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The world is rich in tar-rich coal resources, which have broad exploration and development prospects. At present, the academic community has relatively weak research on the impact of shock waves on the permeability of target coal bodies. This article focuses on the development technology of controllable shock waves in tar-rich coal reservoirs. Firstly, a systematic analysis was conducted on the drilling situation and physical parameters of the target coal seam. Secondly, the impact of shock waves on permeability of target coal was systematically analyzed. Thirdly, a systematic summary was conducted on the process parameters and measures of coal seam operations. Research has shown that: (a) The permeability of coal samples after shock wave action is much greater than the permeability of coal samples in their original state. (b) Under the same axial and confining pressure conditions, as the gas pressure increases, the increase in the permeability of coal under the action of shock waves shows a decreasing trend. (c) The shock wave generates a large number of crack structures inside the coal body. These cracks will undergo very obvious closure under external pressure. (d) Under different axial and confining pressures, when the gas pressure is 1 MPa, the maximum increase in permeability of coal samples under shock wave action can reach 90–200 times. When the gas pressure is 2 MPa, the increase in permeability before and after shock wave action is the smallest, reaching 30–90. • The impact of shock waves on the permeability of target coal is analyzed. • The process parameters and measures of coal seam operations is revealed. • In-situ development of oil-rich coal on-site testing operation process is discussed. [ABSTRACT FROM AUTHOR]

AbstractTar-rich coal has the features of high tar yield and great potential for oil production. The underground in-situ pyrolysis technology is an effective way to accomplish the green and efficient utilization of tar-rich coal resources. The pore structures of coal seam have great change in the actual in-situ pyrolysis process. Hence, the physical properties of coal seam also vary with pyrolysis temperature, which affects the heat transfer performance of coal seam. However, previous studies on heat transfer characteristics of coal seam have been seldom paid attention to the physical property dynamic change during the in-situ pyrolysis, which possibly causes deviation between the simulated heat transfer characteristics and the actual performance. It is still unclear whether the deviation is acceptable in practical engineering practice at present. In this article, a modification scheme for the physical property dynamic change of coal seam was proposed. The effects of physical property dynamic change on heat transfer characteristics of coal seam during in-situ pyrolysis of tar-rich coal were analyzed using numerical simulation approach. In addition to single factor correction, the effects of three-factor coupling modification on heat transfer performance of coal seam were also systematically investigated. The simulation results indicated that both the average temperature and the effective heating area of the coal seam increased after the specific heat capacity was corrected individually. Whereas, the heat transfer of coal seam was weakened after the endothermic effect of pyrolysis and the thermal conductivity coefficient were modified, respectively. When the three factors were coupled for correction in the single well and single fracture mode, the correction factor that played a leading role was the thermal conductivity coefficient, accounting for 90%. However, an increase in the number of cracks enhanced the heat transfer, resulting in the specific heat capacity was dominant among the three correction factors. The present study can offer theoretical guidance and technical support for realizing large-scale in-situ pyrolysis of tar-rich coal. [ABSTRACT FROM AUTHOR]

The study of the relationship of in-situ stress and permeability is of great necessity to understand the formation mechanism of high-permeability coal reservoir. However, characteristics of in-situ stress and its relationship with permeability in low-rank CBM blocks of China is poorly understood. This study employs permeability and in-situ stress test experiment to analyze the vertical variation law of in-situ stress and permeability and the control mechanism of in-situ stress on permeability in the Binchang block of southwestern Ordos Basin. It is concluded that the permeability varies from 0.11–6.84 mD, and the entire coal reservoir is in a state of relative tension in the Binchang block. The extended Somerton stress–permeability model composed of six stress parameters and the geological model of in-situ stress–permeability were established. The modeling results indicate that permeability is most sensitive to the maximum horizontal stress and its derived parameters, and least sensitive to vertical stress. This model indicates that highly permeability develop in the two-stage turning burial depth section (480–800 m), which is consistent with the minimum value of stress parameters such as the lateral pressure coefficient, which indicate that the relative tension stress zone in the depth profile provides structural conditions that are conducive to the development of permeability. [ABSTRACT FROM AUTHOR]

In this study, to investigate the mechanical properties and microscopic geological features, a series of uniaxial compression, petrophysical properties, X-ray diffraction, elements, and conductivity tests were conducted. The results indicate that quartz and plagioclase are essential minerals determining Upper Triassic sandstone compositions, while quartz plays a dominant role in the Lower Permian sandstones. Due to microcracks, the relationships between porosity and permeability of Upper Triassic sandstones are better than those of Lower Permian sandstones. Observations have shown that silicon and aluminum are the main elements of those rocks. The rocks which have large porosity and permeability have a strong capacity of anticompression resistance. There are good consistencies between the mechanical behavior and the high proportion of rigid minerals. Similarly, the rocks which have abundant silicon and aluminum could better support the structural integrity of rocks. High pore structure heterogeneity and abundant clay minerals have positive influence on the mechanical resistance. Shales have distinct plastic deformation features when compared with the sandstones. [ABSTRACT FROM AUTHOR]

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

Micrometer-scale pore and fracture structures of coal seams are the crucial parameters in the case of enhanced coalbed methane (CBM) recovery as they determine permeability and productivity. A significant study has been made in fracture characteristics; however, the detailed structural and fractal characteristics of micro-fractures and micro-pores of tectonically deformed coals are poorly understood. To get deep insight into the variability and heterogeneity of micro-pores and micro-fractures in different tectonically deformed coals collected from the Guojiahe coal mine, the multifractal analysis using the Menger sponge and box-counting model was employed to study deformed coals based on mercury injection porosimetry and scanning electron microscopy. The results show that tectonic deformation changed the structure of the micro-pores by increasing their diameter and that of the micro-fractures by expanding, intersecting, and creating additional microfracture networks at any scale. For the coals investigated in this study, permeability was nonlinearly correlated with their structural fractal. For undeformed coals, with the increase in the fractal dimension of micro-pores, the distribution of coal permeability exhibits a U-shape. However, it exhibited an inverted U-shape as the fractal dimension of micro-fractures increased. The distribution of the permeability of deformed coal samples is characterized by a U-shape as the fractal dimension of micro-fractures increased, while the fractal dimension of micro-pores decreased. Thus, the structural proportional odds of micro-pores and micro-fractures are vital in defining the maximum value of permeability. As a whole, deformed coals have a relatively high permeability as local micro-pores and micro-fractures are well connected. We suggest that coal seams with brittle deformation at syncline, anticline, and folded areas are favorable for CBM exploitation. [ABSTRACT FROM AUTHOR]

AbstractA large amount of pyrolysis residue is left in the coal seam after the in-situ pyrolysis of tar-rich coal, which contains rich heat energy. However, there are few numerical simulations on waste heat utilization from in-situ pyrolysis of tar-rich coal at present. This paper mainly focused on the temperature field distribution and heat transfer characteristics of the coal seam in the process of waste heat utilization of tar-rich coal in large spatial scale and over a long period. The effects of cooling medium type, cooling fluid velocity, cooling fluid temperature, fracture distribution and well pattern on temperature field distribution and heat transfer performance of tar-rich coal seam were analyzed by Fluent numerical simulation. The present study can offer theoretical guidance and technical support for large-scale utilization of residual heat from pyrolysis residue of tar-rich coal. [ABSTRACT FROM AUTHOR]

Research on the relationships among tectonics, micropores, microfractures, and coalbed methane (CBM) content is important for the optimal selection of CBM production areas. In this study, micropore-microfracture structural parameters of coal samples from the Guojiahe coalfield are determined through the use of X-ray photography, an image recognition algorithm, and a liquid nitrogen adsorption method. The relationship between the micropore-microfracture characteristics of the reservoir and the gas content is quantitatively assessed using the Grassberger and Procaccia (GP) algorithm to calculate the correlation dimension of the parameters. Micropore-microfracture development varies in different tectonic zones. Additionally, the CBM content varies according to the characteristic parameters of hysteresis loops and the pore diameter. The correlation dimension is an effective indicator of the nonlinear relationship between reservoir micropore-microfracture characteristics and the gas content. [ABSTRACT FROM AUTHOR]

The Lower Paleozoic reservoir in the Micangshan tectonic belt is a new shale gas exploration area with excellent potential. However, the black shale's sedimentary environment and tectonic setting and their effects on pore-scale fluid flow have not been thoroughly elucidated and urgently require further research. A large amount of trace and rare earth element data of black shale in this area was tested to analyze the sediment's provenance, tectonic setting, and coupling process, and nuclear magnetic resonance was applied for fluid flow ability determination. The characteristics of trace elements and rare earth elements in shale samples were drilled from the Upper Sinian Doushantuo Formation, the Lower Cambrian Niutitang Formation, and the Lower Silurian Longmaxi Formation in the Micangshan tectonic belt. The results reveal that the provenance of the shales was dominated by island arc volcanic rocks and some ancient fine-grained sediments. The provenances of shale in the Micangshan, Huijunba, and Zhenba areas may have a strong genetic relationship with the igneous rocks from the nearest Hannan massif. The provenance of the Niutitang shale in the Ningqiang area may comprise sediments derived from the Longmenshan continental rift system. The provenance of the shale tends to transition from the island arc to the continental margin from the depositional stage of the Doushantuo Formation to the Longmaxi Formation. The Micangshan tectonic belt inherited the tectonic evolution of the South China block, North China block, and Qinling, and those blocks provided the igneous provenance of the shale by island arcs, which were formed due to weathering caused by the process of subduction and collision orogeny. The movable fluid distribution of the Longmaxi Formation and Niutitang Formation is relatively uniform, and excessive pursuit of large amount of movable fluid will be counterproductive to the shale reservoir. The proportion of the cracks was generally less than 3% except for one sample from the Doushantuo Formation with 12.06%. Based on the centrifugation force in this research, the low fluid flow limits were less than 1.47, and there are strong positive relationships between movable fluid saturation and saturated porosity. In this study, the coupling relationship between the material source and pore scale flow is discussed for the first time, and the results can provide theoretical references for local provenance analysis and tectonic movement. [ABSTRACT FROM AUTHOR]

Biomass from agricultural production is a renewable energy source with a high-volatile content. Semicoke (SC) and gasification residual carbon, which are ultralow volatile carbon-based fuels (LVFs), are by-products of the coal chemical industry, which are over capacity and urgently need to be cleanly and effectively consumed. Biomass and LVFs exhibit complementary fuel characteristics, particularly the volatile content, which can potentially be coprocessed efficiently. Furthermore, oxyfuel combustion technology can not only realize carbon dioxide (CO2) capture and benefit from carbon neutrality, but also effectively reduce NOx emissions. The cocombustion of biomass and ultralow volatile fuel under oxyfuel conditions can both use individual benefits and effectively control pollutant emissions. However, the cofiring characteristics and interaction mechanisms of LVF with biomass in an oxyfuel atmosphere are yet to be fully understood. In this study, three ultralow volatile fuels and one typical biomass were selected to explore the oxyfuel cocombustion characteristics via thermogravimetric experiments. The experimental results indicated that blending with wheat straw (WS) improved the overall combustion performance, the ignition point was close to that of WS, and the burnout temperature was close to the average value of the two individual fuels. An increase in the oxygen content can effectively enhance the combustion feature of ultralow volatile fuels while it has little effect on the burnout performance of biomass. In the presence of high-content CO2 , the decomposition of carbonate in ultralow volatile fuel is impeded and the decomposition temperature is slightly increased. The present investigation can promote the efficient co-utilization of inferior LVFs and biomass, which also benefits CO2 capture, carbon neutrality, and even negative carbon emissions. [ABSTRACT FROM AUTHOR]

This study aims to show how fractal analysis can be effectively used to characterize the pore structure of porous tar-rich coal. In this study, tar-rich coal pores were obtained by mercury intrusion porosimetry (MIP). The results showed that the sample had a high porosity and large pore diameter after pyrolysis, and the porosity of tar-rich coal was more than 35% at 600°C. The pore-throat ratio at high temperature was large, resulting in the high mercury retention rate. The pore distribution curves of samples at pyrolysis temperatures of 500 and 600°C were unimodal, and those of samples at room temperature and 150, 300, and 400°C were bimodal. The models for calculating fractal dimension based on MIP include Menger sponge model and thermodynamic model. Experiments show that the Menger sponge method is more reasonable when the pore size is less than 50 μm and greater than 350 nm. For the fractal dimension calculation of the whole pore curve section, the thermodynamic method was more reasonable. The use of fractal analysis in conjunction with the results of classical characterization methods leads to a better understanding of pore evolution in the pyrolysis process of tar-rich coal. The average fractal dimension could also be used as a valid parameter to monitor the textural evolution of the coals. [ABSTRACT FROM AUTHOR]

Coal reservoirs are highly heterogeneous, and different macrolithotypes of coal have differences in material composition and pore structure, resulting in different coalbed methane (CBM) adsorption/desorption performance and gas-water migration characteristics. In order to explore the pore development characteristics of different macrolithotypes in low-rank coal and their influence on adsorption and desorption, the low-rank coal of the Yan'an Formation in the Huanglong coalfield is taken as the sample of the research object. Vitrain and durain are collected and then separated. The results show that the porosity of coal is 2.92%–11.81%, and the total porosity is relatively developed. The porosity of durain is slightly higher than that of vitrain, the pore throat of durain is thicker, and the macropores are more developed. The Brunauer-Emmett-Teller (BET) specific surface area (SSA) and Barrett-Joyner-Halenda (BJH) total pore volume (TPV) of vitrain is slightly larger than durain, the micropores are more developed, and most of them are semiclosed and ink-bottle-shaped pores. The pore-fracture connectivity of the durain is better than vitrain via nuclear magnetic resonance (NMR). Vitrain has more developed micropores than durain, with larger specific surface area and stronger adsorption capacity. In the process of methane adsorption/desorption, the phenomenon of desorption hysteresis generally exists. Moreover, the pore connectivity of durain is relatively good, the degree of desorption hysteresis is weaker than vitrain, and the theoretical desorption efficiency is higher than vitrain. Temperature has a significant influence on adsorption and desorption of methane. The increase in temperature is beneficial to desorption and weakens the hysteresis effect of desorption. At the same temperature, the isosteric adsorption heat in the desorption process is greater than the adsorption process, which further explains the incomplete reversibility of the adsorption and desorption process. Under the same conditions, the isosteric adsorption heat of vitrain in the adsorption process is greater than durain, and adsorption capacity of vitrain is stronger than durain. Similarly, the isosteric adsorption heat of vitrain in the desorption process is greater than durain, and desorption needs to absorb more heat. The desorption capacity is weaker than durain. [ABSTRACT FROM AUTHOR]

The theory of coalbed methane distribution controlled by tectonism is a hot issue in the field of geofluid-geotectonic interaction research. Taking the geological structure in the scale range of the 1302 working face of a Guojiahe wellfield in a Yonglong mining area as the background, this paper focuses on the basic research problem of the influence of geological structure on the control of coal reservoir gas content and uses a THM coupling model to analyze the change of coalbed gas content and distribution characteristics of different tectonic positions. The change of CBM content and permeability in the anticline, syncline, and faults is analyzed. Accordingly, the variation distance of gas content and reservoir permeability controlled by tectonism of different geological structures is quantified to provide guidance for the selection of CBM-favorable areas. The research results show that the gentle dip syncline hinge zone is a potential gas-rich area with heat preservation and low permeability, while the gentle dip anticline hinge zone is a gas-poor area with low temperature and low pressure and high permeability. The thick coal seam zone of the syncline hinge zone is the potential gas accumulation zone, and the high-permeability area is near the fault plane of a normal fault. The coal matrix near the normal fault is subjected to tensile tectonic stresses to form tensional fissures, and the coal seam in the fault plane area is susceptible to coalbed gas dispersion and increases the permeability of the coal reservoir. The variation distance of gas content and reservoir permeability controlled by the normal fault within the Guojiahe wellfield is 37 m and 54 m from the fault plane, respectively. [ABSTRACT FROM AUTHOR]

The separation of low-temperature coal tar (LTCT) containing solid particles by vacuum distillation is essential in various applications, such as in-situ pyrolysis. However, the separation characteristics of solid-containing LTCT using vacuum distillation have yet to be fully understood. Therefore, explorations were conducted with variations in pressure and solid particle concentration by analyzing pitch with Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The FTIR demonstrated that the organic constituents exhibit greater regularity at higher temperatures, leading to a 2 % reduction in the tightly bound cyclic OH tetramers, an intricate hydrogen bonding. The structural parameters indicate a 2 % decrease in f ar H, alongside a decline in AR from 0.332 to 0.293. This suggests that the low-pressure distillation conditions contribute to an increased retention of aromatic hydrocarbons in pitch, potentially complicating its subsequent utilization. The reduction in F from 2.273 to 2.029 signifies a shorter aliphatic chain, indicative of an intensified degradation effect. The elevated content of solid particles results in a more rigid texture of pitch in micromorphology. This study examined LTCT pitch to clarify the separation mechanism of solid-containing LTCT, offering insights into appropriate vacuum distillation conditions for LTCT separation and aiding in the effective utilization of pitch products. [Display omitted] • Analyzing pitch using FTIR and SEM reveals the LTCT vacuum distillation procedure. • Elevated distillation temperatures lead to deeper deoxygenation of the pitch. • Organic components are arranged more regularly at elevated temperatures. • Too low distillation pressure results in aromatic hydrocarbons remaining in pitch. • Increasing solids content makes the pitch harder and less friable texture. [ABSTRACT FROM AUTHOR]

• The connectivity of adsorption and seepage pore increased after FCJ. • The adsorption pore heterogeneity of coal increased with R r and inertinite content. • The V L is controlled by maceral, and R r , before and after FCJ, respectively. • Bitumen has an important effect on micropores and adsorptivity of bituminous coal. Bituminous coal rich in inertinite is widely distributed throughout the world, but the effect of the first coalification jump (FCJ) on the reservoir properties of high-volatile bituminous coal rich in inertinite, has rarely been studied. Therefore, nine coal samples were collected from the southwestern Ordos Basin and used in a series of experiments designed to study the adsorptivity of high-volatile bituminous coal rich in inertinite and factors affecting adsorptivity before and after FCJ. Such information is helpful for further understanding the evolution of coal physical properties in the key stage of early coalification. The pyrolytic parameters indicate that the FCJ of the coal samples was between 0.60% and 0.65% (R r). The NMR-T 2 spectrum type of the samples was bimodal differentiation before FCJ, and after FCJ, it was a bimodal connection or adsorption single peak. The fractal dimension of adsorption pore increased with increased R r and inertinite content. The adsorptivity of coal were affected by coalification and macerals. Langmuir pressure was mainly controlled by the maceral composition and exhibited a positive linear relationship with the ratio of vitrinite to inertinite. Langmuir volume was mainly controlled by maceral composition, and coalification, before and after FCJ, respectively. The increase in the adsorption pore volume of high-volatile bituminous coal caused by the increase in inertinite content had little effect on the increase in the adsorption capacity of the coal. In the stage of high-volatile bituminous coal, bitumen is adsorbed by vitrinite, which blocks and fills the pores of vitrinite, thereby reducing the adsorption capacity of coal. [ABSTRACT FROM AUTHOR]