Longwall mining of coal seams affects a large area of overburden by deforming it and creating stress-relief fractures, as well as bedding plane separations, as the mining face progresses. Stress-relief fractures and bedding plane separations are recognized as major pathways for gas migration from gas-bearing strata into sealed and active areas of the mines. In order for strata gas not to enter and inundate the ventilation system of a mine, gob gas ventholes (GGVs) can be used as a methane control measure. The aim of this paper is to analyze production performances of GGVs drilled over a longwall panel. These boreholes were drilled to control methane emissions from the Pratt group of coals due to stress-relief fracturing and bedding plane separations into a longwall mine operating in the Mary Lee/Blue Creek coal seam of the Upper Pottsville Formation in the Black Warrior Basin, Alabama. During the course of the study, Pratt coal's reservoir properties were integrated with production data of the GGVs. These data were analyzed by using material balance techniques to estimate radius of influence of GGVs, gas-in-place and coal pressures, as well as their variations during mining. The results show that the GGVs drilled to extract gas from the stress-relief zone of the Pratt coal interval is highly effective in removing gas from the Upper Pottsville Formation. The radii of influence of the GGVs were in the order of 330-380 m, exceeding the widths of the panels, due to bedding plane separations and stress relieved by fracturing. Material balance analyses indicated that the initial pressure of the Pratt coals, which was around 648 KPa when longwall mining started, decreased to approximately 150 KPa as the result of strata fracturing and production of released gas. Approximately 70% of the initial gas-in-place within the area of influence of the GGVs was captured during a period of one year.

中文翻译：

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长壁开采地层瓦斯控制采空区气孔生产性能分析

随着工作面的推进，煤层的长壁开采通过使覆盖层变形和产生应力释放裂缝以及层理平面分离来影响大面积的覆盖层。应力释放裂缝和层理平面分离被认为是气体从含气地层运移到矿山封闭和活动区的主要途径。为了不让地层气体进入并淹没矿井的通风系统，采空区瓦斯通风孔 (GGV) 可用作甲烷控制措施。本文的目的是分析在长壁板上钻孔的 GGV 的生产性能。由于应力释放压裂和层理平面分离进入在阿拉巴马州 Black Warrior Basin 的 Upper Pottsville 组的 Mary Lee/Blue Creek 煤层中作业的长壁矿，这些钻孔是为了控制 Pratt 煤组的甲烷排放. 在研究过程中，普拉特煤的储层特性与 GGV 的生产数据相结合。通过使用材料平衡技术对这些数据进行分析，以估计 GGV、原位瓦斯和煤炭压力的影响半径，以及它们在采矿过程中的变化。结果表明，从 Pratt 煤层段应力释放带中开采的 GGV 对从上 Pottsville 地层去除气体非常有效。GGV 的影响半径约为 330-380 m，由于层理平面分离和压裂释放的应力，超过了面板的宽度。物质平衡分析表明，普瑞特煤的初始压力（长壁开采开始时约为 648 KPa）由于地层压裂和释放气体的产生而下降至约 150 KPa。在 GGV 影响区域内，大约 70% 的初始气体在一年内被捕获。