Effect of protective coal seam mining and gas extraction on gas transport in a coal seam

保护煤层开采与瓦斯抽放对煤层中瓦斯运移的影响

International Journal of Mining Science and Technology, Volume 26, Issue 4, July 2016, Pages 637-643

Abstract: A gas–solid coupling model involving coal seam deformation, gas diffusion and seepage, gas adsorption and desorption was built to study the gas transport rule under the effect of protective coal seam mining. The research results indicate: (1) The depressurization effect changes the stress state of an overlying coal seam and causes its permeability to increase, thus gas in the protected coal seam will be desorbed and transported under the effect of a gas pressure gradient, which will cause a decrease in gas pressure. (2) Gas pressure can be further decreased by setting out gas extraction boreholes in the overlying coal seam, which can effectively reduce the coal and gas outburst risk. The research is of important engineering significance for studying the gas transport rule in protected coal seam and providing important reference for controlling coal and gas outbursts in deep mining in China.

CBM drainage engineering challenges and the technology of mining protective coal seam in the Dalong Mine, Tiefa Basin, China

中国大龙矿煤层气排采工程挑战及保护煤层开采技术

Journal of Natural Gas Science and Engineering, Volume 24, May 2015, Pages 412-424

Abstract: As mining depths increase, new challenges, such as gas drainage, gas outbursts and rock bursts induced by mining extraction, have emerged, particularly in gassy, multiple-seam coal mines. The primary reason for this phenomenon is that increased depths lead to higher stresses, higher gas pressures and lower permeabilities in coal seams. Stress and pressure relief require temporal and spatial pre-drainage protection. However, the pursuit of increased coal production, along with economic interests, has resulted in a lack of time and space for CBM pre-drainage engineering. To solve these challenges, protective seam mining is the best way to reduce stress, increase coal permeability and increase CBM extraction efficiency. The stress relief of the rock mass below the protective seam generated five zones and three belts, providing the time and space for CBM pre-drainage engineering. Based on simulations of the outburst risk and mining economics using LaModel, the gassy No. 12 coal seam was selected as the first-mined protective seam. The floor rock roadway and cross boreholes were designed to drain mining-induced stress-relief gas from the protected seam. Therefore, they should be constructed before the protective seam mining begins. The time and space allocation pattern of CBM drainage can be summarized as follows: CBM drainage before coal seam mining using long bedding boreholes, stress-relief gas drainage during mining using floor roadway crossing boreholes and CBM extraction after mining using the roof roadway. Finally, field applications indicate that the remnant gas pressure and content of the protected seam significantly decreased after mining the protective seam. The permeability coefficient increased 1465-fold. The simultaneous extraction of CBM and coal was realized. The gas drainage rate increased from 45% to 70%, and the CBM utilization rate improved from 23% to 90%. These CBM drainage practices could provide insight and guidelines for other coal mines under similar conditions.

A protective seam with nearly whole rock mining technology for controlling coal and gas outburst hazards: a case study

防治煤与瓦斯突出灾害的几乎全岩保护层开采技术：实例研究

Natural Hazards, December 2016, Volume 84, Issue 3, pp 1793–1806

Abstract: Coal seams with low gas permeability and high gas outburst hazards are becoming more serious as coal mines extend deeper, but there are no appropriate protective coal seams for this kind of coal seam in China. In this paper, mining technology using a protective seam with nearly whole rock (PSNWR) is used to improve gas drainage and ensure safety during production. The characteristics of the distribution and occurrence of PSNWRs and their mechanical properties are analyzed. A theoretical mechanics model and three-dimensional numerical model are established to study the controlling effect of PSNWR mining on pressure-relief gas drainage. In this context, the mining process, system and gas extraction design for PSNWRs are introduced. The results for Pingdingshan No. 12 Coal Mine show that mining with a PSNWR 2.0 m thick can effectively reduce the danger of coal and gas outbursts and improve gas drainage and utilization. The gas drainage rates are >80 %, which significantly increases the social, economic and environmental benefits of Pingdingshan No. 12 Coal Mine.

Gas outburst disasters and the mining technology of key protective seam in coal seam group in the Huainan coalfield

淮南煤田瓦斯突出灾害及煤层组关键保护层开采技术

Natural Hazards, June 2013, Volume 67, Issue 2, pp 763–782

Abstract: Coal and gas outburst disasters in coal seams are becoming more serious as coal mines extend deeper underground in China. To aid gas control in high-gas outburst coal seam group, this study performed research based on the geological conditions of the Xinzhuangzi coal mine in the Huainan coalfield. The laws of gas occurrence, the strength of the coal outburst, and the regional partition were studied. Simultaneously, we introduced the key protective seam mining technology and confirmed the mining sequence of coal seam groups. The results indicate that (1) each seam absorbs gas well, and the currently measured gas content is up to 15.0 m3/t. (2) Although some differences about coal seams outburst intensity remain, the differences in the same group are very small. (3) The coal seam B10 was chosen as the key protective seam and was mined first; then adjacent seams were mined from bottom to top by layer within the roof of B10 and from top-to-bottom within the floor of B10 to guarantee each adjacent coal seam received the good effects of pressure-relief and increasing permeability. (4) The main methods of gas extraction in each protected seam are surface boreholes and net-like penetrating boreholes in the floor roadway, and related technical parameters were determined according to the degree of pressure-relief in coal seam. This in situ experiment indicates a method aiding the gas control problem and guaranteeing safe and highly efficient exploitation of high-gas outburst seams.

Mining thin sub-layer as self-protective coal seam to reduce the danger of coal and gas outburst

降低煤与瓦斯突出危害的薄次层（自保护煤层）开采

Natural Hazards, March 2014, Volume 71, Issue 1, pp 41–52

Abstract:In response to the severe situation of coal mine gas disaster in China, a new method of reducing the danger of coal and gas outbursts and improving gas drainage and utilization in coal mines was introduced in this paper. The main idea of this method is to mining thin sub-layer as self-protective coal seam to eliminate or reduce the danger of coal and gas outburst. This method can be implemented by drills along seam and hydraulic jet when the mined seam with a relatively weak risk of coal and gas outbursts is soft or has a soft layer. This method was first applied in the Yian mine to verify its effectiveness. The results of application showed that mining thin sub-layer as self-protective coal seam can effectively eliminate the danger of coal and gas outburst and improve gas drainage and utilization. As this method needs less time and lower cost than conventional protective layer mining, it is of great significance for mining coal seam with the danger of coal and gas outburst.

Keywords: Coal miningSelf-protective coal seamThin sub-layerThe danger of coal and gas outburst

Permeability distribution characteristics of protected coal seams during unloading of the coal body

煤体卸载期间保护煤层的渗透率分布特征

International Journal of Rock Mechanics and Mining Sciences, Volume 71, October 2014, Pages 105-116

Abstract: Based on the engineering practices used in mining the protective coal seam at Huainan coalfield in China, this study investigates the temporal evolution of three-dimensional stress of the protected coal seam by numerical simulation. The coal body of the protected coal seam should first be loaded and then unloaded with the three-dimensional stress being reduced simultaneously. In accordance with the forcing process around the coal body of the protected coal seam, permeability experiments were performed under fixed axial displacement with unloading confining pressure (FADUCP) and fixed deviatoric stress with unloading confining pressure (FDSUCP) stress paths. During the loading stage before the unloading point, the permeability and axial strain have a negative exponential relationship. However, during the unloading stage after the unloading point, the permeability and axial strain have a positive exponential relationship. Using the relationship between permeability and axial strain, along with the swelling and deformation of the protected coal seam, the permeability distribution characteristics for the protected coal seam was obtained. According to the permeability distribution characteristics of the protected coal seam, drilling designs for gas drainage can be optimized. Finally, the rationality of the drilling design was verified using gas data measured during mining of the protected coal seam.