Practice of enhanced gas extraction by staged fracturing with comb-shaped long hole in coal mine roof
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摘要: 为了解决碎软煤层本煤层钻孔施工困难，瓦斯抽采浓度低，抽采效果差，无法实现大面积区域预抽的问题，在现有煤矿井下定向钻进技术和水力压裂技术的基础上，结合前期研究成果，提出了顶板梳状长钻孔分段水力压裂技术，并在韩城矿区桑树坪二号井进行了现场试验。现场施工顶板梳状长钻孔主孔长度588 m，包含8个分支孔，钻孔总进尺1 188 m，主孔距煤层0~3.28 m，平面上覆盖约12.5 m。采用不动管柱分段水力压裂工艺，分4段进行水力压裂施工，累计注水2 012 m3，最大泵注压力8.74 MPa。压裂后最大影响半径大于30 m，且裂缝主要位于钻孔下方，向煤层延伸。压裂钻孔稳定抽采阶段瓦斯抽采纯量1.18 m3/min，抽采瓦斯体积分数平均43.54%。顶板梳状长钻孔分段水力压裂钻孔瓦斯抽采纯量是水力割缝钻孔的1.2倍，是本煤层顺层钻孔的4.0倍。试验结果表明，顶板梳状长钻孔分段水力压裂技术可有效避免本煤层常规钻孔施工过程中存在的塌孔、卡钻、喷孔等问题，实现了碎软低渗煤层大面积区域瓦斯预抽，为碎软低渗煤层区域瓦斯预抽提供了新思路和新方法。Abstract: In order to solve the problems of drilling construction difficulty, low gas drainage concentration, poor drainage effect and inability to realize the predrainage of large area, based on the existing directional drilling technology and hydraulic fracturing technology in coal mines, combined with the previous research results, the staged hydraulic fracturing technology of comb-shaped long borehole in roof was proposed, and the field test was carried out in Sangshuping No.2 well in Hancheng mining area.The test results show that the main hole length of comb-shaped hole in roof was 588 m, including 8 branch holes, the total drilling footage was 1 188 m, the distance between main hole and coal seam was 0-3.28 m, and the plane coverage was about 12.5 m. The hole was divided into 4 stages with staged hydraulic fracturing technology of immovable pipe string. The cumulative injected water volume was 2 012 m3 and the maximum pumping pressure was 8.74 MPa. After fracturing, the maximum influence radius was more than 30m, and the fractures were mainly located under the borehole and extended to the coal seam. During the stable drainage stage of fracturing borehole, the pure gas drainage volume was 1.18 m3/min, and the average drainage concentration was 43.54%. The pure gas drainage quantity of segmented hydraulic fracturing borehole with comb-shaped long borehole in the roof was 1.2 times of that of hydraulic slotting borehole and 4.0 times of that of bedding borehole in this coal seam. The test results show that the staged hydraulic fracturing technology of comb-shaped long borehole in roof can effectively avoid the problems such as collapsed hole, stuck hole, spray hole, etc in the conventional drilling process of this coal seam, and realize the pre-drainage of gas in a large area of broken soft coal seams of low permeability. It provides a new idea and new method for gas pre-drainage in the area of broken soft coal seams of low permeability.
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