Study on the law of multi-fracture through-layer propagation of crushed soft coal seam roof by hydraulic fracturing
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摘要:
为了研究碎软煤层顶板水力压裂过程中多裂缝穿层扩展规律，应用扩展有限元方法，分析了间接压裂时不同地质参数、施工参数、压裂顺序等因素对多裂缝穿层扩展形态和缝间应力干扰的影响。结果表明：顶板岩层最小水平主应力越大，裂缝在煤层中扩展效果越好；煤岩层厚度越大，裂缝穿层扩展的可能性越小；压裂液注入速率越快，裂缝在煤层中长度增加，但缝间应力干扰范围不断变大；裂缝起裂角度越大，压裂裂缝转向距离增加，使裂缝主要在顶板内扩展；较大的裂缝间距能够保证压裂后多裂缝均匀扩展进入煤层；不同的起裂顺序下，多裂缝起裂顺序依次为左侧、右侧、中间时，压裂后在煤层中形成的水力裂缝长度和面积是起裂顺序依次为中间、左侧、右侧时的2倍；相比裂缝起裂顺序依次为左侧、中间、右侧时，同时起裂会增大应力干扰范围，裂缝起裂压力变高，在煤层中形成的面积更小。
Abstract:The effects of different geological parameters, construction parameters, fracturing sequence and other factors on the propagation patterns of multiple cracks and stress interference between cracks in indirect fracturing were analyzed by the extended finite element method (FEM) in order to study the law of multi-fracture through-layer propagation during hydraulic fracturing of crushed soft coal seam roof. The results show that the larger the minimum horizontal principal stress of the roof layer is, the better the spreading effect of the crack in the coal seam is. The greater the thickness of coal layer, the less the possibility of fracture propagation. As the injection rate increases, the length of crack in coal seam increases, but the interference range of stress between cracks increases continuously. The larger the fracture initiation angle, the larger the fracture turning distance, the fracture mainly spreads in the roof. The larger fracture spacing can ensure the uniform expansion of multiple fractures into the coal seam after fracturing. The length and area of hydraulic fractures formed in coal seam after fracturing are twice as long as those in middle, left and right fractures in different fracture initiation sequences. When the fracture initiation sequence is left side, middle side and right side, the stress interference range will be increased, the fracture initiation pressure will be higher, and the area formed in the coal seam will be smaller.
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图 3 不同最小水平主应力下裂缝形态及孔压云图
Figure 3. Fracture morphologies and pore pressure diagrams at different minimum horizontal principal stress conditions
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图 4 不同煤岩厚度下裂缝形态及孔压云图
Figure 4. Fracture morphologies and pore pressure diagram under different coal and rock thickness conditions
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图 5 不同注入速率下裂缝形态及孔压云图
Figure 5. Fracture morphologies and pore pressure diagrams at different injection rates
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图 6 不同起裂角度下裂缝形态及孔压云图
Figure 6. Fracture morphologies and pore pressure diagrams at different initiation angles
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图 7 不同裂缝间距下裂缝形态及孔压云图
Figure 7. Fracture morphologies and pore pressure diagrams under different fracture spacing conditions
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图 8 不同压裂方案下最小主应力变化云图
Figure 8. Diagrams of minimum principal stress variation under different fracturing schemes
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图 9 不同压裂方案下各裂缝在煤层中扩展面积曲线图
Figure 9. The area of fracture propagation in the coal seam under different fracturing schemes
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图 10 不同压裂方案下裂缝在煤层中扩展总长度和总面积曲线图
Figure 10. Total area and length of fractures in coal seams under different fracturing schemes
表 1 数值模型基本参数
Table 1 Numerical model parameters
岩层 抗拉强度/
MPa弹性模量/
GPa泊松比 孔隙比 垂向地应力/
MPa煤层 0.2 2 0.30 0.2 15 顶板 0.5 5 0.25 0.1 15 
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表 2 数值模拟方案
Table 2 Numerical simulation schemes
组号 顶板最小水平主应力/MPa 煤岩厚度/m 注入速率/(m3·min−1) 起裂角度/(°) 裂缝间距/m 裂缝起裂顺序 1 6、7、8、9 10 2 0 5 ①② 2 8 10、12、14、16 2 0 5 ①② 3 8 10 0.5、1、2、3 0 5 ①② 4 8 10 2 0、15、30、45 5 ①② 5 8 10 2 0 10、8、5、2 ①② 6 8 10 2 0 5 同步 7 8 10 2 0 5 ①②③ 8 8 10 2 0 5 ①③② 9 8 10 2 0 5 ②①③
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