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Now showing items 65 - 72 of 72

  • Pore morphology characterization and its effect on methane desorption in water-containing coal:An exploratory study on the mechanism of gas migration in water-injected coal seam

    Zhang, Kaizhong   Cheng, Yuanping   Wang, Liang   Dong, Jun   Hao, Congmeng   Jiang, Jingyu  

    Water-injected technique was once considered as an effective way to solve the gas-induced problems in the mining industry, but has been stagnant due to its unstable conditions. A systematic knowledge on the micro-mechanism of water injection into coal seams is beneficial to better understand water-based techniques. However, the laboratory investigations considering the engineering background of water injection as well as the pore morphology of targeted coal seam have been rarely studied. In this paper, characterization of pore morphology and the effect of injected water on gas desorption characteristic were carried out using pore structure analyzers (N-2 adsorption and mercury intrusion methods), high-resolution scanning electron microscopy and a home-made instrument (water-injecting desorption test). The pore morphology results from pore size distribution, fractal dimension, pore shape and connectivity indicate that the essential configuration of pore structure is well-developed larger pores containing abundant smaller pores with extensive distribution of constricted pores that are inaccessible to fluid migration. The influence of pore morphology on desorption process may be attributed to the microporous constrictions with non-effective pores, which are geometrically interpreted by pore blocking mechanism. The desorption test results show the total desorption volumes and initial effective diffusion coefficient have the reduce rates of 26.65% and 38% with the moisture content increasing from dry to 1.8% while have a little change as moisture increases (1.8%-11.2%), demonstrating the obstruction of water molecule on gas desorption pathway and the existence of extremity moisture content. Water injection has a remarkable effect on the average desorption rates in the initial period of 10 min; however, the ultimate desorption volume of 0.81 mL/ g with higher moisture content of 11.2% is not sensitive to adsorption equilibrium pressure. Moreover, combined with the mature water-injected technique in the actual coal seam, a conceptual design was summarized to consider the effect of the constricted pore morphology on the interactions of "water-gas-coal", which may demonstrate the micro-mechanism of gas migration in the far water-injected coal seam. Meanwhile, in the near water injection zone, due to more energetic gas molecules forming gas bubble in the aqueous condition, nucleation appears to be imagined to explain the pressure-insensitive phenomenon. These findings are of great guiding significance to the theoretical studies and field applications of actual water-injected coal seam.
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  • Hydraulic flushing in soft coal sublayer:Gas extraction enhancement mechanism and field application

    Zhang, Hao   Cheng, Yuanping   Yuan, Liang   Wang, Liang   Pan, Zhejun  

    In China, one or several ultrathin soft coal sublayers are widely developed in the coal seam. Therefore, a method of using hydraulic flushing in soft sublayer to enhance the gas extraction in these particular coal seams is developed in this work. We first established a new fully coupled gas extraction model by combing gas diffusion, gas flow, and a permeability model that considers the effect of stress change and plastic failure. By adopting this model, the gas extraction enhancement mechanism and its main influence factors were studied using the numerical simulation method based on the engineering and geological background in the Yangquan No.5 coalmine. Thereafter, a hydraulic flushing equipment, which could move freely in the underground coalmine, was developed to apply the hydraulic flushing method in soft coal sublayers used in the 8402 working face. Its gas extraction effect was systematically investigated. Our simulation results match well with the field results, suggesting that our model is feasible. Meanwhile, after adopting this method, the gas extraction condition in this coalmine improves significantly. The borehole number decreases by 66.7%, while the gas extraction rate and gas extraction concentration increase by 1.33 times and 3 times, respectively. Moreover, during the coal mining process, the gas adsorption index of drilling cuttings, the quantity of drilling cuttings, and the CH4 concentration also decrease dramatically.
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  • A novel technology for high-efficiency borehole-enlarging to enhance gas drainage in coal seam by mechanical cutting assisted by waterjet

    Hao, Congmeng   Cheng, Yuanping   Liu, Hongyong   Wang, Liang   Liu, Qingquan  

    Although the effect of pressure relief and permeability increasing achieved by the technology of traditional hydraulic flushing (enlarging hole by high-pressure water jet) is great significance, it has some shortcomings, such as low efficiency, large consumption of water resources, and being difficult to guarantee the sizes of holes. The limitations of water jet are known as the main factors that would restrict the effective pressure relief in soft and low-permeability coal seam. For the purpose of solving the problems existing in the traditional technology, a new technology of mechanical cutting assisted by waterjet has been developed. Then, a numerical simulation method was employed to illustrate the flow field and the force characteristics of equipment equipped with new technology. Finally, the new technology and its supporting system were tested in the Xinjing Coal Mine, Shanxi Province, China. The test results obtained are as follows: the efficiency of mechanical cutting is much higher than that of hydraulic flushing; the maximum equivalent to enlarge hole radius is increased by 83.3% (t =3D 30 min), and the efficiency of the new technology is 3 times more than that of the traditional hydraulic flushing; the time used for gas drainage is increased by 19.3%, and the drainage efficiency is increased by 28%; the cost spent on gas control in one working face decreases by more than 14.7%. It is concluded that the new technology can efficiently enlarge borehole so as to enhance gas drainage in coal seam and decrease the cost of coal mining.
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  • Non-uniform Distributions of Gas Pressure and Coal Permeability in Coalbed Methane Reservoirs Induced by the Loess Plateau Geomorphology:A Case Study in Ordos Basin,China

    Liu, Qingquan   Chu, Peng   Hao, Congmeng   Cheng, Yuanping   Wang, Haifeng   Wang, Liang  

    Coal mine methane (CMM) accumulation and permeability distribution characterization are the bases for a successful CMM drainage design. Before extending degassing activities, the prediction of the gas pressure and permeability distributions of the pre-drainage area is recommended. The effects of CMM geology on gas occurrence and coal permeability have drawn much attention and contributed to numerous research findings. However, the loco-regional distribution characteristics of the gas occurrence and coal permeability induced by landform characteristics have barely been investigated. This paper presents a quantitative case study on the non-uniform distributions of the gas pressure and coal permeability induced by the Loess Plateau geomorphology. Based on the collection and analysis of a large amount of data, the non-uniform distribution of the burial depth was first obtained. The relationship between the burial depth and gas pressure was analyzed by using the safety line method. A series of efforts were made to perform experimental tests for determining the competing influences of the effective stresses and sorption-induced strain on the coal permeability, and the relationship between the burial depth and coal permeability was analyzed by using the modified PM model. Finally, a quantitative evaluation of the non-uniform distributions of the gas pressure and coal permeability induced by the Loess Plateau geomorphology was achieved. The results showed that the statistical burial depth, gas pressure, and permeability of the No. 3 coal seam were irregular with large loco-regional changes, and the range of the burial depth was from 349 to 479 m. Further, the predicted pressure was from 0.81 to 1.66 MPa, and the predicted permeability was from 0.0155 to 0.01968 mD. When using both the gas pressure and coal permeability as the evaluation indices of the difficulty of gas drainage, the study area could be divided into six parts and classified into two groups. The division of a coal seam influenced by the Loess Plateau geomorphology will provide essential information formulation and economical plan to degas it.
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  • Optimal selection of coal seam pressure-relief gas extraction technologies:a typical case of the Panyi Coal Mine,Huainan coalfield,China

    Shang, Zheng   Wang, Haifeng   Cheng, Yuanping   Li, Bing   Dong, Jun   Liu, Qingquan  

    Pressure-relief gas extraction from a protected seam can be realized by surface well drilling and the use of net-like penetrating boreholes (NPB). The selection of the extraction mode is affected by the geological conditions, characteristics of the coal seam occurrence and inherent characteristics of the extraction mode. Based on a numerical simulation and field test, the Panyi Coal Mine in the Chinese Huainan coalfield is used as an example to study the effective extraction range, efficiency, cost, total amount extracted and applicable conditions of two methods. The results show that the average gas extraction concentration (93.8%) and average extraction purity (28 m(3)/min) of surface wells are higher than those (41.8% and 17 m(3)/min) of NPB by approximately 124.4% and 64.7%, respectively. The superiority of the extraction efficiency from the use of surface wells is obvious from intact wells. In addition, the unit methane control costs, methane drainage costs per cubic meter and construction period of surface drillings are less than those of the NPB. However, Surface drilling is easily restricted by poor wellbore stability and high surface environment requirements. This case study can provide valuable examples for other coal mines with similar geological conditions for the selection of pressure-relief gas treatment methods.
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  • Gas extraction challenge and the application of hydraulic cavity technology in the Shijiazhuang coalmine,Qinshui basin

    Zhang, Hao   Cheng, Yuanping   Yuan, Liang   Wang, Liang   Jiang, Jingyu   Li, Guofang  

    Hydraulic cavity has been demonstrated to be an efficient gas extraction technology in the single, soft, and low-permeability coal seam in China. However, the permeability evolution around hydraulic cavity has not been fully understood due to the lack of the permeability model in the post-peak stage, which leads to a great challenge for the gas extraction simulation and thus the borehole design. Therefore, we developed a fully coupled dual-porosity gas extraction model that considers the effects of coal damage on the permeability evolution. Based on this model, we analyzed the permeability evolution and the gas migration characteristics around the hydraulic cavity and designed a hydraulic cavity scheme for the Shijiazhuang coalmine. The field application results prove that our simulation results are correct.
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  • Study on the stress relief and permeability increase in a special low-permeability thick coal seam to stimulate gas drainage

    Zhang, Rong   Cheng, Yuanping   Yuan, Liang  

    Underground coal seam gas drainage is an effective method to lower the risk of coal and gas outburst disaster. However, it is difficult to drainage gas in the low-permeability coal seam, especially low-permeability thick coal seam with a tectonic coal sub-layer. Due to the special structure of this coal seam, many conventional technologies are inefficient. In this paper, to stimulate the coal seam gas drainage, we proposed a hydraulic flushing cavitation technology. Theory analysis, numerical simulation and field tests ware carried out to study the stress relief and permeability increase after adopting this technology. The results indicated that a large hole was formed in the coal seam, which enlarged the scope of stress relief and plastic damaged, and the coal seam permeability and gas drainage efficiency increased significantly.
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  • Experimental study of the influences of water injections on CBM exploitation

    Chen, Xiangjun   Li, Liyang   Cheng, Yuanping   Qi, Lingling  

    In order to study the influence of water injections into a coal body on coalbed methane (CBM) recovery following hydraulic fracturing, experiments have been done in which water is injected into coal samples at adsorbate equilibrium pressures of 0.5 MPa, 0.84 MPa, 1.5 MPa and 2.5 MPa using a high-pressure adsorbate-injecting water-desorption device. The results show that the injected water can replace the adsorbed methane. The replacement ratio and quantity of adsorbed methane gradually increase with increasing quantities of injected water. Under the adsorbate equilibrium pressure of 2.5 MPa, the maximum displacement of the adsorbed methane is 11.88 mL/g when the moisture content of the coal is 10.03%. Under the adsorption equilibrium pressure of 0.84 MPa, the maximum replacement rate of the absorbed methane is 59.14% when the moisture content is 8.39%. The increase of the displacement and replacement rates shows that the adsorbed methane can be replaced by water, which further promotes improved CBM recovery.
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