Tectonically Deformed Coals Research Articles

Multi-Scale Characterization of Pores and Fractures in Coals with Different Coal-Body Structures from the Jincheng Mine, Qinshui Basin, Northern China

The Qinshui Basin is located in the southeast of Shanxi Province, China. It is one of the most abundant coal resources from Permo-Carboniferous North China. It is rich in coal and coalbed methane resources. However, the accumulation of coalbed methane is complex and the enrichment law has not been fully understood because of the high heterogeneity of coal reservoirs in the Qinshui Basin. The examination of dissimilarities between tectonically deformed coals (TDCs) and primary coals at multiple scales holds paramount importance in advancing our understanding of the occurrence and flow patterns of coalbed methane, and in providing guidance for exploration efforts. In the present study, the samples from the Jincheng Mine, Qinshui Basin, were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), CO2 gas adsorption and 3D X-ray micro-computed tomography. The results showed that the dominant minerals in coal were illite, kaolinite, and calcite, with minor amounts of quartz and ankerite. In comparison to primary coal, tectonism could increase the microfractures density of type A (the fracture of width ≥ 5 μm and length > 10 mm) in TDCs. In CO2 gas adsorption in mylonite coal, it was observed that the volume of micropores (<2 nm) was significantly reduced leading to a decrease in gas adsorption capacity. The result of Micro-CT scanning revealed that the minerals occurred as veins in primary coal, but as irregular aggregates in TDCs. Moreover, tectonism had a staged impact on fracture structure, which was initially closed in cataclastic coal and then formed into granulated coal during the tectonic evolution. The effects of tectonism on coal structure had an impact on the connectivity of micropores at the micrometer scale by the destruction of the pore throat structure, increasing the heterogeneity of the reservoir. These findings help to better understand the changes in TDC structure at different scales for developing effective strategies for coalbed methane exploration and production.

Coalbed methane reservoir properties assessment and 3D static modeling for sweet-spot prediction in Dahebian block, Liupanshui Coal field, Guizhou Province, southwestern China

Guizhou Province has many multi-layer, thin-thick coal seams; however, complex geology, incomplete reservoir characterization, and sweet-spot selection technology prevent large-scale coalbed methane (CBM) development. This study evaluates the CBM reservoir properties within the Dahebian block using logging data, coal sample analysis, and well-testing data and develops a 3D static reservoir properties model to analyze their spatial and vertical propagation. A sweet spot evaluation model was established using a multi-level fuzzy method based on 9 parameters extracted from a 3D static reservoir properties model. The coal measure has 22 coal seams, and seams >2 m thick have 2 or 3 thin non-coal layers intercalated. Coal seams 1#, 7#, and 11# are thin to thick, deeply buried, widely distributed, and have high gas content and saturation. Undeformed and cataclastic coal predominates the coal seam 1# and 7#, whereas coal seam 11# is dominated by cataclastic and granulated coal. The southern and central parts of coal seam 7# and 11# have less tectonically deformed coal (TDC). Coal seams 1# and 7# have low permeability relative to seam 11# and are localized, while coal seams 11# have high permeability, are extensively distributed, and contain substantial gas concentrations. Comparative analysis of evaluation scores and CBM production statistics shows that high scores indicate sweet spots for CBM development. Sweet-spot potential was classified as high, medium, and low. Scattered sweet spots are found in single layers, while combined development (1# + 7#+11#) reveals a wider high-potential area in the south-central region. This area, featuring deep, thick coal seams, high permeability, gas saturation, reservoir pressure, and low TDC proportion, indicates significant development potential. This study validates CBM development statistics, identifies future development areas, and guides the development of geologically complex Guizhou CBM.

Influence of Tectonically Deformed Coal-Based Activated Carbon and Its Surface Modification on Methane Adsorption.

A series of coal-based activated carbons (CACs) were synthesized from mylonitized fat coal, a type of tectonically deformed coal (TDC) and symbiotic primary structural coal (PSC), followed by oxidative modification. The pore structure, surface oxygen-containing functional groups, and their influence on methane adsorption by CAC as the simplified coal model were investigated by using low-temperature nitrogen adsorption, Fourier transform infrared spectroscopy, Boehm titration, and X-ray photoelectron spectroscopy. The results showed that tectonic deformation fostered smaller pores, particularly ultramicropores in TDC, dominating methane adsorption. Acid-modified TDC-based activated carbons (ACs) showed higher pore parameters and oxygen-containing functional groups than those of PSC-based ACs. Nitric acid introduced abundant carboxyl groups concurrently increasing the pore volume and specific surface area (SSA), while sulfuric acid-ammonium persulfate treatment resulted in increased lactone groups and a partial collapse/blockage of nanopores. Methane adsorption experiments confirmed the importance of micropores and revealed a significant decrease in capacity owing to increased oxygen-containing functional groups as the primary role, with pore wall corrosion playing a secondary role. Thus, the study highlights the surface effects of TDC on methane adsorption and the potential for producing high-performance methane storage materials from China's tectonic coal resources.

Numerical Simulation Study on the Mechanics and Pore Characteristics of Tectonically Deformed Coal under Multi-Level and Multi-Cycle Loading and Unloading Conditions

Horizontal well cavern completion and stress release is considered a potential technique for efficient development of coalbed methane in tectonically deformed coal (TDC). Pulsating loading and unloading is a key technique for the controlled expansion of caverns and broader stress release within the reservoir. However, current understanding of the mechanical characteristics and pore network structure evolution of TDC under cyclic loading and unloading conditions is still limited. This paper employs numerical simulation methods to study the mechanical behavior and damage characteristics of TDC under cyclic loading and unloading. After obtaining a set of micromechanical parameters reflecting the behavior of TDC samples under triaxial compression in high-stress states, the effects of different stress gradients and cyclic amplitudes on the stress–strain curve, porosity changes, and crack propagation in TDC samples were analyzed. The study results indicate that under various cyclic loading and unloading conditions, the mechanical response characteristics of TDC samples are broadly similar, primarily divided into compression, slow expansion, and accelerated expansion phases. Under low unloading level conditions, the volume expansion of TDC samples is minimal. Also, at the same unloading level, the strain increment decreases with an increasing number of cycles. Correspondingly, under these conditions, the porosity and microcrack expansion in TDC are less than in high-stress gradient scenarios. Under the same unloading level but different amplitudes, the volume expansion rate at 50% unloading amplitude is higher than at 1 MPa unloading amplitude for TDC, with an increased number of crack expansions. Therefore, under cyclic loading conditions, the sensitivity of crack propagation within TDC samples to amplitude is greater than that to unloading level. Under actual pulsating excitation conditions, a low-amplitude, low-stress gradient pulsation method should be used to maintain the stability of horizontal well caverns, and gradually increase the cyclic amplitude to achieve the efficient extraction of coalbed methane in TDC reservoirs. The findings of this study can serve as an important reference for optimizing process parameters in cyclic pulsating stress release engineering for TDC.

Multiscale Fractal Characterization of Pore–Fracture Structure of Tectonically Deformed Coal Compared to Primary Undeformed Coal: Implications for CO2 Geological Sequestration in Coal Seams

The tectonically deformed coal (TDC) reservoirs with abundant gas resources and low permeability are expected to become one of the target coal seams for carbon dioxide geological storage-enhanced coalbed methane recovery (CO2-ECBM). The pore–fracture structure plays a crucial role in determining the effectiveness of CO2 storage. Fractal analysis provides a valuable approach to quantitatively describe the complex and heterogeneous pore–fracture structures across various scales in coal matrixes. Accordingly, the TDC samples in the Huainan–Huaibei coalfield and primary-undeformed coal (PUC) samples in the Qinshui Basin were selected for pore–fracture structure parameter tests using the mercury intrusion porosimetry (MIP) and low–temperature nitrogen adsorption (LNA) methods. Their multiscale pore–fracture parameters were analyzed using different fractal methods based on pore diameter. According to the fractal results, a multiscale classification standard for pore–fracture structures was devised in this study that is suitable for the controlling gas migration process. A parameter of 8 nm is set as the separating pore diameter for gas migration and storage. It was observed that the connectivity of migration pores (>8 nm) in TDC samples was stronger compared to PUC samples, reflected in larger pore volumes and smaller fractal dimensions. However, its complex development of seepage pores (150–300 nm) may hinder the flow of CO2 injection. As for the storage pores (<8 nm), the fractal dimension of the 2–8 nm pores in TDC was found to be similar to that of PUC but with larger pore volumes. The fractal dimension of the filling pores (<2 nm) in TDC samples was relatively lower, which facilitates efficient gas volume filling. Therefore, the pore–fracture structure of the TDC samples is found to be more advantages for CO2 injection and storage compared to the PUC. This suggests that TDC reservoirs holds promising geological potential for CO2-ECBM implementation.

Expulsion of small molecule hydrocarbons and expansion of nanopores effect in tectonically deformed coal evolution

This work aimed to investigate the impact of the evolution of small molecular structures during the deformation of coal on the constraints imposed on nanoscale pores and adsorption behaviors. Both undeformed and tectonically deformed coal (TDC) from the Huaibei mining area in China were collected for analysis. The pyridine was employed for the extraction of small molecules from coal, which was further complemented by Fourier-transform infrared spectroscopy (FTIR) and low-pressure CO2 and N2 adsorption experiments (LP-CO2/N2AD). The study yielded the following results: (1) Intense tectonic stresses facilitate the removal and recombination of small molecular side chains within the basic structural units (BSU) of coal. As deformation occurs, hydroxyl groups, aliphatic hydrocarbons, C=O, C=C, aromatic hydrocarbons, and 'C' structures tend to accumulate, resulting in an enhanced extraction rate. (2) The deformation process of the coal demonstrates a significant pore-expansion effect, with the most substantial increase observed in micropores (surface area increased by over 70 %), followed by mesopores and macropores. This enlargement facilitates the aggregation of small molecules and gas storage. (3) Intense deformation stages involving shear slip, ductile deformation, and maceral fragmentation result in small molecular fractures and an increased number of micropores. The removal and recombination of I, DOC, and CH2/CH3 within BSUs lead to the formation of larger interconnected pores accompanied by the aggregation of hydroxyl groups (OH–), C=O, C=C, aliphatic hydrocarbons, and 'C' structures. (4) The adsorption capacity of residual coal is lower than that of the raw coal before extraction, indicating that small molecular structures contribute to increased adsorption. This likely plays a significant role in the higher gas content observed in structurally deformed coal. The removal of small molecular structures effectively diminishes the adsorption capacity, providing novel perspectives for coalbed methane development and gas extraction in TDC areas.

Advantages of Structure and Electrochemical Properties of Graphene Prepared from Tectonically Deformed Coal.

Asa low-cost carbon-rich resource, coal has been widely used to prepare excellent electrochemical energy-storage carbon materials such as graphene. However, the different structures of carbon source will affect the performance of carbon materials. To explore the feasibility of preparing high-performance graphene from the carbon source affected by tectonic stress in coal, in this paper, series products of coal-based graphene are prepared by tectonically deformed coal (TDC) and normal structural coal (NSC). The structural parameters are characterized by HRTEM, XRD, Raman, and low-temperature CO2 and N2 adsorption, and the electrochemical performance of coal-based graphene lithium battery is tested by galvanostatic charge-discharge and cyclic voltammetry. The results show that tectonic stress makes the proportion of the medium-long aromatic fringes, preferred orientation degree (POD), and multilayer stacking in TDC aromatic fringes slightly higher than those in NSC. At the same temperature, the relatively large microcrystalline size, the high order degree, and more pore structures make the local molecular oriented (LMO) domain vertical height (d) and graphitization degree (G) of the coal-based graphite microcrystalline structure prepared by TDC better than those of NSC, which indicates that the carbon source in TDC contains more graphitizable carbon structures. This makes the graphene prepared by TDC not only possess perfectly ordered crystal planes but also relatively abundant nanochannels. High lithium-storage capacity and low charge-transfer resistance make the electrochemical performance of graphene prepared by TDC as an anode electrode material for lithium-ion batteries superior to that by NSC.

Response in coal reservoirs and in-situ stress control during horizontal well coal cavern completion and stress release

Widely developed tectonically-deformed coal (TDC) blocks the recovery of coalbed methane (CBM). Horizontal well cavern completion and stress release (HWCCSR) is expected to extract TDC in-situ CBM efficiently. Systematic investigations on reservoir response during stress release are essential for evaluating stress-relief performances and guiding engineering practices. However, the dynamic-controllable cavern enlargement and the influence of high stress and intense tectonic stress were mainly ignored in previous work. Additionally, the mechanism of permeability variation remained unclear under complex stress surroundings. In this work, the geological-engineering numerical model of coal cavern completion was constructed, and the equivalent method was adopted to investigate the dynamic HWCCSR. The stress, plastic deformation, and permeability responses were analyzed, and the impact of in-situ stress on reservoir response was discussed. The mechanism and model of permeability changes under complex stress were revealed.The results showed that the equivalent method could imitate the dynamic-controllable enlarging of the cavern properly with easy operation and good convergence. The reservoir responded significantly during the HWCCSR. With increasing cavern diameter (Dc), the plastic deformation zone (PDZ) increased, permeability could be expanded to 1–105 times the initial value, and the effective permeability increase zone (EPIZ) was linear with Dc. Laboratory and field data confirmed the rationality of simulation results. High vertical principal stress and significant lateral stress coefficient were conducive to generating more complex PDZ and larger EPIZ and enhancing more considerable permeability in TDC reservoirs due to more energy and a strong damage-deformation trend. Four models of permeability variation and possible division of permeability distribution were proposed according to the integrated control of elastic-plastic deformation, stress difference, and volume stress. Stress difference was the direct reason for coal plastic deformed-failure. Plastic deformed-failure coupled with volume stress-relief could significantly increase permeability. Plastic deformed-failure and volume stress concentration might cause a permeability decrease. Coal elastic deformation with volume stress concentration might induce a similar outcome. Additionally, elastic deformation coupled with volume stress-relief might enhance permeability and expand the EPIZ under tectonic stress conditions. In conclusion, larger caverns had better stress-relief performance, expected to attain a high recovery of coalbed methane. The HWCCSR was suitable for recovering TDC in-situ CBM in areas with intense tectonic stress and large burial depths. This work confirmed the feasibility of efficient recovery of TDC in-situ CBM through HWCCSR. It provided basics for evaluating reservoir responses to stress-relief and selecting layers.

Molecular dynamics simulation of shear friction process in tectonically deformed coal

Shear friction is an important deformation process in tectonically deformed coals (TDCs) and is closely related to the dynamic metamorphism of coal. In the current study, we perform a molecular dynamics (MD) simulation of the shear friction process on primary structure coal. The simulation results show that coal friction is a process of energy transformation. The mechanical energy of shear friction work can lead to temperature increases and chain motion. Chain diffusion and reorientation are the two main chain motion modes during friction. Chain diffusion behavior is regular in the initial friction stage and becomes irregular in the later friction stage. The orientation change is different for various fused aromatic chains. The orientation changes of pentacenes and naphthacenes are more significant than those of the other fused aromatic chains, indicating that fused aromatic chains with a higher aspect ratio are preferentially reorientated by shear friction. It is also demonstrated that the C-O and C-N bonds in coal are more easily disassociated by shear friction. The research results directly confirm the molecular evolution during coal friction caused by shear stress.

Permeability Response Characteristics of Primary Undeformed Coal and Tectonically Deformed Coal under Loading-Unloading Conditions in Huainan Coalfield, China.

Reservoir pressure relief is a practical method to enhance permeability for coalbed methane (CBM) extraction in tectonically deformed coal (TDC) reservoirs. To explore the coal permeability response to stress changes, the primary undeformed coal (PUC) and TDC from the same coal seam were sampled for the pore–fissure structure analysis, mechanical property test, and permeability experiments under different stress loading–unloading methods in this study. The experimental results demonstrated that the coal permeability is more sensitive to the changes in confining pressure (perpendicular to airflow) than axial stress (parallel to airflow). Coal permeability decreases negatively exponentially as the confining pressure increases, and its change process with increased axial pressure can be divided into five stages in this study. The pore structures and mechanical properties of coal samples affected their permeability response to stress changes. Under the stress loading condition, the coal matrix and fractures of PUC samples were compressed simultaneously, and the permeability was regulated by the pore–fissure structures in the coal matrix. Due to the deformation and displacement of coal particles, the permeability of the TDC sample is predominantly dependent on changes in intergranular pores. At the initial stress unloading stage, the fissure recovery and expansion lead to a rapid increase in permeability, but the permeability cannot rereach the original value when the stress is fully released. Furthermore, the influencing factors of coal permeability in response to stress loading–unloading also include confining pressure conditions and coal matrix adsorption swelling. Research on the permeability response characteristics of the stress loading–unloading process can provide some clarifications for the reservoir depressurization and permeability enhancement of CBM extraction in the TDC reservoir.

Evolution of the Hierarchical Molecular Structures of Tectonically Deformed Coals: Insights from First-Order Raman Spectra.

The dynamic metamorphism of tectonically deformed coals (TDCs) is closely correlated with the occurrence of coalbed methane. Here, a Raman detection technique was used to characterize two primary coals and 14 TDCs sampled from the Huaibei coalfield, allowing for profound insight into the dynamic metamorphism caused by tectonic stress in coals. A nine-peak curve fitting method was applied to deconvolute the first-order Raman spectra, and six structural parameters were used to characterize the hierarchical evolution of TDC molecules. The lower AS/ATotal and AS/AD ratios in TDCs indicate that the cross-links in secondary and aggregated (outer) structures are cleaved by tectonic stress. Once the aromatic structures are released through the cleavage of cross-links, the outer structures can be rearranged into a more ordered configuration, as indicated by the higher IG/IA2 ratio in TDCs. Additionally, the basic structural units (including aromatic structures and side chains) in TDC are also altered by stress. The increasing values of AD/AG and A(D+A1+A2)/ADs illustrate that the size of aromatic structures and the ratio of small aromatic structures both increase with increasing coal deformation intensity. The value of AC/ATotal decreases with increasing deformation intensity, indicating that oxygen functional groups are disassociated by tectonic stress. However, as the basic structural units are commonly rigid and cannot be altered as readily as the outer structures, the evolution of basic structural units is not always obvious in weakly deformed coals (such as cataclastic, mortar, and schistose types) but is more significant in strongly deformed coals (such as granulitic, scaly, and wrinkle types).

Experimental Study on the Influence of Effective Stress on the Adsorption–Desorption Behavior of Tectonically Deformed Coal Compared with Primary Undeformed Coal in Huainan Coalfield, China

In order to explore the influences of effective stress change on gas adsorption–desorption behaviors, primary undeformed coal (PUC) and tectonically deformed coal (TDC) from the same coal seam were used for adsorption–desorption experiments under different effective stress conditions. Experimental results showed that gas adsorption and desorption behaviors were controlled by the coal core structure and the pore-fissure connectivity under effective stress. The coal matrixes and fissures were compressed together under effective stress to reduce connectivity, and it was difficult for gas to absorb and desorb as the stress increased in primary undeformed coal. The loose structure of tectonically deformed coal cores can help gas to fully contact with the coal matrix, resulting in higher adsorption gas volumes. The support of coal particles in tectonically deformed coal cores weakens the compression of intergranular pores when effective stress increases, which in this study manifested in the fact that while the volumetric strain of the coal matrix change rapidly under low effective stress, but the adsorbed gas volume did not decrease significantly. The reduction in effective stress induced the rapid elastic recovery of the coal matrix and the expansion of cracks, and increased desorption gas volumes. The stress reduction significantly increased the initial gas volume of the tectonically deformed coal, while promoting slow and continuous gas desorption in primary undeformed coal. Therefore, the promotion effect of the reservoir pressure reduction on gas desorption and coal connectivity enhancement can help to improve coalbed methane recovery in primary undeformed coal and tectonically deformed coal reservoirs.

Nanopore Structure and Mechanical Properties in Brittle Tectonically Deformed Coals Explored by Atomic Force Microscopy

Tectonically deformed coals (TDCs) are of great importance to coalbed methane exploitation and coal mining safety. Compared to primary coals, reservoir properties of TDCs have been transformed greatly by tectonic stress. Here, the pore structure and mechanical properties of primary coal and brittle TDCs were obtained with atomic force microscopy and software. The results showed that tectonic stress generally promotes pore development and Young’s modulus of brittle TDCs. According to the variation in pore structure and Young’s modulus, two stages were identified: weak brittle deformation stage (primary coal–cataclastic coal–schistose coal–mortar coal) and strong brittle deformation stage (mortar coal–granulitic coal–flaky coal). The extent of tectonic impact varies greatly between these two stages. In weak brittle deformation stage, tectonic stress has little impact on coal pore structure. The mean pore number increases slowly, and the mean pore size decreases slowly. In this stage, half of the pore number increment is provided by macropores of 100–200 nm diameter. In strong brittle deformation stage, tectonic stress has a major impact on coal pore structure. The mean pore number increases quickly, and the mean pore size decreases quickly. Most of the pore number increment comes from mesopores of 10–50 nm diameter and macropores of 50–100 nm diameter. In addition, the Young’s modulus of primary coal and different brittle TDCs varies from 1.5 to 2.0 GPa. In weak brittle deformation stage, the Young’s modulus of different samples levels off. While in strong brittle deformation stage, the Young’s modulus increases gradually. Combined with former studies, it is inferred that tectonic stress can influence coal’s Young’s modulus by transforming its chemical structure.

The Influence Mechanism of Pore Structure of Tectonically Deformed Coal on the Adsorption and Desorption Hysteresis

Pore is the main adsorption and desorption space of coalbed methane (CBM). Pore size configuration and connectivity affect the adsorption/desorption hysteresis effect. Using tectonically deformed coal (TDC) and original structure coal of medium- and high-rank coal as the research objects, through the N2/CO2 adsorption experiment to analyze the pore size distribution and connectivity of different scales. We investigate the control mechanism of heterogeneous evolution in the key pore scales against adsorption/desorption hysteresis characteristics during coal metamorphism and deformation by combining the CH4 isothermal adsorption/desorption experiment under 30°C equilibrium moisture. The findings indicate that the super micropores (<2 nm) are mainly combination ink bottle-shaped pores and have worse connectivity as the degree of metamorphism and deformation increases. The super micropores occupy the vast majority of pore volume and specific surface area; its pore size distribution curve change presents an “M” bimodal type and is mainly concentrated in two pore segments of 0.45–0.70 nm and 0.70–0.90 nm. The effect of ductile deformation exerts a significantly greater effect on super micropores than brittle deformation. The exhibited adsorption–desorption characteristics are the result of the combined effect of the unique pore structure of the TDCs and different moisture contents. The presence of a large number of super micropores is the most important factor influencing the degree of gas desorption hysteresis. The “ink-bottle effect” is the primary cause of gas desorption hysteresis. For CBM development, some novel methods to increase desorption and diffusion rate at the super micropores scale should be considered.

Comparison of Nanopore Structure Evolution in Vitrinite and Inertinite of Tectonically Deformed Coals: A Case Study in the Wutongzhuang Coal Mine of Hebei Province, North China

The understanding of the nanopore structure in tectonically deformed coal (TDC) could be expanded from the perspective of a single maceral. A total of 10 TDCs with an increasing deformation degree (in the order of unaltered, cataclastic, porphyroclast, scaly, and powdery coal), were collected in a single coal seam and stripped into 10 vitrinite and 10 inertinite samples. Carbon dioxide (CO2) adsorption and nitrogen (N2) adsorption/desorption experiments were conducted to reveal the nanopore structure, whereas 13C solid-state nuclear magnetic resonance and X-ray diffraction experiments were conducted to detect the macromolecular structure. The results reveal that the macromolecular structures of both vitrinite and inertinite can be altered by tectonic stress. As the deformation degree increases, the aliphatic carbons decrease, the aromatic carbons increase, and the aromatic interlayer spacing decreases, whereas the crystallite stacking height and the average number of crystallites in a stack increase. For mesopores, the pore volume of vitrinite slowly decreases and then increases at the stage of scaly coal, whereas that of inertinite fluctuates with no obvious regularities. For micropores, the pore volume of vitrinite deceases as the deformation degree increases, whereas that of inertinite decreases since the deformation stage of porphyroclast coal, and little changed from the deformation stage of unaltered to cataclastic coal. As the coal deformation degree increases, the aromatic interlayer spacing decreases, resulting in a decrease in the micropore volume; however, the average crystallite stacking height and the average number of crystallites in a stack increase, leading to an increase in the micropore volume. Therefore, the micropore decreases and then increases as the coal deformation degree increases. However, the macromolecular changes weakly affect the mesopore evolution in the coal deformation process.

Methane Adsorption Behavior and Energy Variations of Brittle Tectonically Deformed Coal under High Temperature and High Pressure.

This paper aims to reveal the methane adsorption characteristics of brittle tectonically deformed coal (TDC) under high temperature and high equilibrium pressure and the surface free energy changes during methane adsorption. Taking the anthracite of the Yangquan mining area of China as the research object, methane adsorption tests of high temperature and high pressure were conducted to discuss the influences of coal deformation, temperature, and pressure on the gas adsorption behaviors of coal under the in situ conditions of deep coal seams. Results indicated that coal deformation and pressure had positive effects on the methane adsorption capacity of coal, whereas temperature showed negative effects. Meanwhile, the negative influences of temperature rise on gas adsorption gradually increased with the equilibrium pressure and the enhancement of coal deformation. In a given adsorption system, the adsorption potential decreased with increasing pressure while increased with temperature. In contrast, the adsorption space increased with increasing pressure and decreased with temperature rise. Meanwhile, adsorption potential decreased with the increase in adsorption space. In the process of methane adsorption, with increasing adsorption pressure, the cumulative reduction of surface free energy gradually increased, whereas the surface free energy reduction at each pressure point decreased. The cumulative and incremental reduction of surface free energy at each pressure point gradually decreased with the increase in temperature. The reduction of adsorption space and cumulative surface free energy and the changes in surface free energy at each pressure point of brittle TDCs were higher than those of primary structure coal, and the changes in the strongly deformed coal (granulitic and mortar coal) were more significant than those of the weakly deformed coal (cataclastic coal). Based on the adsorption potential theory, we drew the adsorption characteristic curves and established adsorption capacity prediction models of primary structure coal and brittle TDCs.

Dynamic Characterization during Gas Initial Desorption of Coal Particles and Its Influence on the Initiation of Coal and Gas Outbursts

The law of gas initial desorption from coals is greatly important for understanding the occurrence mechanism and predicting coal and gas outburst (hereinafter referred to as ‘outburst’). However, dynamic characterization of gas initial desorption remains to be investigated. In this study, by monitoring the gas pressure and temperature of tectonically deformed (TD) coal and primary-undeformed (PU) coal, we established the evolution laws of gas key parameters during the initial desorption. The results indicate that the gas pressure drop rate, mass flow rate, initial desorption rate, and gas velocity increase with increasing gas pressure, with stronger gas dynamic effect, generating a high pressure gradient on the coal surface. Under the same gas pressure, the pressure gradient formed on the TD coal surface is greater than that formed on the surface of the PU coal, resulting in easily initiating an outburst in the TD coal. Moreover, the increased gas pressure increases temperature change rates (falling rate and rising rate) of coal mass. The minimum and final stable temperatures in the TD coal are generally lower compared to the PU coal. The releasing process of gas expansion energy can be divided into two stages exhibiting two peaks which increase as gas pressure increases. The two peak values for the TD coal both are about 2–3 times of those of the PU coal. In addition, the total gas expansion energy released by TD coal is far greater than that released by PU coal. The two peaks and the total values of gas expansion energy also prove that the damage of gas pressure to coal mass increases with the increased pressure, more likely producing pulverized coals and more prone to initiate an outburst.

Influence of mechanical grinding on characterization of nanopores of tectonically deformed coal: a comparative study between coal chunks and crushed coal

Accurately characterizing the pore structure of tectonically deformed coal (TDC) is an essential prerequisite for investigating the porosity change rules of coal during coal deformation. This paper aims to discuss the possibility of conducting gas adsorption on original coal chunks of TDCs instead of ground coal particles and to investigate the influence of mechanical grinding during coal preparation on pore structure characterization. First, we conducted N2 and CO2 adsorption tests on coal chunks and crushed coal (60 mesh particles) of primary structure coal and ductile TDCs (Ro, max = 0.93%~1.15%). Then, the influences of grinding coal into 60 mesh particles on the micro-mesopore structure of coal were investigated by contrastive analysis. N2 adsorption tests showed that the pore volume of the > 10 mesopores (especially the > 40 nm mesopores) significantly increased in crushed coal from that of coal chunks, and the observed increases in pore volume decreased progressively with the enhancement of ductile deformation. The difference in pore volume (by N2 adsorption), pore surface area, and surface fractal dimension (DFHH) of < 10 nm pores was insignificant, indicating that mechanical grinding hardly affected < 10 nm pores. During CO2 adsorption, incomplete equilibrium was observed in partial coal chunks of primary structure coal and weakly crumpled coal. After coal being ground into 60 mesh particles, incomplete equilibrium was effectively eliminated as grinding significantly shortened the distance between adsorbate molecules and micropores. Apart from that, there were no noticeable influences on the total pore surface area and micropore size distribution (by CO2 adsorption). Thus, during characterizing the pore structure of different types of TDCs (a micro- and mesoporous material), it is recommended to conduct N2 adsorption on the original state and conduct CO2 adsorption on 60 mesh particles if possible.

Macromolecular Structure Changes of Tectonically Deformed Coal: Evidence from Coal Pyrolysis, 13C NMR, and XRD Experiments

Research on tectonically deformed coal (TDC) is of significance to gas outburst prevention and coalbed methane development. Despite numerous works focusing on the structural characteristics of TDC, the influence of tectonic stress on coalification is a long-term controversial issue. In this study, the X-ray diffraction (XRD), 13C solid-state nuclear magnetic resonance (NMR), and coal pyrolysis techniques were applied to compare the macromolecular structural characteristics and gas generation properties between weak TDC (WTDC) and strong TDC (STDC). The results show that the strong tectonic stress obviously promotes the evolution of coalification in advance. The basic structural unit (BSU) of the WTDC and STDC both increases with increasing coal rank, but the d(002) values of the STDC are usually less than those of the WTDC, and the Lc, La, and N values of the STDC are larger than those of the WTDC. Furthermore, the impact of tectonic deformation on the BSU of coals becomes weaker as the coal rank increases. The aromaticity in coals is accelerating and increasing with the enhancement of deformation degree. The fractions of protonated aromatic carbon (fH a) and aromaticity (fa) in the STDC are higher than those in the WTDC of the same coal rank, while the relative contents of the oxygen-substituted aromatic carbon (faO), aliphatic-substituted aromatic carbon (faS), and aliphaticity (fal) in the STDC are lower than those in the WTDC. The gas generation potential of the STDC is lower than that of the WTDC of the same coal rank from the coal pyrolysis data. Furthermore, with an increase of temperature, the difference in the generation capacity of CH4 and H2 between STDC and WTDC increases. The research results help us to illustrate the effect of tectonic stress on coalification and supply guidance on the prevention and management of gas outburst disasters.

Multifractal Behavior of the Micro- and Mesopore Structures of Brittle Tectonically Deformed Coals and Its Influence on Methane Adsorption Capacity

The well-developed tectonically deformed coals (TDCs) and the high heterogeneity of coalbed methane (CBM) occurrence severely restrict the sustainable development of the CBM industry in structurall...