Stress sensitivity has significant negative effects on the permeability and production of coalbed methane (CBM) reservoirs. To effectively minimize these negative effects, the degree of stress sensitivity during the CBM production process should be carefully studied. In this work, the curvature of the stress-sensitivity curve was adopted to explore the degree of stress sensitivity, dividing the stress-sensitivity curve and the drainage process into five stress stages: sharp decrease, rapid decrease, low-speed decrease, slower decrease and harmless with four critical stress points—transition, sensitivity, relief and harmless. The actual stages were determined by the initial permeability, stress-sensitivity coefficient and difference between the reservoir pressure and desorption pressure. The four critical stress points did not completely exist in the stress-sensitivity curve. With an increase in the initial permeability of coal, the number of existing critical stresses increases, leading to different gas-water drainage strategies for CBM wells. For reservoirs with a certain stress-sensitivity coefficient, the permeability at the sensitive stress point was successively greater than that at the transition, relief and the harmless stresses. When the stress-sensitivity coefficient is different, the stage is different at the beginning of drainage, and with an increase in the stress-sensitivity coefficient, the decrease rate of the permeability increases. Therefore, the stress-sensitivity coefficient determines the ability to maintain stable CBM production. For well-fractured CBM reservoirs, with a high stress-sensitivity coefficient, permeability damage mainly occurs when the reservoir pressure is less than the relief stress; therefore, the depressurization rate should be slow. For CBM reservoirs with fewer natural fractures, the reverse applies, and the depressurization rate can be much faster. The higher the difference between the reservoir and desorption pressures, the higher the effective stress and permeability damage after desorption, resulting in a much longer drainage time and many difficulties for the desorption of coalbed methane. The findings of this study can help better understand and minimize the negative effects of stress sensitivity during the CBM production process.

Pores are the main accumulation sites and migration pathways for coalbed methane (also referred to as CBM). Pore structure restricts the content and recoverability of CBM from coal reservoirs. In this study, 12 representative coal samples with different ash yields that have similar tectonic characteristics and burial depths were collected from different mining areas in the Jiergalangtu and Huolinhe depressions in the Erlian Basin. These samples were used to study the restrictions of ash yield on the characteristics of coal pore structures and the recoverability of CBM through macroscopic and microscopic structure observation, scanning electron microscope observations, vitrinite reflectance tests, low-temperature N₂ adsorption, nuclear magnetic resonance (NMR), and micro-computed tomography. The results show that coal reservoirs in the study area vary greatly in ash yield, based on which they can be divided into three types, i.e., low-ash-content, ash-bearing, and high-ash-content coal reservoirs. In addition, the ash yield has a certain impact on the development of coal pores; coal samples with lower ash yields indicate the presence of well-developed medium-large pores and better connectivity. Ash yield also has a certain impact on the brittleness of coal wherein a lower ash yield implies the development of brittle coal that is more liable to fracture as compared to less brittle samples at the same pressure. Absorbed gas content also varies significantly with ash yield; a low ash yield impacts the gas saturation of coal. Overall, for coal reservoirs in the study area, their porosity, pore diameter, movable fluid porosity, adsorbed gas amount, and recoverability decrease as the ash yield increases.

Research on the characteristics of faults and their evolutionary history since the Cretaceous in the Suhongtu-Dagu depressions can provide a theoretical basis for geological evaluation of the coal seams in the Suhongtu Formation in the northern-central region of the Yin’e Basin. Using 3-D seismic-logging inversion techniques, seismic stratigraphic calibration, stratigraphic sequence delineation, and thickness calculations on the Suhongtu-Dagu depressions were carried out to clarify the planar and profile distributions of the faults, as well as the evolutionary history of these faults and the tectonic history of the depressions. The results of this study revealed that the distribution of the faults in the Suhongtu-Dagu depressions in the northern part of the Yin’e Basin varies with region, and the fault system was multi-period, orthotropic, north-east-trending, and north-north-east-trending, with a certain degree of inheritance in terms of the geological setting. Three types of faults were identified: Y-shaped fractures, reverse Y-shaped fractures, and parallel fractures, which can be classified as Paleozoic-Cenozoic continuous syncline faults and intra-depression faults from the top of the Permian to the Upper Cretaceous series and inter-stratigraphic adjustment faults within the Cretaceous System, respectively. The evolution of these faults can be divided into three phases: the controlling faults were the faults that existed before the Early Cretaceous and had been active since then; synclinal faults that formed during the Early Cretaceous; and modified faults that formed since the Early Cretaceous. The development and modification of the coal seams in the Cretaceous Suhongtu Formation in the Hari, Kuanzihu, and Babei sags were strongly controlled and influenced by a multi-phase complex fault system.

The mechanical characteristics of coal reservoirs are important parameters in the hydraulic fracturing of coal. In this study, coal samples of different ranks were collected from 12 coal mines located in Xinjiang and Shanxi, China. The coal ranks were identified with by the increased Maximum vitrine reflectance (R_o,max) value. The triaxial compression experiments were performed to determine the confining pressure effect on the mechanical properties of coal samples of different ranks. The numerical approaches, including the power function, arctangent, and exponential function models, were used to find the correlation between coal elastic modulus and the confining pressure. The fitting equations of compressive strength and elastic modulus of coal ranks were constructed under different confining pressures. The results showed that the coal compressive strength of different ranks has a positive linear correlation with the confining pressure. The coal elastic modulus and confining pressure showed an exponential function. Poisson’s ratio of coal and confining pressure show negative logarithmic function. The stress sensitivity of the coal elastic modulus decreases with the increase of confining pressure. The coalification jump identifies that the compressive strength, elastic modulus, and stress sensitivity coefficient of coal have a polynomial relationship with the increase of coal ranks. The inflection points in coalification at R_o,max = 0.70%, 1.30%, and 2.40%, are the first, second, and third coalification jumps. These findings provide significant support to coal fracturing during CBM production.

To understand the natural gas characteristics of multi-thin coal seam, this study selected the desorbed gas of coal seams in different layers of Well A in the Wujiu depression, Hailar Basin in northeast Inner Mongolia. The results show that the heavy hydrocarbon content of desorbed gas increases significantly with the increasing depth. Methane carbon (δ¹³C₁) and ethane carbon (δ¹³C₂) isotope values are vertically become heavier downwards, while the δ¹³{\mathrm{C}}_{{\mathrm{C}\mathrm{O}}_2} values did not change significantly. The kerogen is close to the III–II mixed type with the source rocks mainly deposited in a shore/shallow lake or braided-river delta front, and the gas produced has certain characteristics of oil associated gas. However, the characteristics of oil associated gas produced by the organic formed in the shallow-water environment (braided-river delta plain) are not obvious. The sandstone pore and fracture systems interbedded with multi-thin coal seam are well developed. And it is conducive to the migration of methanogenic micro-organisms to coal seams via groundwater, making it easier to produce biogenic gas under this geological condition. During the burial evolution of coal-bearing strata in the study area, when the burial depth reaches the maximum, there are significant differences in the paleotemperature experienced by different vertical coal seams, caused by a high-paleogeothermal gradient, increasing the δ¹³C₂ of desorbed gas with increasing depth. The above research indicates that there is less biogenic gas in the multi-thin coal seams with relatively developed mudstone, and the multi-thin coal seams with relatively developed sandstones have obvious biogenic gas characteristics. Therefore, for the exploration and development of biogenic gas in low-rank multi-thin coal seams, it is necessary to give priority to the layer with high sandstone content.

Gas diffusion in the coal matrix plays a significant role in forecasting the production performance of coalbed methane (CBM) wells. To better understand methane diffusion behavior, a systematic study was performed on various rank coals with vitrinite reflectance (R_o,m) ranging from 0.46% to 2.79%. Multiple experiments, including coal petrographic analysis, field emission scanning electron microscopy (FESEM), low-temperature N₂ adsorption/desorption, and mercury intrusion porosimetry (MIP), were conducted to quantitatively characterize the multiscale micro-nano pore system in different rank coals, which showed that the pore structure of coals exhibited a multimodal pore size and volume distribution. Isothermal adsorption-diffusion experiments using the volumetric method were also performed to understand the methane diffusion characteristics in the micro-nano pores of the coal reservoir. The applicability of the multiporous diffusion model is verified, and methane diffusion in the multi-scale pores of coal reservoirs exhibits the characteristics of early fast diffusion, transitional diffusion in the medium term, and slow diffusion in the later period. In addition, the factors affecting methane diffusion in coals were systematically analyzed, and gray relational analysis (GRA) was employed to analyze and identify the importance of these factors on methane diffusion. The results show the impact ranking of factors, in order from the most important to the least: particle size > moisture > surface area > pore volume > pressure > coal rank > temperature in all of three diffusion stages. These findings are helpful for forecasting production performance and enhancing the production efficiency of CBM.

Coal-derived natural graphite (CDNG) has multiple industrial applications. Here, ten metamorphic coals from anthracite to CDNG were obtained from Lutang and Xinhua in the Hunan Province and Panshi in the Jilin Province. Bulk characterization (proximate and ultimate analyses, X-Ray powder diffraction (XRD), and powder Raman spectroscopy), along with optical microscopy, scanning electron microscope (SEM) and micro-Raman spectroscopy were utilized to examine the transitions from anthracite to semi-graphite to CDNG. The XRD and Raman spectroscopy data indicate that from anthracite to highly ordered graphite the average crystal diameter (L_a) and height (L_c) increased from 6.1 and 4.6 nm to 34.8 and 27.5 nm, respectively. The crystalline parameters of the CDNG samples from Panshi and Lutang varied slightly when closer to the intrusive body. Optical microscopy and SEM indicated that in the anthracite samples there were thermoplastic vitrinite, devolatilized vitrinite, and some “normal” macerals. In the meta-anthracite, pyrolytic carbon, mosaic structure, and crystalline tar were present. In the CDNG there were flake graphite, crystalline aggregates, and matrix graphite. The crystalline aggregates show the highest structural ordering degree as determined from Raman spectral parameters (full-width at half maxima (G-FWHM) ~20 cm⁻¹, D1/(D1 + D2 + G) area ratio (R2) value < 0.5). The flake graphite is less ordered with G-FWHM ~28 cm ⁻¹ and 0.5 < R2 < 1, but a larger grain size (up to 50 μm). The mosaic structures were likely the precursors of the matrix graphite through in situ solid-state transformation. The pyrolytic carbon and crystalline tars are the transient phase of gas-state and liquid-state transformations. This study is beneficial to realize the rational utilization of CDNG.

The acoustic response of gas and/or water saturated coal rock is fundamental for establishing the correspondence between the physical properties of the coal reservoir and the characteristics of the well-logging response, which is the technology essential for the geophysical exploration of coalbed methane (CBM). This acoustic response depends on water (S_w) and gas (S_g) saturation among other factors. In this study, we performed acoustic tests on dry and different gas-water saturated coal samples with different degrees of metamorphism and deformation, collected from several coal mining areas in China. These tests enabled us to analyze the influence of coal type and gas-water saturation on the acoustic response of CBM formations. Our results show that the acoustic velocity of P-wave and S-wave (V_p and V_s, respectively), and the relative anisotropy of and V_s, increased with increasing vitrinite reflectance, density, V_p and S_w. WithS_w increasing from 0 to 100%, the growth rate of the acoustic velocity decreased with increasing vitrinite reflectance. The V_p/V_s ratio of tectonic coal was generally higher than that of primary coal. The growth rate of the relative anisotropy in tectonic coal was markedly higher than that in primary coal.

The behavior of coalbed methane (CBM) diffusion considerably influences gas productivity. Based on the multi-porous diffusion model and on-site CBM desorption data of coal cores, the behavior of CBM diffusion and its implications on the gas productivity of No. 3 coal seam in the southern Qinshui Basin (SQB) were elaborately analyzed. Results indicate that CBM diffusion of No. 3 coal seam demonstrates noticeable three-stage characteristics, including the fast diffusion, transitional diffusion, and slow diffusion stages. During the gas diffusion process, the gas content and/or the degree of developed pores and fractures/cleats in coal seams can affect the desorption of CBM and the amount of diffused CBM by influencing the changes in gas pressure in pores, thus controlling the behavior of gas diffusion in different stages. Because gas content and the developed degree of pores and fractures/cleats are closely associated with the deformation degree of the coal seams, variably deformed coal seams exhibit unique characteristics of gas diffusion. The low-deformation degree of the coal seams have a relatively uniform distribution of gas production over the history of a well. By contrast, the moderate-deformation degree of the coal seams have a relatively high rate and amount of gas diffusion in the fast and transitional diffusion stages, producing most of the gas in the early-to-intermediate stages of the wells. Finally, the high-deformation degree of the coal seams has a high rate and amount in the fast diffusion stage, indicating that most of the production stage occurs during the early stage of the gas production history of a well. In summary, the behavior of gas diffusion can be used for predicting gas production potential.

In this work, CH₄ isothermal adsorption measurements were carried out on 64 coal samples collected from western Guizhou Province of China, and the coalbed methane (CBM) desorption processes were quantitatively analyzed. The results show that the Langmuir volume and the Langmuir pressure are controlled by coalification, and tend to increase as the vitrinite reflectance changes from 0.98% to 4.3%. Based on a division method of CBM desorption stages, the CBM desorption process were divided into four stages (inefficient, slow, fast and sensitive desorption stages) by three key pressure nodes (the initial, turning and sensitive pressures). The fast and sensitive desorption stages with high desorption efficiency are the key for achieving high gas production. A theoretical chart of the critical desorption pressure (P_{\mathrm{c}\mathrm{d}}) and its relationship with different pressure nodes was established. The higher-rank coals have the higher initial, turning and sensitive pressures, with larger difference between pressure nodes. Most CBM wells only undergo partial desorption stages due to the differences in P_{\mathrm{c}\mathrm{d}} caused by the present-gas content. Under the same gas content conditions, the higher the coal rank, the less desorption stages that CBM needs to go through. During coalbed methane co-production from multiple coal seams within vertically superposed pressure systems, the reservoir pressure, the P_{\mathrm{c}\mathrm{d}}, the initial working liquid level (WLL) height, and coal depth are key factors for evaluating whether coal seams can produce CBM simultaneously. It must be ensured that each production layer enters at least the fast desorption stage prior to that the WLL was lower than the depth of each layer. Only on this basis can all layers achieve the maximum gas production.

During the coalbed methane (CBM) exploitation, the reservoir permeability can be affected by the effective stress that varies with the reservoir fluid pressure, which is a complex, dynamic and significant engineering problem. To analyze the response characteristics of the pore-fracture system by the changing stress, this work simulated reservoir and fluid pressures during the exploitation by adjusting confining pressure and displacement pressure. Stress sensitivity experiments under different effective stresses were conducted to systematically study the stage variation characteristics of porosity and permeability of coal. The results show that the permeability decreases exponentially with the increase in effective stress, consistent with previous studies. However, the porosity shows a V-shaped trend, which is different from the traditional understanding that it would decrease continuously with rising effective stress. These variation characteristics (of porosity and permeability above) therefore result in a phased porosity sensitivity of coal permeability (P_PS). Moreover, the stress sensitivity of the samples was evaluated using the permeability damage rate method (M_PDR) and the stress sensitivity coefficient method (M_CSS), both of which showed that it ranges from the degree of strong to extremely strong. When the effective stress is lower than 5–6 MPa, the stress sensitivity of the coal reservoir drops rapidly with effective stress rising; when it is higher than 5–6 MPa, the change in stress sensitivity tends to flatten out, and the stress sensitivity coefficient (C_SS) goes down slowly with rising effective stress. Finally, suggestions are proposed for the drainage scheme of CBM wells based on the experimental results.

High- and low-rank coalbed methane (CBM) are both important fields of CBM development in China, but their formation and production mechanisms differ considerably. The adsorption/desorption behavior of high- and low-rank coals under the coupling of coal–water–gas was investigated using two series of samples. Coal samples from Zhangjiamao (ZJM) coal mine, Ordos basin, and Sihe (SH) coal mine, Qinshui basin, were tested by isothermal adsorption–desorption experiment, natural imbibition experiment, nuclear magnetic resonance, mercury injection porosimetry, contact angle test, and permeability test. Isothermal adsorption and desorption experiments under dry, equilibrium water, and saturated water, were performed to explore the differences between the adsorption and desorption characteristics. The results show that the wettability and permeability of the ZJM low-rank coal sample was considerably higher than that of the SH high-rank coal sample. The imbibition process of the ZJM sample exhibited a high imbibition rate and high total-imbibition volume, whereas the SH sample exhibited a slow imbibition rate and low total-imbibition volume. The ZJM sample had a complex pore structure and diverse pore-size distribution with a lower mercury withdrawal efficiency at 59.60%, whereas the SH sample had a relatively uniform pore-size distribution with a higher mercury withdrawal efficiency at 97.62%. The response of adsorption and desorption of the ZJM sample to water was more significant than that of the SH sample. The desorption hysteresis of the ZJM sample was stronger than that of the SH sample and was more prominently affected by water, which was consistent with its strong wettability and complex pore-throat configuration. A comprehensive adsorption and desorption mode was constructed for high- and low-rank coal samples under coal–water–gas coupling condition. The research results are important to enrich the geological theory of high- and low-rank CBM and to guide efficient CBM recovery.

The Ordos Basin is an important intracontinental sedimentary basin in China, containing a significant amount of coal, oil, and natural gas. This study analyzed the sedimentary environment, sedimentary facies, parent material type, maturity, and carbon isotopic composition of the coal-bearing organic matter using gas chromatography–mass spectrometry (GC–MS) and stable isotope ratio mass spectrometry. The source of oil occurring in the No. 2 coal seam of the Jurassic Yan’an Formation (An-1 oil) and its accumulation model were also investigated. The results show that the relative abundances of C₂₇, C₂₈, and C₂₉ steranes in the An-1 oil are 43.8%, 33.0%, and 23.2%, respectively. The tricyclic terpanes, C₂₉20S/(20S + 20R), and C₂₉ββ/(ββ+αα) contents of the An-1 oil are 31.4%−34.8%, 0.85 and 0.81, respectively. Pr/n-C₁₇, Ph/n-C₁₈, and Pr/Ph values are 0.34, 0.42, and 0.87, respectively. Biomarker parameters indicate that the An-1 oil mainly comes from the plankton source rock deposited in the freshwater lake facies and a reducing environment, which has evolved to maturity. The correlation of oil-oil indicates that the An-1 oil is homologous to the Chang-7 oil (Chang-7 member of the Triassic Yanchang Formation). The correlation of oil-source rock presents that the An-1 oil is generated from the Yanchang Formation (Chang-6 and Chang-7 source rocks) and occurred in the coal seam during the stage of stratum uplift since the Early of Late Cretaceous. The distribution characteristics of δ¹³C_{group components} in the An-1 oil and Chang-7 oil also reveal the fractionation phenomenon during the migration of crude oil.

Acoustic testing is a widely used technique to measure the coal mechanical properties under high temperature and pressure in situ conditions. This study compared the acoustic wave characteristics of briquette and raw coal under various temperature and pressure conditions. The results show that the longitudinal wave velocity (V_p) decreases with an increasing vitrinite content. A large number of the vitrinite content enhances the process in which the temperature and pressure changed the V_p. The V_p of briquette decreases approximately linearly with the temperature compared to raw coal. The V_p of raw coal experiences initially a rapid, then gradual, and finally the moderate increasing trend with the increase in confining pressure. However, in briquette, the V_p increases approximately linearly with the confining pressure. The results indicate that the V_p is more sensitive to temperature under low confining pressure and peaks at 50°C−60°C than high confining pressure. However, the V_p is less sensitive to temperature under higher confining pressure, and the positive effect of high confining pressure is dominant. Understanding the mechanical properties of coal under high pressure and temperature develops better insight into coalbed methane (CBM) exploration from deep reservoirs.

The study of microbial communities in the produced water of coalbed methane (CBM) wells is an important aspect of microbial-enhanced methane production. Water produced from 15 CBM wells in four synclines in eastern Yunnan and western Guizhou was collected. Through the use of 16S ribosomal RNA (16S rRNA) amplicon sequencing and realtime fluorescence quantitative polymerase chain reaction (PCR), the characteristics of bacterial and archaeal communities before and after enrichment culture were studied. The methanogenic pathways of secondary biogas were discussed, and potential microbial-enhanced methane production was preliminarily evaluated. The results showed that the bacterial DNA content in uncultured produced water was low, so it is difficult to detect. After enrichment, the dominant bacteria phyla were Proteobacteria, Bacteroidetes, and Firmicutes. A total of seven phyla were detected in the uncultured produced water, and the dominant archaeal phylum was Euyarchaeota. Methanogens were the main component of archaea. The dominant archaeal genera were Methanobacterium, Methanoculleus and Methanobrevibacter. The community structure of the archaea changed noticeably after four days of enrichment culture. The relative abundance of Euryarchaeota increased to 99% in most samples after enrichment culture. It was found that there was a transition from Methanoregula to Methanobacterium within genera. The relative abundance of Methanobacterium increased, which can produce hydrogenotrophic methane. Combined with the isotopic composition of the produced water and gas, it is considered that the CBM in the Tucheng and Enhong synlines consists of a mixture of thermogenic gas and biogas. The proportion of secondary biogas in the Tucheng and Enhong synlines are estimated to range from 10.89% to 49.62%. There are mainly hydrogentrophic methanogens in the study area, and CO₂ reduction is the main way of microbial gas production. After enrichment culture of produced water in the study area, the hydrogenotrophic methanogens were enriched. These two areas have strong potential for microbial-enhanced methane production.

Commercial exploration and development of deep buried coalbed methane (CBM) in Daning-Jixian Block, eastern margin of Ordos Basin, have rapidly increased in recent decades. Gas content, saturation and well productivity show significant heterogeneity in this area. To better understand the geological controlling mechanism on gas distribution heterogeneity, the burial history, hydrocarbon generation history and tectonic evolution history were studied by numerical simulation and experimental simulation, which could provide guidance for further development of CBM in this area. The burial history of coal reservoir can be classified into six stages, i.e., shallowly buried stage, deeply burial stage, uplifting stage, short-term tectonic subsidence stage, large-scale uplifting stage, sustaining uplifting and structural inversion stage. The organic matter in coal reservoir experienced twice hydrocarbon generation. Primary and secondary hydrocarbon generation processes were formed by the Early and Middle Triassic plutonic metamorphism and Early Cretaceous regional magmatic thermal metamorphism, respectively. Five critical tectonic events of the Indosinian, Yanshanian and Himalayan orogenies affect different stages of the CBM reservoir accumulation process. The Indosinian orogeny mainly controls the primary CBM generation. The Yanshanian Orogeny dominates the second gas generation and migration processes. The Himalayan orogeny mainly affects the gas dissipation process and current CBM distribution heterogeneity.

Abundant unminable coal in deep strata and abandoned mines are also precious sources of clean gas energy, under which biotransformation is a potential path. In recent years, substantial progress has been made in laboratory research on coal degradation to produce methane by microbial metabolism. This paper systematically reviews the research progress of microbial enhancement and microbial stimulation of coal, physicochemical pretreatments of coal, and environmental factors affecting coal biotransformation. The research idea of coal biotransformation should aim at field production increase and gradually clarify the microbial mechanism of coal degradation and the regional distribution and functional composition of microbial communities on the block scale. The research on coal biotransformation helps improve the development level of coalbed methane and the sustainable development of unconventional natural gas resources.

Rare earth elements and yttrium (REY) in coal deposits are considered promising alternative sources for these resources owing to their increasing global demand. This paper reports the geochemical characteristics of REY in the Late Permian coals from an underground K1a seam section of the Zhongliangshan coalfield in Chongqing, southwestern China. The mineralogy, degree of enrichment, distribution patterns, modes of occurrence, and sediment origin of REY were investigated. Compared with the average of world coals, the concentration of REY in the K1a coals were normal, dominated by light REY (LREY), with less medium and heavy REY (MREY, HREY). The fractionation degree of the MREY and HREY are higher than that of LREY in most K1a coal samples, deduced from the mixed enrichment type of REY, mainly including M-H-type, and a few L-M type and H-type. In addition, the combination of anomalies of Ce, Eu, Gd, and Al₂O₃/TiO₂ parameters, the terrigenous materials in the K1a coal were derived from the felsic-intermediate rocks at the top of the Emeishan basalt sequence, and the samples were affected by seawater intrusion during early peat accumulation. Although the minerals primarily consist of kaolinite, illite, pyrite, and small amounts of quartz, calcite and anatase, REY are correlated with ash yield, SiO₂, and Al₂O₃, revealing that the REY mainly occur in aluminosilicate minerals, especially kaolinite and illite. Meanwhile, REY positively relate to P₂O₅ and Zr, which may exist in phosphate-containing minerals or zircon. Furthermore, most samples in the K1a coal or ash do not reach the cut-off grade for the beneficial recovery of REY. With the exception of central Guizhou, southwestern Chongqing, and the junction of western Guizhou and northeastern Yunnan, the REY content in coals from southwestern China are high, and its by-products are suitable as potential REY sources.

The sharp increase in the demand for lithium (Li) for high-energy-storage battery materials due to its high specific energy and low negative chemical potential render Li a geopolitically significant resource. It is urgent to develop a low-cost, efficient method to improve lithium extraction. Herein, Li ion (Li⁺) adsorption in coal-bearing strata kaolinite (CSK) was studied. The effects of pre-activation acid leaching (meta-kaolinite/H₂SO₄, MK-HS) and dimethyl sulfoxide intercalation (coal-bearing strata kaolinite/dimethyl sulfoxide, CSK-DMSO) on the Li⁺ adsorption capacity were studied under the same adsorption conditions. The results indicated that the adsorption was completed in 60 min under alkaline conditions (pH = 8.5), a high solution concentration (400 mg/L), and a low dosage (1 g/100 mL); and the comprehensive adsorption capacity is MK-HS > CSK-DMSO > CSK. Furthermore, the DMSO intercalation caused the interlayer spacing of the CSK to increase, which provided more space for Li⁺ to enter and increase the adsorption capacity. After thermal pre-activation and acid leaching, structural failure and lattice collapse resulted in the presence of more micropores in the MK-HS, which resulted in a 10-fold increase in its specific surface area and caused coordination bond changes (Al(VI) to Al(IV)) and leaching of aluminum (Al) from the lattice. It is proposed that these structural changes greatly improve the activity of CSK so that Li⁺ cannot only adsorb onto the surface and between the layers but can also enter the lattice defects, which results in the MK-HS having the best adsorption performance. Combined with the adsorption kinetics analysis, the adsorption methods of CSK and two modified materials include physical adsorption and chemical adsorption. In this study, the adsorption capacity of CSK and its modified products to Li were explored, providing a new option for the reuse of CSK and the extraction of Li.

The adsorption, diffusion, and aggregation of methane from coal are often studied based on slit or carbon nanotube models and isothermal adsorption and thermodynamics theories. However, the pore morphology of the slit model involves a single slit, and the carbon nanotube model does not consider the molecular structure of coal. The difference of the adsorption capacity of coal to methane was determined without considering the external environmental conditions by the molecular structure and pore morphology of coal. The study of methane adsorption by coal under single condition cannot reveal its mechanism. In view of this, elemental analysis, FTIR spectrum, XPS electron energy spectrum, ¹³C NMR, and isothermal adsorption tests were conducted on the semi-anthracite of Changping mine and the anthracite of Sihe Mine in Shanxi Province, China. The grand canonical Monte Carlo (GCMC) and molecular dynamics simulation method was used to establish the coal molecular structure model. By comparing the results with the experimental test results, the accuracy and practicability of the molecular structure model are confirmed. Based on the adsorption potential energy theory and aggregation model, the adsorption force of methane on aromatic ring structure, pyrrole nitrogen structure, aliphatic structure, and oxygen-containing functional group was calculated. The relationship between pore morphology, methane aggregation morphology, and coal molecular structure was revealed. The results show that the adsorption force of coal molecular structure on methane is as follows: aromatic ring structure (1.96 kcal/mol) > pyridine nitrogen (1.41 kcal/mol) > pyrrorole nitrogen (1.05 kcal/mol) > aliphatic structure (0.29 kcal/mol) > oxygen-containing functional group (0.20 kcal/mol). In the long and narrow regular pores of semi-anthracite and anthracite, methane aggregates in clusters at turns and aperture changes, and the adsorption and aggregation positions are mainly determined by the aromatic ring structure, the positions of pyrrole nitrogen and pyridine nitrogen. The degree of aggregation is controlled by the interaction energy and pore morphology. The results pertaining to coal molecular structure and pore morphology on methane adsorption and aggregation location and degree are conducive to the evaluation of the adsorption mechanism of methane in coal.

In this study, a group of overmature coal-measure shale core samples was collected in situ from an exploration well located in the Wuxiang area of the Qinshui Basin, north China. The pore water contents (C_PW) of the shales under as-received conditions, equilibrium water contents (C_EW) of the shales under moisture equilibrium conditions (relative humidity: 100%), and nanopore structures of the shales under both as-received and dried conditions were measured. The results indicate that the C_PW values of these shales are much lower than their C_EW values, which implies that the bulk pore systems of these shales have low water-bearing extents. In addition, approximately half of the total pore volumes and surface areas of the as-received shales are occupied by pore water, and the effects of pore water on shale nanopores with various pore types and widths are different. The average water-occupied percentages (P_W) are 59.16%−81.99% and 42.53%−43.44% for the non-micropores and micropores, respectively, and are 83.54%−97.69% and 19.57%−26.42% for the inorganic-matter hosted (IM) and organic-matter hosted (OM) pores, respectively. The pore water in shales not only significantly reduces the storage of shale gas by occupying many pore spaces, but also causes the shale gas, especially the absorbed gas, to be mostly stored in the OM pores; meanwhile, the IM pores mainly store free gas. Therefore, the water-bearing characteristics and their effects on the pore structures and gas-bearing properties of coal-measure shales should be noted for the evaluation and exploration of shale gas in the Qinshui Basin.

Coal-bearing shale shows great potential for unconventional gas resources in China, while its exploration and development have been challenging for a long time. Gas-in-place (GIP) is critical to shale gas evaluation, but the major factors controlling the GIP content of coal-bearing shale remain unclear. To address this issue, the coal-bearing shales of the upper Carboniferous-lower Permian Taiyuan and Shanxi formations in the Zuoquan Block, Qinshui Basin, China, were collected for GIP measurements and an integrated investigation, including organic geochemistry, inorganic mineral compositions, and pore characterizations, was carried out. Our results show that the GIP content of the studied shales displays relatively low values and wide variations, which range from 0.30 to 2.28 m³/t. The GIP is dominated by desorbed gas and residual gas. Total organic carbon (TOC) contents of the studied shales vary from 0.92% to 16.91%, and inorganic minerals are dominated by clays that mainly consist of illite/smectite mixed layer (I/S) and kaolinite. Inorganic pores have been widely observed in the studied shales, while the organic matter-hosted pores are rarely found using SEM observations. Total porosity of the studied shales is primarily contributed by clay minerals, followed by organic matter and quartz. Weak positive relationships between the GIP content and pore structure parameters imply that the adsorption of methane to nanopores is relatively weak, which may be attributed to the hydrophilicity of clay-hosted pores. Moreover, hydrophobic organic pores are not well developed. Positive correlations between the GIP contents and contents of TOC, clays, and the I/S indicate that major factors influencing the GIP contents of the coal-bearing shales are clays (especially I/S) and TOC content. In summary, these findings would be very helpful to reveal the enrichment mechanism of coal-bearing shale gas and provide a scientific basis for the exploration and development of coal-bearing shale gas.

Organic matter pores are considered to be the most important type of pore for preserving hydrocarbon gases in shale gas reservoirs. The organic matter in each over-mature marine shale sample was separated into two organic fractions with densities of greater than and less than 1.25 g/cm³, and then their molecular compositions and pore characteristics were quantitatively evaluated using solid state ¹³C-nuclear magnetic resonance (NMR) and gas (N₂ and CO₂) adsorption analyses, respectively. The results revealed that aromatic carbon is the dominant molecular composition of the over-mature organic matter in the Lower Cambrian Niutitang shale. During the over-mature stage, the organic fractions with densities of greater than and less than 1.25 g/cm³ have no significant differences in molecular composition. The organic fractions with densities of greater than and less than 1.25 g/cm³ do have significant differences in their organic pore characteristics. In contrast to the high density organic fraction, the low density fraction contained abundant micropores and lacked mesopores and macropores. The organic pore structures of the different occurrence states of organic matter were significantly different. The C/O of organic matter in different occurrence states are obviously different, which proves that the organic pore structure is closely related to both the occurrence state and density of the organic matter. However, these relationships are still unclear and require further study.

Recently, deeply-buried shale (depth > 3500 m) has become an attractive target for shale gas exploration and development in China. Gas-in-place (GIP) is critical to shale gas evaluation, but the GIP content of deep shale and its controlling factors have rarely been investigated. To clarify this issue, an integrated investigation of deep gas shale (3740–3820 m depth) of the Lower Paleozoic Wufeng–Longmaxi Formations (WF–LMX) in the Dingshan area, Sichuan Basin had been carried out. Our results show that the GIP content of the studied WF–LMX shale in the Dingshan area ranges from 0.85 to 12.7 m ³/t, with an average of 3.5 m³/t. Various types of pores, including organic matter (OM) pore and inorganic pore, are widely developed in the deep shale, with total porosity of 2.2 to 7.3% (average = 4.5%). The OM pore and clay-hosted pore are the dominant pore types of siliceous shale and clay-rich shale, respectively. Authigenic quartz plays a critical role in the protection of organic pores in organic-rich shales from compaction. The TOC content controls the porosity of shale samples, which is the major factor controlling the GIP content of the deep shale. Clay minerals generally play a negative role in the GIP content. In the Sichuan Basin, the deep and ultra-deep WF–LMX shales display the relatively high porosity and GIP contents probably due to the widespread of organic pores and better preservation, revealing great potentials of deep and ultra-deep shale gas. From the perspective of rock mechanical properties, deep shale is the favorable exploration target in the Sichuan Basin at present. However, ultra-deep shale is also a potential exploration target although there remain great challenges.

Natural fractures are of crucial importance for oil and gas reservoirs, especially for those with ultralow permeability and porosity. The deep-marine shale gas reservoirs of the Wufeng and Longmaxi Formations are typical targets for the study of natural fracture characteristics. Detailed descriptions of full-diameter shale drill core, together with 3D Computed Tomography scans and Formation MicroScanner Image data acquisition, were carried out to characterize microfracture morphology in order to obtain the key parameters of natural fractures in such system. The fracture type, orientation, and their macroscopic and microscopic distribution features are evaluated. The results show that the natural fracture density appears to remarkably decrease in the Wufeng and Longmaxi Formations with increasing the burial depth. Similar trends have been observed for fracture length and aperture. Moreover, the natural fracture density diminishes as the formation thickness increases. There are three main types of natural fractures, which we interpret as (I) mineral-filled fractures (by pyrite and calcite), i.e., veins, (II) those induced by tectonic stress, and (III) those formed by other processes (including diagenetic shrinkage and fluid overpressure). Natural fracture orientations estimated from the studied natural fractures in the Luzhou block are not consistent with the present-day stress field. The difference in tortuosity between horizontally and vertically oriented fractures reveals their morphological complexity. In addition, natural fracture density, host rock formation thickness, average total organic carbon and effective porosity are found to be important factors for evaluating shale gas reservoirs. The study also reveals that the high density of natural fractures is decisive to evaluate the shale gas potential. The results may have significant implications for evaluating favorable exploration areas of shale gas reservoirs and can be applied to optimize hydraulic fracturing for permeability enhancement.

Mosses form a diverse land plant group in modern vegetation but have rarely showed up in the fossil record compared with vascular plants. Here, we report an extraordinarily-preserved early Miocene moss fossil from the lower Laoliangdi Formation in the Pingzhuang Coal Mine in Chifeng, Inner Mongolia Autonomous Region, northern China. Although lacking rhizoids and most reproductive organs, the well-preserved fossil allows us to assign it to Platydictya cf. jungermannioides (Amblystegiaceae) based upon its detailed gross and micro-morphology. The diagnostic characteristics include a small-sized body with slender stems bearing spirally arranged ovate-lanceolate leaves that lack costae. Leaf margins are mostly partly entire and partly dentate, a few dentate, and rarely completely entire. It represents the first fossil record of Platydictya in Asia. The specific living microenvironment of the extant P. jungermannioides enriched our understanding of the early Miocene environment that was previously based upon vascular plant fossils and sedimentary lithofacies in the area. Our early Miocene Platydictya cf. jungermannioides fossil lived in a warm and humid lush forest with a dense understory that received adequate water supplies.