Tectonically Deformed Coals Research Articles

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...

Macromolecular Structure Controlling Micro Mechanical Properties of Vitrinite and Inertinite in Tectonically Deformed Coals—A Case Study in Fengfeng Coal Mine of Taihangshan Fault Zone (North China)

In order to study the evolution of the mechanical properties and macromolecular structures in different macerals of tectonically deformed coal (TDC), vitrinite and inertinite samples were handpicked from six block TDCs in the same coal seam with an increasing deformation degree (unaltered, cataclastic, porphyroclast, scaly and powdery coal). The micro mechanical properties were tested by the nanoindentation experiment and the macromolecular structures were measured using 13C nuclear magnetic resonance (NMR) and Fourier transform infrared spectroscopy (FTIR). The results show that the range of hardness and elastic modulus of inertinite is 0.373–1.517 GPa and 4.339–12.158 GPa, respectively, which is significantly higher than that of vitrinite with values of 0.278–0.456 GPa and 4.857–7.810 GPa, respectively. From unaltered coal to powdery coal, the hardness of vitrinite and inertinite gradually decreases, with the difference between these macerals becomes smaller and the elastic modulus of vitrinite shows an increasing trend, while that of inertinite was more variable. Both the NMR and FITR results reveal that the macromolecular structure of inertinite has similar structural transitions as vitrinite. As the degree of deformation increases, the aliphatic side chains become shorter and the aromaticity is increasing. Macromolecular alterations caused by tectonic stress is expected to produce defects in the TDCs, therefore there should be more interspacing among the macromolecular groups for the extrusion of macromolecules caused by the indenter of the nanoindentation experiment, thereby reducing the hardness. The elastic modulus of coal is believed to be related to intermolecular forces, which are positively correlated to the dipole moment. By calculating the dipole moments of the typical aromatic molecular structures with aliphatic side chains, the detachment of the aliphatic side chains and the growth of benzene rings can both increase the dipole moment, which can promote elastic modulus. In addition, the increasing number of benzene rings can create more π-π bonds between the molecules, which can lead to an increase in the intermolecular forces, further increasing the elastic modulus.

Structural and Fractal Characterizations of Nanopores in Middle-Rank Tectonically Deformed Coals - Case Study in Panguan Syncline.

The reservoir properties of tectonically deformed coals (TDCs) differ significantly compared with their neighboring primary coals which are also known as unaltered or underformed coals. However, the heterogeneity of nanopores in TDCs under the syncline control has been seldom reported, and also the middle-rank level was minimally investigated to date. Thus, in this paper, the structures and multiscale fractal characteristics of nanopores in middle-rank TDCs under the controlling effect from Panguan Syncline were investigated via high-pressure mercury injection (HPMI), low-pressure CO2/N2 adsorption (LPCO2/N2GA), and fractal theory. The results show that both the pore volume (PV) and specific surface area (SSA) of macropores increase significantly in the stage of cataclastic–schistose coal. For ductile deformed coals, the PV increases, while the SSA remains stable. The SSA of mesopores increases slightly in the brittle deformation stage, but significantly in the ductile deformation stage. For micropores, both the PV and SSA for TDCs are significantly higher than primary coals. Moreover, the ductile deformation has a more significant promotion effect for the microporous PV and SSA than the brittle deformation. The fractal dimension of the adsorption pore (induced from the Sierpinski model) increases; however, that of seepage pores (Sierpinski model) decreases with the enhancement of tectonic deformation. The fractal dimension for mesoporous (induced from the FHH model, Frenkel–Halsey–Hill) at 2–6 nm keeps stable in the stage of cataclastic–schistose coal but significantly increases in the ductile deformation stage. For mesopores of 6–100 nm, their heterogeneities were also enhanced in the ductile deformation stage. The fractal dimension of 0.3–0.6 nm micropores is close to 3 and changes slightly with the enhancement of tectonic deformation, indicating that the heterogeneity of smaller micropores is stronger than that of larger micropores. The results are of broad interest for CBM exploration and gas outburst prediction.

Influence of pore structure and surface free energy on the contents of adsorbed and free methane in tectonically deformed coal

Differences in pore and surface free energy affect the occurrence of adsorbed and free methane in tectonically deformed coal (TDC), which influences coalbed methane extraction and coal mine safety. Therefore, eight middle-rank TDC samples from the Yueliangtian coal mine (Guizhou Province, Southwest China) were screened. Pore volume and specific surface area were explored via mercury intrusion, nitrogen adsorption/desorption, and carbon dioxide adsorption approaches. Based on the low-field nuclear magnetic resonance analysis for powdered TDC, the relaxation time (T2) amplitudes and the contents of adsorbed and free methane, the surface free energy reduction (SFER), and adsorption potential were measured. 1) The pore specific surface area, pore volume, T2 amplitudes and contents of adsorbed and free methane, SFER, and adsorption potential of the strongly deformed coal are obviously larger than those of the weakly deformed coal. Adsorbed methane content is dominantly affected by micropore specific surface area, and free methane content is primarily influenced by macropore volume. 2) The methane T2 spectra of the TDC contain three distinctive peaks; the first, P1 (T2 < 2 ms), represents adsorbed methane, and the second, P2 (2–40 ms), corresponds to free methane. As the pressure increases, both the T2 amplitude rate and content rate of adsorbed methane decrease, while those of free methane rise. 3) With the enhancement of tectonic deformation, the specific surface area and volume of pores increase (especially those of micropores and macropores), more adsorption positions and storage space for adsorbed and free methane become available, and the SFER and adsorption potential of the TDC are enhanced, leading to increases in the T2 amplitudes and contents of adsorbed and free methane. The present research will help improve the accuracy of coalbed methane evaluation and the prevention of coal and gas outbursts.

Effects of pore-fracture structure of ductile tectonically deformed coals on their permeability: An experimental study based on raw coal cores

The distribution area of ductile tectonically deformed coals (TDCs) usually coincides with the forbidden zones for coalbed methane (CBM) development. Permeability of coal reservoirs, an important indicator to evaluate the energy potential of CBM, is directly affected by the pore-fracture structures. To study the controlling mechanism of pore-fracture structures on permeability, we investigated the micropore - micro-fracture structures of various ductile TDCs by adopting mercury intrusion porosimetry (MIP) and low-pressure gas adsorption. The permeability and the pressure sensitivity of the standard TDC cores were tested by the pressure pulse decay method. The results showed that the total pore volume of ductile TDCs, especially the mesopores and macropores, significantly increased from that of primary structure coals (PSCs), while the volume of transitional pores only slightly increased in strongly crumpled and mylonitized coals, and thus, the dominant pores in coals gradually changed from micro-fractures to mesopores and macropores. The pore-fracture structure of ductile TDCs showed a poorer connectivity and a higher roughness than PSCs. The permeability of typical ductile TDCs obviously decreased, the permeability heterogeneity significantly increased, and the pressure sensitivity of permeability generally showed a pattern of “weakly crumpled - > PSCs > strongly crumpled - and mylonitized coals”. Correlation analysis showed that the weakly crumpled coals that contain well-developed and well-connected micro-fractures and fewer <10 μm coal particles usually display high permeability. In strongly crumpled – and mylonitized coals, the connected pore-fracture structures were damaged and many pore throats and flask-shaped pores formed, which is the main reason for the reduced permeability and the high permeability damage rate.

Thickness Distribution Prediction for Tectonically Deformed Coal with a Deep Belief Network: A Case Study

Thickness of tectonically deformed coal (TDC) has positive correlations with the susceptible gas outbursts in coal mines. To predict the TDC thickness of the coalbed, we proposed a prediction method using seismic attributes based on the deep belief network (DBN) and dimensionality reduction. Firstly, we built a DBN prediction model using the extracted attributes from a synthetic seismic section. Next, we transformed the possibly correlated seismic attributes into principal components through principal components analysis. Then, we compared the true TDC thickness with the predicted TDC thicknesses to evaluate the prediction accuracy of different models, i.e., a DBN model, a support vector machine model, and an extreme learning machine model. Finally, we used the DBN model to predict the TDC thickness of coalbed No. 8 in an operational coal mine based on synthetic experiments. Our studies showed that the predicted distribution of TDC thickness followed the regional characteristics of TDC development well and was positively correlated with the burial depth, coalbed thickness, and tectonic development. In summary, the proposed DBN model provided a reliable method for predicting TDC thickness and reducing gas outbursts in coal mine operations.

Macromolecular transformations for tectonically-deformed high volatile bituminous via HRTEM and XRD analyses

Coalbed methane reservoir properties of tectonically deformed coals (TDCs) vary greatly in comparison to unaltered coals. These differences are attributed, in part, to their macromolecular transformations. The structure of different TDCs at various distances from a fault plane was evaluated using image analysis of aromatic fringes from HRTEM micrographs, and from XRD data. The distribution of fringe length, orientation, stacking, and curvature, were quantified to establish the structural organization. From the fringe length, orientation, and XRD data, two deformation stages were identified a) a weak deformation stage (–cataclastic–schistose–scaly) and b) a strong deformation stage (–wrinkled–mylonitic). In the weak deformation stage, the fringe length distribution was consistent, however the strong deformation coals had longer fringes. The majority (94–98%) of the aromatic fringes were individual layers for both stages. There was a significant improvement in the structural alignments however with strong deformation. The common statement that tectonic stress enhances the stacking of the aromatic clusters (and thus perhaps the graphitization ability) was not observed here. Rather, the enhancement in order was limited to the strong deformation stage. The aromatic fringes of the strong deformation coals had greater curvature extents. The XRD data also supported increased organization (lower d002 values) with strong deformation. These observations indicate there was an improvement in the basic structural unit from poorly ordered to more ordered that occurred in the strong deformation stage. These transformations are likely to impact the coalbed methane production and methane behaviors.

Effects of pore structure on methane adsorption behavior of ductile tectonically deformed coals: An inspiration to coalbed methane exploitation in structurally complex area

The distinctive pore structure of tectonically deformed coals (TDCs) has a considerable influence on their methane adsorption behavior, resulting in higher coalbed methane (CBM) pressure and higher CBM content, and thus increasing the difficulty of CBM development in TDCs distribution area (especially ductile TDCs). In this paper, the supermicropore-microfracture structures of primary structure coals (PSCs) and ductile TDCs from the Suxian mining area were characterized by using mercury intrusion porosimetry (MIP) and gas adsorption methods (N2 and CO2). Methane adsorption test was conducted to study the methane adsorption behavior. Then, the control mechanism of pore structure on methane adsorption behavior of ductile TDCs was investigated. Results show that the supermicropores (<2 nm) surface area generally increases and then decreases. The micropores (2–10 nm) and small transitional pores (10–40 nm) volume of ductile TDCs is slightly larger than that of PSCs, while the volume of big transitional pores (40–100 nm) experiences a steady increase. The mesopores (100–1000 nm) and macropores (1000–10000 nm) volume displays a similar pattern of “steep increase-slightly decrease -stable increase” with the enhancement of deformation. The microfractures (>10000 nm) volume first increases and then decreases from the PSCs to mylonitized coals. Langmuir volume (VL, daf) displays no obvious change rules with the increasing ductile deformation, but the Langmuir pressure (PL, daf) of ductile TDCs is smaller than that of PSCs and gradually decreases to its minimum in the mylonitized coals. The Pearson correlation coefficients show that the volume of big transitional pores (40–100 nm), mesopores and macropores both shows obvious negative correlations with the PL, daf, which indicates that the development of 40–10000 nm pores in ductile TDCs causes a lower PL, daf, resulting in higher CBM pressures and contents in the ductile TDCs distribution area under the in-situ pressure condition.

Productivity Evaluation of Coalbed Methane Well with Geophysical Logging-Derived Tectonically Deformed Coal

Tectonically deformed coal (TDC) has a significant influence on coalbed methane recovery. Well-logging is a reliable and efficient way to predict the development of TDCs for understanding the role of TDC on the productivity of coalbed methane (CBM) wells. However, there has been insufficient research regarding both the static physical properties and dynamic invasion of mud-filtrate related to TDCs. Therefore, a new TDC-detecting approach using two indicators of the relative physical properties and fluid attributes was proposed. Through the data normalization and optimization of correlation coefficients and factor analysis, five logs were chosen to construct the two indicators, and three types of TDC were recognized: I—undeformed or cataclastic; II—granulated; and III—mylonitized. It was found out that the identification error rate decreased from 30% to 15%. Furthermore, the thickness ratio of a well-preserved coal layer derived from TDC interpretation was adopted to correlate the gas production of a coal seam. An application in the Hancheng block demonstrated that the thickness ratio of 60% is an explicit threshold value to distinguish between high-yield well (&gt;1000 m3/d) and low-yield one (&lt;750 m3/d). The development of granulated and mylonitized coals mainly exerts negative influence on CBM well production.

Dissolution behavior and chemical characteristics of low molecular weight compounds from tectonically deformed coal under tetrahydrofuran extraction

Tectonically deformed coal (TDC) is formed by complex and variable tectonic stresses. The induced physical-chemical effects result in a variety of chemical compositions and structures in TDCs. In this work, medium volatile bituminous coals with four types of tectonic deformation were selected and extracted with tetrahydrofuran to investigate the dissolution behavior and distribution of low molecular weight compounds in TDCs. The results revealed that a higher degree of tectonic deformation in the coal produces a higher total extraction yield as the total extraction time increases. A diffusion model also establishes a good relationship between the extraction mass and extraction time, and the diffusion coefficient Dobs can be used to describe the extraction kinetics. Chemical analysis of the extracts using column chromatography and GC/MS further found that there are more binding points for forming non-covalent bonds in long chain aliphatic hydrocarbons. So the alkanes with more than 20 carbon atoms are often dissolved at the medium and late stages. Aromatic compounds, which are mostly dissolved at the medium stage, are essentially combined into the coal macromolecular structure by π–π stacking interactions or hydrogen bonds. While a large number of heteroatomic compounds are distributed widely in coals, resulting in diverse dissolution behaviors. Under the tectonic stresses, the type and content of low molecular weight compounds in TDCs increase compared to intact coal. Particularly, an increase in tectonic deformation appeared to produce more aromatic hydrocarbons and long chain alkanes in TDCs, whilst the presence of heteroatomic compounds becomes more complex.

Variation of Surface Free Energy in the Process of Methane Adsorption in the Nanopores of Tectonically Deformed Coals: A Case Study of Middle-Rank Tectonically Deformed Coals in the Huaibei Coalfield

Methane adsorption of coal is essentially a process of energy and material state transformation. It is a complex process especially for tectonically deformed coals (TDCs). In order to clarify this ...

Structural and evolutionary characteristics of pores-microfractures and their influence on coalbed methane exploitation in high-rank brittle tectonically deformed coals of the Yangquan mining area, northeastern Qinshui basin, China

The Yangquan mining area is an important anthracite production base in China, but various types of tectonically deformed coals (TDCs) are well developed in its mines and gas outburst accidents take place frequently. By using methods such as optical microscope observation, mercury intrusion porosimetry (MIP) and low pressure nitrogen/carbon dioxide gas adsorption (LP-N2/CO2GA), we tested the structural and evolutionary characteristics of pores-microfractures and their influence on coalbed methane exploitation in high-rank (Ro,max = 2.00–2.72%) primary structure coals and brittle TDCs (i.e., cataclastic-, mortor-, and granulitic coals) from the Yangquan mining area. The results show that the tectonic deformation can change the pore-microfracture structure of coals and, thus, their reservoir physical properties. In general, with the enhancement of tectonic deformation, the density of microfractures in coal, which become more and more mussy, gradually increases, the total pore volume (TPV) of coal (pores with a diameter of 5.37–27000 nm) and the pore volume of each pore size range increase, with the greatest increase occurring in pores of 100–1000 nm, and the connectivity between pores is gradually reduced as well. From primary structure coals to granulitic coals, the TPV by the Barret-Joyner-Halenda (BJH) model increases with a greater increase in mesopores, and the specific surface area (SSA) by the Brunauer-Emmett-Teller (BET) model shows a trend of “increase-decrease-increase”, while the average pore diameter changes follow the pattern of “decrease-increase-decrease” in 0.7–206 nm. However, tectonic deformation has little effect on micropores with a diameter of less than 1 nm in brittle TDCs. The difference in pore-microfracture structures of different brittle TDCs indicates that the cataclastic coals distribution area is favorable for coalbed methane (CBM) exploitation, while the granulitic coals distribution area is a gas outburst-prone area.

Experimental investigation into the damage-induced permeability and deformation relationship of tectonically deformed coal from Huainan coalfield, China

Knowledge of the mechanical behavior and permeability evolution of tectonically deformed coal (TDC) is the foundation for the successful design of enhanced gas drainage. However, our understanding of the effect of progressive damage on the fluid flow in TDC is limited due to its unique structure and low visibility. We report measurements of the deformation, strength and permeability evolution during triaxial compression of soft coal samples made by TDC. According to the experimental results, the soft coal can generate an obvious nonlinear-elastic deformation before the yield strength, leading to the formation of unique features of the fracture volume evolution of soft coal. The closure of the existing microcracks in soft coal continues until the reversal of the total volume when the confining pressure is greater than or equal to 8 MPa. The fracture compressibility constants of the three soft coal samples were 4.71E-2 MPa−1, 4.63 E−2 MPa−1 and 4.89 E−2 MPa−1, which are much smaller than the literature values reported for other intact coals, which range from 6.21E−2 MPa-1 to 2.71E−1 MPa−1, indicating that the stress sensitivity of TDC is weaker than that of the intact coal. During the progressive deformation of soft coal, the permeability only reverses when the stress state exceeds the fracture damage stress. The study's achievements show that the simplification of the pre-failure behavior as a perfectly elastic mode is not applicable to the soft coal and would lead to mistakes in further permeability calculations. Further work should be extended to develop a more appropriate constitutive model and a permeability model to describe the pre-failure non-linear (elastic) deformation and the corresponding permeability evolution of soft coal.

Macromolecular evolution and structural defects in tectonically deformed coals

Here, the differential macromolecular evolutions and the structural defects in various TDCs (tectonically deformed coals) were profoundly investigated utilizing the 13C NMR, FTIR, Raman, and DFT (density function theory) calculations. The increasing aromatics provides evidences for advance evolution for aromatic clusters induced by dynamic metamorphism. The variations of fal∗ (non-protonated carbons) and falH (CH or CH2) indicates that the brittle could promote the aliphatic carbon diversifications while ductile deformation could significantly promote the loss of aliphatic carbons. The I2 (=A900–700/A3000–2800) gradually increases while the I3 (=A1703+1745/A1618) decreases. The comparisons of the D1 peak in TDCs with those in perfect- and defective graphene indicated the existences of the structural defects in TDCs. The ductile TDCs have the lowest D1 peak positions, indicating that ductile deformation can promote the macromolecular disorder degrees to the maximum extent. The gradual increase of ID1/IG and the good linear relationship (R2 = 0.90) between ID1/IG and AD1/AG indicated the gradual increasing content of structural defects with the enhancement of the tectonic deformation. The geological conditions and the Raman results indicated the existences of the Stone-Wales defect (SW) and vacancy-like defects, including the single vacancy (SV), double vacancies (DV), and multi vacancies (MV1 and MV2) in TDCs. The evolution paths of DV (formation energy, Ef: 11.68 eV) → MV1 (Ef: 9.74 eV) → MV2 (Ef: 18.34 eV) and initial structures → SW defects have witnessed the increasing defects and disorder degrees with the enhancement of the tectonic deformation without damaging the continuity of coal matrix structures, which macroscopically manifests as ductile deformation. However, the evolution paths of initial structure → DV (Ef: 11.68 eV) and initial structure → SV (Ef: 9.08 eV) have damaged the continuity by missing carbons, which are consistent with the brittle TDCs.

Methane Adsorption Characteristics and Adsorption Model Applicability of Tectonically Deformed Coals in the Huaibei Coalfield

Based on methane isothermal adsorption experiments, the supercritical methane adsorption characteristics of middle-rank tectonically deformed coals (TDCs) screened from the Huaibei coalfield were analyzed. The applicability of different adsorption models to different kinds of TDCs was discussed using the standard deviation method, and the mechanism of methane adsorption in TDCs was explored. The following results were obtained. First, the experimental maximum adsorption capacity of TDCs increases gradually with enhanced tectonic deformation, and the experimental maximum adsorption capacity of ductile TDCs is significantly higher than those of primary coal and brittle TDCs. The adsorption potential of TDCs gradually decreases with increasing adsorption space, and the adsorption potential of ductile TDCs is generally higher than those of primary coal and brittle TDCs. Second, for the applicability of adsorption models to TDCs, the highly applicable models of primary, cataclastic, and scaly coals are the Toth, Langmuir–Freundlich, and Dubinin–Astakhov models; the highly applicable models of schistose coals are the Langmuir, Toth, Langmuir–Freundlich, three-parameter Brunauer–Emmett–Teller, Dubinin–Radushkevich, and Dubinin–Astakhov models; the highly applicable models of wrinkle coals are the Toth, Langmuir–Freundlich, Dubinin–Radushkevich, and Dubinin–Astakhov models; and the highly applicable models of mylonitic coals are the Langmuir, Toth, Langmuir–Freundlich, expand-Langmuir, three-parameter Brunauer–Emmett–Teller, and Dubinin–Astakhov models. Third, the three-parameter Brunauer–Emmett–Teller model is suitable to study the adsorption state of TDCs. As the deformation degree increases, the adsorption state of TDCs transforms from monolayer unsaturated adsorption to multilayer adsorption. TDCs have larger adsorption potential and adsorption space with the enhancement of tectonic deformation, which increases the number of adsorption layers on the coal surface.

Influence of coal moisture on initial gas desorption and gas-release energy characteristics

Coal and gas outbursts (hereinafter referred to as ‘outbursts’) result in serious damage and often occur in tectonically-deformed coal that is rich in gas. It has been demonstrated that the level of outburst risk declines with increasing coal moisture content. Moreover, an outburst generally lasts for mere tens of seconds and the required energy is mainly provided by the gas expansion energy. However, the gas desorption and energy release characteristics of tectonically deformed coal with different moisture contents during the first dozen seconds have not often been studied. In this study, a set of self-designed gas desorption equipment was used to carry out gas adsorption-desorption experiments in the first dozen seconds (about 13 s) on coal with different moisture contents. The results show that the drop rate of the gas pressure increases with an increase in coal moisture content, and the total amount of gas desorption and the mass flow rate of gas desorption reduce correspondingly in the gas desorption process. Moreover, the gas velocity also slows with increasing moisture content. Under different gas pressures, the total gas expansion energy (TGEE) and the total gas energy (TGE) released from the coal decrease with the increasing moisture content. Correspondingly, it takes a shorter time for coal to release 90% of the TGEE and the TGE. For all of the coal samples, the time taken for releasing 90% of the TGEE is shorter than that for releasing 90% of the TGE. Meanwhile, the ratio of TGEE in the TGE increases with the increasing moisture content. The TGEE accounts for 14–16% of the TGE released from coal samples of different moisture contents under different gas pressures.

Detection of Tectonically Deformed Coal Using Model-Based Joint Inversion of Multi-Component Seismic Data

Tectonically-deformed coal (TDC) is a potential source of threats to coal-mining safety. Finding out the development and distribution of TDCs is a difficult task in coalfield seismic explorations. Based on the previous investigations, the P- to S-wave velocity ratio (α/β) is a stable parameter for the identification of TDCs and most TDCs have α/β values of less than 1.7. Here, a TDC detection method using a model-based joint inversion of the multi-component seismic data is proposed. Following the least square theories, the amplitude variation with offset gathers of the PP- and PS-waves are jointly inverted into the corresponding α/β values. The prior models generated from the P- and S-wave velocity and density logs are employed in the joint inversion to enhance the inversed models. Model test results show that the model-based inversion is of high anti-noise ability and has a good recognition ability of TDCs. The proposed method is applied to a work area of the Guqiao mine in China. The TDCs developed in coal seam 13-1 are effectively identified according to their inverted α/β values of less than 1.7. The detection result is verified by the well and tunnel excavation information.

Macromolecular response to tectonic deformation in low-rank tectonically deformed coals (TDCs)

The nano-porous and optical properties of tectonically deformed coals (TDCs) are well characterized while their macromolecular structures have been minimally reported in recent years. In this study, the structural transformations of low-rank TDC macromolecular structures were identified utilizing 13C nuclear magnetic resonance (13C NMR) and Fourier transform infrared spectroscopy (FTIR). Both the NMR aromaticity (fa’) and the ratio of aromatic bridge carbon to aromatic peripheral carbon (xBP) increase with the increasing deformation intensity, indicating the aromatization process induced by the stress condensation. The carbonyl first increases then decreases with the increasing deformation intensity, leading the maximum value in granulitic coal. The decrease of A (CH2)/A (CH3) (defined by FTIR A2940-2900/A3000-2940) indicates the increasing branching degree and shorting aliphatic chains especially for the ductile- and strong brittle deformation processes. The Al/-OX (defined by FTIR A3000-2800/A1800-1520) firstly increases until the granulitic coal and then decreases, indicating that the loss rate of the oxygen functional groups is higher than that of the aliphatic carbons during the brittle deformation process but lower than the latter during the shear- and ductile deformation stages. The ductile- and strong brittle deformations (especially for the deformation process of mortar- to granulitic coal) have a more significant promote effect for aliphatic loss than the weak brittle deformations (i.e. deformation process of primary-, cataclastic-, and mortar coal). The loss of oxygen functional groups is significant during shear- and brittle deformation stages but rather slight during the ductile deformation stage. The strong shear TDCs (i.e. mortar- and granulitic coal) have a relative abnormal high content of ether, hydroxybenzenze, and CH2CO, indicating that the shear deformation process has a significant promote effect for the diversification in the aliphatic carbons and oxygen functional groups. Then, the relative content of ether, hydroxybenzenze, and unsaturated carboxylic acids overall decreases with the increasing deformation intensity and the reduction rate all manifests as high at the shear deformation stage and low at the ductile deformation stage.

Permeability characteristics of remolded tectonically deformed coal and its influence factors

The characterisations of Tectonically deformed coals (TDCs) are different from those of undeformed coals. Permeability characteristics of TDC and its influence factors should be studied deeply. In this paper, five samples of TDCs in various degrees of deformation and one undeformed coal were selected. The microstructure was observed by using scanning electron microscope. The pore structure characteristics were analyzed through a mercury injection experiment. Permeability was tested by steady flow. In TDCs, the crumpled and cracked microstructures are well developed and the mesopores and macropores account for large proportions. The permeability of TDC is mainly controlled by structural deformation degree, effective stress (or effective confining pressure), adsorption swelling effect and slippage effect. On the premise of only effective stress increasing, the internal space of deformed coal is compressed and the slippage effect is weakened. The both factors contribute to the permeability reducing in an exponential form. On the premise of only pore pressure increasing, the gas seepage channels are narrowed and obstructed because of the adsorption swelling effect and the slippage effect is also weakened. In this case, the permeability will also be reduced. In TDCs, the pores and fractures are strongly developed, the gas seepage channels are markedly unobstructed and the slippage effect is enhanced. Thus, the permeability of TDC is much larger than that of undeformed coal. Generally, the permeability change rate of TDCs is larger than that of undeformed coal and shows a negative correlation with effective confining pressure. The stress sensitivity coefficient of permeability also shows a negative correlation to the deformation degree of coals.

Variations in stress-sensitive minerals and elements in the tectonic-deformation Early to Middle Permian coals from the Zhuxianzhuang mine, Anhui Province

The variations in stress-sensitive minerals and elements in tectonically deformed coals (TDCs) were revealed in this paper. Results show that the clay minerals, carbonates and sulfides in the Middle to Lower Permian TDCs of the no. 8 and no. 10 coal seams of the Zhuxianzhuang mine are dominated by kaolinite, iron dolomite and calcite, and pyrite, respectively. The contents of Th are significantly higher than U, indicating that the sedimentary paleo-environment was almost independent of seawater and was characterized by hypoxia during peat accumulation periods. The light rare earth elements (LREEs) are enriched, yielding (La/Lu)N values of 1.00–12.30 and slightly positive Ce anomalies in the no. 8 (0.95–1.44, δCe/δEu=1.30–2.62) and no. 10 coal seams (1.02–1.31, δCe/δEu=1.66–2.17). The cluster results show that stress-sensitive elements in TDCs can be classified as aggregation-type (Si, Al, Ti, K, Fe, Sc, Li, Rb, Nb, Ag, Cd, Ta, Tl, Bi, Th, U, W), dissipation-type (N, H, Cl, Mo, Ni, Y, Sr), and mutant-type (O, Na, Ca, Be, Mn, Co, Ga, Ge, Pb, Bi, As). The enrichment effects of ductile deformations for major elements are more significant than those of brittle deformations. The combinations of (Nb, U), (Th, Rb), and (Li, Sc, Ag, Ta) are good brittle-, ductile-, and brittle-ductile enrichment-type elements, respectively, which can be used as good indicators of the coal's tectonic deformation. Sr records higher loss rates in brittle TDCs than in wrinkle coals. The Mo content gradually decreases with increasing tectonic deformation intensity, which is related to the dynamic metamorphism caused by tectonic deformation. Nucleophilic clay mineral elements (Be, Ga, Pb, Ge) in the no. 8 and no. 10 coal seams record similar trends and are highest in scaly coals, indicating that strong shearing effects can promote the enrichment of these elements. The contents of LREE and heavy rare earth elements (HREEs) both increase with the increasing tectonic deformation intensity. For various TDCs, LREE record higher degrees of fractionation than the HREE.