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Now showing items 17 - 32 of 72

  • Laboratory study of the displacement coalbed CH4 process and efficiency of CO2 and N2 injection.

    Wang, Liguo   Cheng, Yuanping   Wang, Yongkang  

    ECBM displacement experiments are a direct way to observe the gas displacement process and efficiency by inspecting the produced gas composition and flow rate. We conducted two sets of ECBM experiments by injecting N2 and CO2 through four large parallel specimens (300 * 50 * 50 mm coal briquette). N2 or CO2 is injected at pressures of 1.5, 1.8, and 2.2 MPa and various crustal stresses. The changes in pressure along the briquette and the concentration of the gas mixture flowing out of the briquette were analyzed. Gas injection significantly enhances CBM recovery. Experimental recoveries of the original extant gas are in excess of 90% for all cases. The results show that the N2 breakthrough occurs earlier than the CO2 breakthrough. The breakthrough time of N2 is approximately 0.5 displaced volumes. Carbon dioxide, however, breaks through at approximately 2 displaced volumes. Coal can adsorb CO2, which results in a slower breakthrough time. In addition, ground stress significantly influences the displacement effect of the gas injection.=20
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  • Non-Darcy Flow in Hydraulic Flushing Hole Enlargement-Enhanced Gas Drainage:Does It Really Matter?

    Liu, Qingquan   Cheng, Yuanping   Dong, Jun   Liu, Zhengdong   Zhang, Kaizhong   Wang, Liang  

    In recent years, the non-Darcy flow has seen a significant increase in interest in conventional and unconventional gas developments. The nonlinear behavior can be described by adding a quadratic term of the velocity with the permeability-dependent beta factor in Darcy's law. The methods for improving permeability such as the hydraulic flushing hole enlargement (HFHE) technique can remarkably enhance the coal permeability thus changing the beta factor. However, few studies have been done previously to seek how this non-Darcy flow impacts the permeability-enhanced gas drainage. In this study, a fully coupled coal deformation and damage, the non-Darcy flow of free methane gas in the fractures, and Fickian diffusion of adsorbed methane in the coal matrix model were developed. The Klinkenberg effect has been taken into account as a reference object. A series of simulation scenarios were carried out to evaluate and compare the influences of the non-Darcy effect, gas-coal interaction effect, and Klinkenberg effect on the HFHE-enhanced gas drainage. Results of the simulation illustrate that the HFHE technique can significantly improve the gas drainage efficiency by permeability enhancement. But the HFHE-enhanced gas drainage is unique to the other gas developments with high or low flow rates; a slight pressure gradient and short drainage time make the non-Darcy effect fail to generate cumulative influence on the HFHE-enhanced gas drainage.
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  • Non-Darcy Flow in Hydraulic Flushing Hole Enlargement-Enhanced Gas Drainage: Does It Really Matter?

    Liu, Qingquan   Cheng, Yuanping   Dong, Jun   Liu, Zhengdong   Zhang, Kaizhong   Wang, Liang  

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  • Size effect on uniaxial compressive strength of single coal particle under different failure conditions

    Wang, Chenghao   Cheng, Yuanping   He, Xinxin   Yi, Minghao   Wang, Zhenyang  

    Strength of coal particles is an important mechanical property that is of great significance in the prevention of coal and gas outburst. The force-displacement curve for each small particle was obtained by the uniaxial compression of 1200 coal particles of sizes in the range 0.2-7 mm and different degrees of deterioration ranging from low-rank bituminous to high-rank anthracite. The experimental data were used to analyze various theories describing the effect of size on material strength; these include the traditional statistical theory, random distribution of structural strength based on the Weibull statistical theory, fracture energy release theory, and crack fractal theory. The results show that one of the traditional statistical models that receives little attention is more suitable than the others to describe the size effect on particle crushing strength. The intact coal strength, i.e the strength when particle size d -> 0, is in the range 10-88.79 MPa, and the coal mass strength, i.e., the strength when d is in the range 0.92-8.03 MPa. The tensile strength of the matrix is 0.7 times the intact coal strength. Based on different failure conditions, the uniaxial compressive strength is subdivided into yield strength, crushing strength, and ultimate strength. When the particle size tends to infinitesimal, the ratios of yield strength to ultimate strength and crushing strength to ultimate strength are 0.73 and 1, respectively. Both ratios decrease with increase in particle size and tend towards constant values thereafter. Various factors affecting the breakage of coal particles were analyzed. The firmness coefficient, maximum vitrinite reflectance, and apparent density have a significant positive correlation with particle strength whereas the proximate analysis parameters have little effect on strength. (C) 2019 Elsevier B.V. All rights reserved.
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  • Size effect on uniaxial compressive strength of single coal particle under different failure conditions

    Wang, Chenghao   Cheng, Yuanping   He, Xinxin   Yi, Minghao   Wang, Zhenyang  

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  • Model development and analysis of the evolution of coal permeability under different boundary conditions

    Lu, Shouqing   Cheng, Yuanping   Li, Wei  

    Coal permeability is an important parameter for coalbed methane (CBM) production and CO2-enhanced coalbed methane (ECBM) recovery. Coal permeability is mainly controlled by the structure of the coal. In this study, based on the pore-elasticity principle and the coal matrix-fracture interactions, we developed an analytical coal permeability model and its five forms under the corresponding specific boundary conditions, and four of them are validated by matching the corresponding field data or experimental data. The study presents a discussion of the permeability evolution for the different forms. The results show that the coal permeability under constant volume and constant effective stress conditions are controlled by the matrix sorption deformation and that the coal permeability under uniaxial strain and constant external stress conditions is governed by both the effective stress and the matrix sorption deformation. The constant volume model may be a special form of the constant effective stress model. For the same pressure drawdown, the permeability that is predicted under constant external stress conditions decreases more than that the permeability that is predicted under uniaxial strain conditions, and the permeability change induced by the matrix sorption deformation depended on the values of the internal swelling partition. The boundary conditions are important to the model, and different boundary conditions will lead to different evolutions of coal permeability. The permeability model can only be used to predict the evolution of permeability under the corresponding boundary conditions. (C) 2016 Elsevier B.V. All rights reserved.
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  • Effect of confining pressure unloading on strength reduction of soft coal in borehole stability analysis

    Liu, Qingquan   Cheng, Yuanping   Jin, Kan   Tu, Qingyi   Zhao, Wei   Zhang, Rong  

    Underground borehole drilling usually causes instability in the surrounding coal due to in situ stress redistribution (including stress concentration and stress release). However, the mechanisms of unloading-induced coal strength reduction are still poorly understood. The primary objective of this study is to investigate the effect of confining pressure unloading on soft coal strength reduction for borehole stability analysis. A series of mechanical tests were conducted on both the traditionally and newly reconstituted coal samples under two different experimental stress paths, including conventional uniaxial/triaxial compression and triaxial compression with confining pressure unloading. The unloading stress path was obtained by analyzing the stress redistribution around a borehole, to capture a more accurate coal mechanical response. According to our experimental results, plastic deformation generated before failure under the unloading stress path is smaller than that generated under the conventional loading stress path. Furthermore, the cohesion of the traditionally and newly reconstituted samples diminishes approximately by 44.77 and 29.66%, respectively, with confining pressure unloading, indicating that there is a significant reduction in coal strength due to confining pressure unloading. The mechanism for unloading-induced coal strength reduction comes from confining pressure unloading-induced increase in shear stress on the fracture surface and a decrease in shear strength. This effect increases the shear slipping potential, whose driving force generates tension fractures at both ends of the preexisting fractures.
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  • Evolution of gas transport pattern with the variation of coal particle size:Kinetic model and experiments

    Zhao, Wei   Wang, Kai   Cheng, Yuanping   Liu, Shimin   Fan, Long  

    Gas desorption laws varies with coal partide size. Their proper description is of great importance to natural gas engineering. This paper tries to summarize the internal links in commonly used models and combine them into a more general form. The combined general model is capable of describing fracture flow, matrix diffusion and surface sorption processes in pores with shapes like flat plates, cylinders and spheres. It can also be simplified into a power function which can be readily implemented into simulation of sorption and calculation of dynamic Fickian diffusion coefficients. Experiments including low-temperature liquid N-2 adsorption, proximate analysis, isothermal methane sorption and desorption experiments with different particle sizes were conducted to validate the model. Other sorption data from literature were also collected for validation. The fitting results can help explain the size dependence of desorption characteristics and show the flow type evolutions during the damage of pore system. (C) 2020 Elsevier B.V. All rights reserved.
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  • Effects of Supercritical CO2 Fluids on Pore Morphology of Coal:Implications for CO2 Geological Sequestration

    Zhang, Kaizhong   Cheng, Yuanping   Jin, Kan   Guo, Haijun   Liu, Qingquan   Dong, Jun   Li, Wei  

    A systematic knowledge of the pore morphology of coal treated with supercritical CO2 (ScCO2) is critical for the process of CO, geological sequestration. To better understand the desorption mechanism and to evaluate the storage capacity of target coal seams, the changes in pore volume, pore size distribution, fractal dimension, pore shape, and connectivity in high-, middle-, and low-rank coals were analyzed using N-2/CO2 adsorption and mercury intrusion porosimetry. The results indicate that micropores of high- and middle-rank coals decreased after ScCO2 treatment, whereas an increasing trend was found in low rank coals, and ScCO2 promoted the accessibility of the macropore spaces for all coals. With ScCO2 treatment, the roughness of smaller pores in both high- and middle-rank coals decreased, whereas larger pores became more complex for high-rank coals. Although no significant change was observed in the pore shapes, ScCO2 facilitated the development of effective pore spaces and improved the connectivity of the pore system. Additionally, the gas desorption properties of these samples were enhanced by ScCO2, verifying the pore morphology results. A conceptual model was proposed to explain the mechanism of the desorption process in relation to the constricted pore spaces of the coal matrix under ScCO2 and higher-pressure conditions. The results contribute to the understanding of long-term CO2 storage and enhanced coalbed methane recovery.
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  • Numerical assessment of the critical factors in determining coal seam permeability based on the field data

    Liu, Qingquan   Chu, Peng   Zhu, Jintuo   Cheng, Yuanping   Wang, Deyang   Lu, Yanfei   Liu, Yuanyuan   Xia, Lei   Wang, Liang  

    Accurate acquisition of the coal seam permeability is of great significance to gas drainage. Field measurements are more reliable to determine the coal permeability than laboratory. The Radial Flow Permeability (RFP) measurement is the most used field measuring method to measure the coal seam permeability in Chinese coal mines. The theoretical basis of the RFP method is the single-porosity medium flow theory and the gas content in coal seams is described by a parabolic equation. However, coal is a typical dual-porosity medium. The simplifications may lead to nonnegligible influence on the accuracy of the RFP method, which are not well documented in the literature. In this paper, three different mathematical models of gas flow in coal seams are established. The field gas flow data was matched by these three models to prove the reliability of these models. Then the influences of single-porosity simplification and the parabolic gas content simplification on the accuracy of the RFP method were investigated by comparing the differences of gas flow rates and gas pressures. Results show that there are apparent deviations of gas flow rate and gas pressure between these three models. The precision of the RFP method rapidly reduces with the increase of cleat spacing, which is induced by the single-porosity simplification; the precision decreases with the increase of Langmuir volume, first increases and then declines with the increase of Langmuir pressure, which are led by the gas content simplification. Moreover, because of the imperfections of the numerical calculus in the derivation of the RFP method, the calculated value is always smaller than the true value of coal seam permeability.
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  • Surface energy of coal particles under quasi-static compression and dynamic impact based on fractal theory

    Wang, Chenghao   Cheng, Yuanping   Yi, Minghao   Hu, Biao   Jiang, Zhaonan  

    The crushing energy of coal plays an important role in mining, gas extraction, occurrence of outburst accidents, and coal pulverisation engineering. To calculate the crushing energy of coal, a fractal model considering the crack area was established. The fractal fragmentation distribution and the fractal crack distribution, with their fractal dimensions (D-f and D-s) calculated from the particle size distribution, were used to calculate the new specific surface area (SSA) of coal after crushing. Two different experimental methods, quasi-static compression on a single particle (strain rate: 0.0005-0.001 s(-1)) and dynamic impact on particles (strain rate: 300 s(-1)), were used to crush coal particles to particle size of 2-3 mm. The experimental results show that D-f for coal particles under quasi-static compression (QSC) and dynamic impact (DI) are 1.44-1.85 and 1.9-2.74, respectively. D-f increases significantly with the crushing ratio, and there is a limit value, which is defined as D-s. D-s measured by QSC and DI were 1.87 and 2.75, respectively. The SSA of a single particle obtained by the proposed fractal model increases with the decrease in the particle size, which can be highly consistent with the BET SSA by fitting. The surface energy of coal particles under QSC and DI calculated by the traditional theory, which does not take into account the crack area, is 240.10 J/m(2) and 180.92 J/m(2), respectively, independent of the crushing ratio. The surface energy calculated by the fractal model is 3.55 J/m(2), taking into account the frictional heat.
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  • An improved method for high-efficiency coal mine methane drainage:Theoretical analysis and field verification

    Liu, Qingquan   Zhou, Hongxing   Cheng, Yuanping   Shu, Longyong   Ullah, Barkat  

    Degassing a coal seam with in-seam boreholes is an important method for mitigating the gas hazards in the underground coal mine. However, the low strength of the outburst-proven coal limits the borehole sealing performance and borehole space maintaining, and thus influences the drainage performance of in-seam boreholes. This study was conducted to seek a method to improve the sealing performance and borehole space maintaining for high-efficiency CMM drainage. A visco-elastic plastic model involving the plastic softening and dilatancy features for soft coal was proposed, and the deformation, shrinkage, and fracture characteristics of the coal surrounded a borehole and a roadway were analyzed. An enormous amount of connective fractures generated in the failure zone and the plastic zone, and the plastic zone develops timely for the creep behavior of coal, which aggravates the difficulties of borehole sealing. A comprehensive approach including the theoretical method and the technical method was developed to determine the proper sealing depth which can significantly influence the sealing performance. The sieve tubes made of high-density polyethylene has been used to maintain the borehole space during the scheduled pre-drainage period. A series of field tests were conducted in an outburst-proven coal seam to verify the feasibility of the comprehensive approach and the borehole space maintaining method. Field tests showed that the comprehensive approach to determine the proper sealing depth and the installing of sieve tubes to protect the borehole space can provide favorable conditions for maximum CMM pre-drainage from outburst-prone coal and maximum utilization of the in-seam boreholes.
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  • Modeling and experiments for transient diffusion coefficients in the desorption of methane through coal powders

    Zhao, Wei   Cheng, Yuanping   Jiang, Haina   Wang, Haifeng   Li, Wei  

    Diffusion is widely thought to be the key factor that controls the desorption of gas through porous media. However, the classical diffusion model with a constant Fick diffusion coefficient is inadequate for describing desorption from coal over the entire timescale. In this study, a new model to describe the time dependent diffusion coefficients during desorption was developed. Experiments including proximate analyses, isothermal adsorptions, mercury intrusions, N-2 adsorptions and methane desorptions were conducted to verify its validity. The results demonstrated that the Fick diffusion coefficients decreased over the desorption period first sharply and then slowly. By introducing the time-dependent relationship into the general unipore model, we obtained a unipore diffusion model with D(t) (UDMD), which had a better degree of fit with the laboratory desorption than that of the original. Moreover, the simplification for the UDMD was acquired for engineering applications. (C) 2017 Elsevier Ltd. All rights reserved.
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  • A fractal theory based fractional diffusion model used for the fast desorption process of methane in coal

    Jiang, Haina   Cheng, Yuanping   Yuan, Liang   An, Fenghua   Jin, Kan  

    Based on the realistic property of the pore structure in coal, we established a fractal theory based Fractional diffusion model (FFDModel) by introducing the fractal dimension d(f) to the Fick's classical model and changing the first-order partial differential equation about time into a upsilon fractional-order partial differential equation. Then, the solution of the FFDModel was obtained with separation variables technique. In order to verify the correctness of the solution, three coal samples with different rank from China were collected to do the methane desorption experiment of the fast desorption stage. The results indicate that the fractal dimension (d(f)) of the coking coal is the lowest of the three coal ranks. By comparing the FFDModel with Fick's classical model, we can see that the FFDModel fits better with the three measured samples. (C) 2013 AIP Publishing LLC.
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  • Definition, theory, methods, and applications of the safe and efficient simultaneous extraction of coal and gas

    Cheng, Yuanping   Wang, Liang   Liu, Hongyong   Kong, Shengli   Yang, Quanlin   Zhu, Jintuo   Tu, Qingyi  

    Simultaneous extraction of the coal and gas is an effective method of eliminating coal mine gas disasters while safely exploiting the coal and achieving efficient gas drainage in China, which is widely accepted by the main coal-producing countries around the world. However, the concrete definition of simultaneous extraction is vague and there is little accurate theoretical support for the simultaneous extraction of coal and gas, which makes it difficult to determine an efficient gas drainage method appropriate to the features of coal seams. Based on theoretical analysis, laboratory tests and field observations, a specific definition of simultaneous extraction of coal and gas is proposed after analyzing the characteristics of coal seam occurrences in China, and we developed the mechanism of mining-enhanced permeability and established the corresponding theoretical model. This comprises a process of fracture network formation, in which the original fractures are opened and new fractures are produced by unloading damage. According to the theoretical model, the engineering approaches and their quantitative parameters of ‘unloading by borehole drilling’ for single coal seams and ‘unloading by protective seam mining’ for groups of coal seams are proposed, and the construction principles for coal exploitation and gas-drainage systems for different conditions are given. These methods were applied successfully in the Tunlan Coal Mine in Shanxi Province and the Panyi Coal Mine in Anhui Province and could assure safe and efficient simultaneous extraction of coal and gas in these outburst coal mines.
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  • Effects of igneous intrusion on microporosity and gas adsorption capacity of coals in the Haizi Mine, China.

    Jiang, Jingyu   Cheng, Yuanping  

    This paper describes the effects of igneous intrusions on pore structure and adsorption capacity of the Permian coals in the Huaibei Coalfield, China. Twelve coal samples were obtained at different distances from a ~120m extremely thick sill. Comparisons were made between unaltered and heat-affected coals using geochemical data, pore-fracture characteristics, and adsorption properties. Thermal alteration occurs down to ~1.3 * sill thickness. Approaching the sill, the vitrinite reflectance (R(o)) increased from 2.30% to 2.78%, forming devolatilization vacuoles and a fine mosaic texture. Volatile matter (VM) decreased from 17.6% to 10.0% and the moisture decreased from 3.0% to 1.6%. With decreasing distance to the sill, the micropore volumes initially increased from 0.0054cm(3)/g to a maximum of 0.0146cm(3)/g and then decreased to 0.0079cm(3)/g. The results show that the thermal evolution of the sill obviously changed the coal geochemistry and increased the micropore volume and adsorption capacity of heat-affected coal (60-160m from the sill) compared with the unaltered coals. The trap effect of the sill prevented the high-pressure gas from being released, forming gas pocket. Mining activities near the sill created a low pressure zone leading to the rapid accumulation of methane and gas outbursts in the Haizi Mine.=20
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