore structure changes, three groups of CO 2 -H 2 O-coal interaction experiments were carried out in parallel at 40 °C and 5 MPa. Based on X-ray diffraction (XRD) analysis, the mineral compositions of coal samples after interaction exhibit significant changes, notably a marked decrease in calcite and dolomite. The pore type, pore size distribution, effective porosity, and spatial configuration of coal samples before and after interaction were studied through combined low-temperature N 2 adsorption and desorption (LTNAD), nuclear magnetic resonance spectroscopy (NMR), and X-ray computed tomography scanning (CT). The results show that new types of pores were formed and pore shapes became more complex due to mineral reactions. The number of macropores and fractures increased significantly, with the average pore diameter also increasing. Overall, the experiments show that CO 2 -H 2 O-coal interaction plays a positive role in pore structure modification, which can effectively enhance the coal bed methane recovery. • CO 2 sequestration in deep coal beds induced CO 2 – H 2 O- coal interaction. • CO 2 – H 2 O- coal interaction causes coal pore structure change. • The mineral reaction significantly increased the porosity and pore connectivity.

Influence of long-term CH4 and CO2 treatment on the pore structure and mechanical strength characteristics of Baijiao coal

In order to study the influence of adsorptive gas on coal pore structures and mechanical strength, coal specimens were placed in a low-pressure environment of CH4 or CO2 for 30 days to ensure that the specimens were saturated and fully interacted with the gas. Low-pressure nitrogen adsorption, scanning electron microscope, X-ray diffraction, and uniaxial compressive strength tests were employed to study coal's microstructure change and strength characteristics. The results show that after CH4 or CO2 treatment, the proportion of macropores increases by 38.87% and 22.89%, and that of mesopores decrease by 6.34% and 3.73%, respectively, which indicates the evolution of mesopores to macropores. The microcrystalline structure parameters change obviously after long-term gas adsorption treatment. The values of d002 and d100 increase, while Lc, La, and Mc's values both decrease, resulting in a loose coal structure. Besides, a conceptual model is proposed to explain the evolution mechanism of pore structures under the effect of long-term gas adsorption.

Experimental research on increasing effect of water injection in coal seam with chelating agent

ABSTRACT Coal seam water injection has a significant effect on the prevention and control of coal mine rock burst, and the pore characteristics of the coal seam is an important factor affecting the effect of coal seam water injection. With the deep mining of coal mine, the physical and mechanical properties of coal body change, the porosity of the coal seam decreases, and the injectability of the coal seam becomes worse. With the passage of time the minerals and CO2 in coal makes it more difficult to inject water. In order to obtain the reason why the water injection in coal seam becomes worse, the water injection additive with excellent performance is found, and a series of experiments on the impact tendency of coal body are carried out. The results show that the mine water used for coal seam water injection was liable to precipitate more ions, which may react with CO2 dissolved in water to form precipitates, plug coal pore and reduce the fluidity of water in the coal seam, resulting in poor water injection effect. Adding chelating agent IDS in aqueous solution can effectively prevent precipitation formed by the combination of easily precipitated ions and carbonate ions in it. At the same time, the chelating agent IDS can decompose precipitated ions from the surface of coal particles, change from insoluble to soluble, promote pore development, increase the effect of water injection and the pore diameter of the coal, and reduce hydrophobic groups on the coal surface. IDS can reduce the impact tendency of coal, and it is an environmental protection additive that can increase permeability. It can provide reference for prevention and control of composite power disasters by coal seam water injection.

Chemical Modification Effect of Compound Solutions of Surfactants with Acetic Acid on Coal Pores.

Coal seam pores are the major places for coalbed methane storage, diffusion, and seepage, and changes in the pore structure cause changes in the porosity. The porosity of coal seams can be effectively improved by applying strongly corrosive and oxidative chemical reagents to coal seam pores, but these reagents may pose threats to coal workers, corrode mining equipment, and pollute the environment. In this study, coal samples were treated with solutions compounded by acetic acid and anionic, cationic, and non-ionic surfactants. The variations of pores in coal samples after the compound modification of surfactants and acetic acid were investigated. Experimental methods of SEM, MIP, LTNA, PAC, and FTIR and fractal theory are applied in this work. The results reveal that the compound modification of surfactants and acetic acid conduces to the transformation of pore shape and affects a wider pore size range. The anionic and cationic surfactants can increase the hydrophilicity and can promote the connection of larger pores. The non-ionic surfactant reduces the hydrophilicity and capillary effect yet increases the porosity. Thus, it promotes the connection of pores and makes the pore surface smooth and the pore structure simple. Comparing the three kinds of surfactants, non-ionic surfactants are more conducive to coal seam pore reconstruction.

The Characteristics of Closed Pores in Coals With Different Ranks

The closed pores in coal seams influence the storage of coalbed methane. The investigation of closed pores characteristics for coals is of great significance in improving the production of coalbed methane and revealing the mechanism of coal and gas outburst. However, due to limitations in analytical techniques, the characteristics and evolution mechanism of closed pores in coals with different ranks are not sufficiently understood. In this paper, eight coal samples with different ranks were collected and characterized by small-angle X-ray scattering (SAXS) and low-temperature nitrogen adsorption (LTNA). The open and closed pores of coals with various ranks were studied, and the mechanism for evolution of closed pores during coalification was proposed. The results show that among eight coal samples with different ranks, the closed porosity of low-metamorphic coals is relatively lower, the closed porosity of medium-metamorphic coals is in the middle, and the closed porosity of high-metamorphic coals is relatively higher. The change in closed porosity for coals with different ranks may be related to varieties of the molecular structure of coals. The low-metamorphic coals have more disordered arrangement of molecular structure and easily form connected pores. Therefore, the closed porosity in low-metamorphic coals is low. The aromatization of medium-metamorphic coals turns aliphatic chains into closed aromatic rings, and the closed porosity of these coals also increases. When coals reach a high degree of metamorphism, polycondensation compacts the coal macromolecular structure, providing for easy formation of closed pores between aromatic condensed rings, so the closed porosity is obviously increased in high-metamorphic coals. This study has dual significance in advancing the understanding of open and closed pores in coals and the mechanism of coal and gas outburst.

Coal Macrolithotype Distribution and Its Genetic Analyses in the Deep Jiaozuo Coalfield Using Geophysical Logging Data

Coal macrolithotypes are closely correlated with coal macerals and pore–fracture structures, which greatly influence the changes in gas content and the coal structure. Traditional macrolithotype identification in coalbed methane (CBM) wells mostly depends on core drilling observation, which is expensive, time-consuming, and difficult for broken core extraction. Geophysical logging is a quick and effective method to address this issue. We obtained coal cores from 75 wells in the deep regions of the Jiaozuo Coalfield, northern China, quantitatively analyzed the logging cutoff number corresponding to various macrolithotypes, and established natural γ (GR), deep lateral resistivity (LLD), and γ–γ log (GGL) response rules for each coal macrolithotype. The formation mechanisms of different coal macrolithotypes are discussed from the perspective of coal facies and pore structures. The results show that GGL decreased but GR and LLD increased from bright coal to dull coal. Most coal macrolithotypes can be distinguished based on the established thresholds of various logging curves. However, excessively high or low ash yields significantly affect the validity of identification. The vertical coal macrolithotypes attributed to the peat marsh environment in Shanxi Formation mostly comprise three to six sublayers; dull or semi-dull coals are predominant close to the 21 coal seam, and the bright or semi-bright types usually appear in the middle part. The semi-bright and bright coals are usually vitrinite rich, whereas the semi-dull and dull coals are primarily inertinite rich. For pore structure arguments, the highest average specific surface area (SBET) and the total pore volume (VBJH) are found in bright coals, followed by dull and semi-bright coals; those of semi-dull coals are the lowest. However, SBET and VBJH change significantly for different samples, even though the coal macrolithotype is the same. Therefore, the macrolithotype is not the key factor determining the coal parameters of pore structures. Rapid and effective identification of coal macrolithotypes can help determine the CBM enrichment area, the CBM well location, and the exploration horizon.

Fractal characteristics of pulverized high volatile bituminous coals with different particle size using gas adsorption

To better understand the effect of particle size on fractal characteristics of pores in coal, three high volatile bituminous coals with mean maximum vitrinite reflectance of 0.82–0.90% collected from the Naryksko-Ostashinskaya coalbed methane field in Kuznetskiy Basin in Russia were crushed and sieved to five progressivelydecreasing particle size fractions: 1.0–0.5 mm, 0.50–0.25 mm, 0.250–0.125 mm, 0.125–0.063 mm and 0.063–0.032 mm. For all particle size fractions, particle size distribution measurement, proximate analysis, maceral analysis and low-pressure N2 adsorption and CO2 adsorption analyses were conducted. Fractal dimensions D1 (pore surface roughness) and D2 (pore structural irregularity) were determined from relative pressure intervals of 0–0.45 and 0.45–1 of N2 adsorption isotherm with the application of the fractal Frenkel-Halsey-Hill theory, respectively. The results demonstrate significant differences in fractal dimensions in different particle size fractions. As particle size of coal samples decreases, D1 decreases, while D2 increases. Some constricted mesopores are transformed into non-constricted mesopores during crushing. The non-constricted mesopores increasingly concentrate in smaller pore sizes, which results in smoother pore surface (decreasing D1) and more heterogeneous pore structure (increasing D2). Mesopores with pore width between 2 nm and 5 nm are the key to fractal dimensions. Individual mesopore structure parameters closely associated with particle size have universal correlations with fractal dimensions. The increasing mesopore specific surface area and volume and decreasing average mesopore width lead to a decrease in D1 and an increase in D2, which further confirms the effect of particle size on fractal dimensions.

Development and application of a new compound wetting agent for coal seam water infusion

To improve the dust-suppression effect of water infusion in coal seams, a compound wetting agent was developed as a water infusion additive by using the synergistic effect between various surfactants and surfactant compounding with inorganic salts. Firstly, single factor experiments were conducted to select three surfactants with good wettability from nine surfactants by taking surface tension and contact angle as research objects. Then, the critical micelle concentration (CMC) of surfactants was selected as the concentration of the compound solution and the three selected surfactants and two inorganic salts were then mixed. The optimum ratio of the new compound wetting agent is 0.1% wt% rapid infiltration T +0.1 wt% OA-12% +0.1 wt% NaCl by comparing the surface tension and contact angle of various compounds in solution. Finally, the wettability of the compound wetting agent was obtained by coal dust settling and low-field nuclear magnetic resonance (L-FNMR) experiments. The results imply that the compound wetting agent can significantly improve the coal dust settling velocity to 18.38 mg/s, and increase the wetting rate of coal pore water. The test indicates that the new compound wetting agent can improve the dust removal efficacy of water infusion. The average water content increment in a water-infused coal seam with the new compound wetting agent is about 2.33%, and the dust reduction rates of total dust and respirable dust are increased from 63.6% and 67.5% to 89.2% and 91.6%, respectively. The new compound wetting agent exerts significant dust reduction effect on the N1105 fully mechanized coal mining face in the Yuwu Coal Mine.

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

Gas injection into coal seams is gaining increasing attention for improving coalbed methane (CBM) production. This technical development requires detailed and reliable information on gas adsorption/desorption-induced deformation and gas occurrence mechanisms. Hence, in this paper, high volatile bituminous coal was taken as the experiment samples from northern Shannxi Province, China, and adsorption-induced swelling experiments with CO 2 and N 2 were carried out under the conditions of one-time injection and step-by-step injection. Then, based on adsorption-induced swelling characteristics, a thermodynamic model was further established to disclose the adsorption/desorption-induced deformation mechanism of confined coals. Results showed that the swelling/shrinkage strain curves of CO 2 and N 2 adsorption/desorption in the coal matrix were generally divided into four stages, including pressurization-deformation, adsorption-induced swelling, depressurization-deformation, and finally desorption-induced shrinkage. It can be seen that the coal swelling effect caused by N 2 adsorption was far less than that caused by CO 2 adsorption. However, both CO 2 and N 2 adsorption displayed that the apparent volumetric swelling weakened with increasing strain when compared with the same adsorption pressure under different strain conditions. In contrast, the absolute volumetric swelling of the confined coal was a coupling result from adsorption-induced swelling and pore pressure effects. Further, based on the thermodynamic model, it was verified that the adsorbate type, the chemical potential of the adsorbate molecule and stress conditions are key factors affecting adsorption-induced swelling of coal under a certain temperature and pressure. In addition, a desorption lag phenomenon during the desorption-induced shrinkage stage was observed, due to incomplete gas desorption and gas retention in the coal macromolecular network. It was also found that there was a shrinkage phenomenon after the peak strain during gas adsorption. That is to say, when fixed adsorption sites on the coal pore surface reached the adsorption saturation state at the peak strain, gas molecules would be stored in an absorption state in the coal macromolecular network under continuous adsorption. These phenomena further explained coal swelling and gas occurrence mechanisms. • Swelling/shrinkage strain curve can be generally divided into four stages. • Absolute volumetric swelling is controlled by adsorption swelling and pore pressure. • Thermodynamic model reflects Gibbs free energy of gas adsorption in coal decreases. • Adsorption-induced swelling effect tends to be dropped with increasing stress. • Shrinkage effect after peak strain and desorption lag are occurred in whole process.

Molecular Simulation of the Adsorption Characteristics of Methane in Pores of Coal with Different Metamorphic Degrees.

In order to study differences in the methane adsorption characteristics of coal pores of different metamorphic degrees, 4 nm pore structure models based on three typical coal structure models with different metamorphic degrees were constructed. Based on the molecular mechanics and dynamics theory, the adsorption characteristics of methane in different coal rank pores were simulated by the grand canonical Monte Carlo (GCMC) and molecular dynamics methods. The isothermal adsorption curve, Van der Waals energy, concentration distribution, and diffusion coefficient of methane under different conditions were analyzed and calculated. The results showed that at the same pore size, the adsorption capacity of CH4 is positively correlated with pressure and metamorphic degree of coal, and the adsorption capacity of CH4 in high metamorphic coal is more affected by temperature. The relative concentration of CH4 in high-order coal pores is low, and the relative concentration at higher temperature and pressure conditions is high. The CH4 diffusion coefficient in high-rank coal is low, corresponding to the strong Van der Waals interaction between CH4 and coal. The research results are of great significance for further exploration of the interaction mechanism between CH4 and coal with different metamorphic degrees and can provide theoretical support for the selection of gas extraction parameters.

Characterization of Full Pore and Stress Compression Response of Reservoirs With Different Coal Ranks

The accurate characterization of coal pore structure is significant for coalbed methane (CBM) development. The splicing of practical pore ranges of multiple test methods can reflect pore structure characteristics. The pore\fracture compressibility is the main parameter affecting the porosity and permeability of coal reservoirs. The difference in compressibility of different coal rank reservoirs and pore\fracture structures with changing stress have not been systematically found. The pore structure characteristics of different rank coal samples were characterized using the optimal pore ranges of high-pressure mercury intrusion (HPMI), low-temperature liquid nitrogen adsorption (LT-N2A), low-pressure carbon dioxide adsorption (LP-CDA), and nuclear magnetic resonance (NMR) based on six groups of different rank coal samples. The compressibility of coal matrix and pore\fracture were studied using HPMI data and NMR T2 spectrum under effective stress. The results show that the more accurate full pore characterization results can be obtained by selecting the optimal pore range measured by HPMI, LT-N2A, and LP-CDA and comparing it with the NMR pore results. The matrix compressibility of different rank coal samples shows that low-rank coal &amp;gt; high-rank coal &amp;gt; medium-rank coal. When the effective stress is less than 6 MPa, the microfractures are compressed rapidly, and the compressibility decreases slowly when the effective stress is more than 6 MPa. Thus, the compressibility of the adsorption pore is weak. Nevertheless, the adsorption pore has the most significant compression space because of the largest proportion in different pore structures. The variation trend of matrix compressibility and pore\fracture compressibility is consistent with the increase of coal rank. The compressibility decreases with the rise of reservoir heterogeneity and mechanical strength. The development of pore volume promotes compressibility. The research results have guiding significance for the exploration and development of CBM in different coal rank reservoirs.

Study on the Microscopic Mechanism of Spontaneous Combustion and Oxidation Kinetics of Water-Leached Coal

In this article, a series of experiments have been carried out to study the spontaneous combustion and oxidation mechanism of coal after water immersion and investigate its tendency to spontaneous combustion, analyze the difficulty of spontaneous combustion of coal samples under different water immersion conditions, and establish a kinetic model of water immersion coal oxidation (taking the Bulianta 12# coal as a case study). They rely on physical oxidation adsorption, scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), thermogravimetry, and oil bath heating. SEM has been used to analyze the characteristics of coal pore structure under different water immersion conditions (water-saturated coal samples under different water loss conditions until the coal samples are completely dried); FTIR served to investigate the characteristics of the molecular chemical structure of the coal surface before and after the coal is immersed in water. Through programmed temperature oxidation experiments combined with FTIR analyses and gas chromatographic (GC) analysis of gaseous products, it has been possible to study the changes of molecular structure and gas products on the surface of coal samples at different temperatures and water immersion conditions. The oxidation reaction rate of the 12# coal samples of Shendong Mine’s Bulianta Mine under different water content conditions during the spontaneous combustion process has been quantitatively studied. The difficulty of spontaneous combustion of coal samples has been correspondingly addressed. A kinetic model from the perspective of oxygen consumption has been proposed. Thermogravimetry-differential scanning calorimetry (TG-DSC) has been used to analyze and study the exothermal oxidation process before and after coal immersion. From the perspective of the exothermic intensity of the coal-oxygen reaction, an oxidation kinetic model for immersed coal samples has been developed to qualitatively determine its spontaneous combustion tendency. Results have shown that the increase in the specific surface area increases the risk of spontaneous combustion, and coal samples after soaking and drying have a stronger tendency to spontaneous combustion than raw coal. The moisture content of the coal sample leading to the easiest ignition conditions is 16.05%. Regardless of the moisture content, the critical temperature is maintained at 65–75°C, and the temperature of the left coal in the goaf should be prevented from exceeding this critical value.

Study on the Applicability of Reservoir Fractal Characterization in Middle-High Rank Coals with NMR: Implications for Pore-Fracture Structure Evolution within the Coalification Process.

In order to evaluate the applicability of the pore-fracture structure fractal characterizations in coal reservoirs and confirm the internal relationships between the porosity, permeability, coal metamorphic grade, and pore-fracture structure, the pore-fracture features of 21 middle–high rank coal samples from Anhe, Jiaozuo, and Huaibei coalfields in northern China were investigated using a low-field nuclear magnetic resonance (NMR). All the coal samples are characterized by low moisture content (Mad), low and medium ash yield (Aad), and high vitrinite (V) in coal maceral. The adsorption space fractal dimension (DA) is positively correlated with the Langmuir volume (VL) under the three-peak transverse relaxation time (T2) spectrum. The fractal dimension of all effective T2 points under saturated water (DNMR) is positively correlated with VL and the adsorption pore volume, but negatively correlated with the volume ratio of seepage pores and fractures. The free flow space fractal dimension (DM) is negatively correlated with the porosity of full saturated water (ΦF) and the porosity of movable water (ΦM). There is a negative correlation between ΦF and the seepage space fractal dimension (DS) in the coal samples with one-peak and two-peak T2 spectra, but a positive correlation can be found with the three-peak T2 spectrum. Therefore, it is necessary to consider the types of T2 spectral peak as a prerequisite to analyze the correlations between pore-fracture parameters and NMR fractal dimensions. With the increase of coal rank, the adsorption pore content, ΦF, and bulk volume immovable (BVI) fraction first increase and then decrease, whereas the seepage pore content, fracture development, bulk volume movable (BVM) fraction, and BVM/BVI first decrease and then increase. The inflection points of these changes correspond to the maximum vitrinite reflectance (Ro,max) at 2.6–2.8%, which would be attributed to the third coalification jump. Generally, DA is the fractal dimension representing the coal pore surface, and DS and DM are closely related to the pore structure. Furthermore, DNMR not only represents the roughness of the pore surface but also the complexity of the pore structure.

Study on the Pore and Crack Change Characteristics of Bituminous Coal and Anthracite after Different Temperature Gradient Baking

At present, there are few studies on the development, evolution, and quantitative analysis of internal pores and cracks after heat treatment of coal. On the basis of this, low-temperature nitrogen ...

Mechanism of solvent extraction-induced changes to nanoscale pores of coal before and after acidification

As the most widely used reservoir stimulation method, hydraulic fracturing requires the injection of large volumes of fluid with certain viscosity, mainly formed by water and various additives in a certain proportion, into coal seam for the purpose of improving the permeability of coal reservoir, and thus the selection of appropriate fracturing fluid is very important to improve the fracturing effect. In the study, using a medium-volatile bituminous coal as the research object, solvent extraction experiments of coal samples before and after acidification under different solvent concentrations were carried out. After the soluble low molecular weight compounds in coal were extracted and dissolved, and the differences in pore structure parameters and their mechanisms in coal were investigated by using low-temperature liquid nitrogen adsorption apparatus and capillary coalescence theory. The research results show that, after tetrahydrofuran (THF) extraction with different concentrations, the pore volume (PV) of each pore size section for raw coal decreases, and with the increase of the pore size, the decreasing PV tends to be stable. The PV of acidified coal increases significantly in all pore size ranges, especially in the range of 5 ∼ 10 nm. Compared with unacidified coals, the low-temperature nitrogen adsorption isotherm of acidified coals after extracted with different concentrations of THF were more prominent in the relative pressure of 0.8 ∼ 1.0. As THF concentration increases, the fractal dimension D of raw coal and nitrogen adsorption capacity decrease, while the fractal dimension D of acidified coal and nitrogen adsorption capacity increase and are significantly bigger than those of raw coal. The nitrogen adsorption capacity of raw coal and acidified coal shows an inverted “N” and “N” trend, respectively, and the nitrogen adsorption capacity of the residue with 50%THF extraction after acidification reaches the maximum value of 1.47 cm3/g. This study suggests that adding a certain proportion of organic solvent (THF) and acid into fracturing fluids can be beneficial to improve reservoir porosity and thus increasing gas production.