Geological adaptability analysis and operational parameter optimization for staged fracturing horizontal wells in coal seam roof
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摘要: 煤层顶板分段压裂水平井是实现碎软低渗煤层煤层气高效抽采的有效技术。依托“十三五”国家科技重大专项课题，围绕煤层顶板分段压裂水平井煤层气高效抽采技术，采用理论分析、数值模拟等手段，对比不同地应力状态下裂缝的穿层扩展形态，研究水平井布井方位与最小水平主应力方向夹角对裂缝转向扩展的影响，分析多簇射孔条件下裂缝的竞争扩展现象。结果表明：(1)煤层顶板分段压裂水平井技术应用于碎软低渗煤层煤层气开发具有避免钻井液污染储层、提高水平井钻井施工安全性、提高固井质量、提高压裂改造效果、控制煤粉产出等优势。(2)地应力是裂缝穿层扩展的关键控制因素。为保证裂缝穿层扩展，垂向应力需大于最小水平主应力，顶板最小水平主应力需大于煤层，并且层间应力差为1~3 MPa时，既能够保证裂缝的穿层扩展效果，也能避免裂缝的起裂和延伸压力过高；“上覆岩层–顶板–煤层”应力剖面为“低–高–中”型时，水平井与煤层顶面的距离对于裂缝的穿层扩展效果影响较大；推荐水平井水平段与煤层顶面距离小于2.0 m。(3)水平井布井方位与最小水平主应力方向夹角越大，裂缝转向半径和转向距离越大，在压裂段间距相同的条件下，夹角为45°时缝间干扰程度比夹角为0°时强，不利于后续压裂段裂缝扩展，建议水平井布井方位与最小水平主应力方向夹角±15°以内。(4)对于分段多簇压裂，缝间应力干扰、压裂液流动摩阻、射孔孔眼摩阻共同导致各射孔簇裂缝非均匀扩展，可通过限流压裂、裂缝暂堵等手段促进裂缝均匀扩展。(5)工程试验取得了良好的产气效果，裂缝延伸特征与理论研究吻合度高。研究成果可为煤层顶板水平井的施工参数优化设计和推广应用提供借鉴。Abstract: The technique of staged fracturing horizontal wells in coal seam roof is effective for efficient extraction of coalbed methane (CBM) in broken soft and low-permeability coal seams. With the help of the major national science and technology projects of the 13th Five Year Plan for National Economic and Social Development, the efficient extraction technology of CBM by staged fracturing horizontal wells in coal seam roof was studied. By using theoretical analysis and numerical simulation, the fracture propagation patterns in different in-situ stress states were compared, and the influence of the angle between the horizontal well layout orientation and the minimum horizontal principal stress on the fracture deflection and propagation was studied. The competitive propagation phenomenon of fractures under the condition of multi-cluster perforation was also analyzed. The results show that: (1) the staged fracturing horizontal well in coal seam roof applied to CBM development in broken, soft and low-permeability coal seams can avoid contamination of reservoirs by drilling fluid, improve the safety in horizontal well drilling, cementing, and the effect of fracturing transformation, and controlling the output of coal fine. (2) In-situ stress is the key controlling factor for fracture propagation. In order to ensure the crack propagation through the bedding plane interface, the vertical stress must be greater than the minimum horizontal stress, and the minimum horizontal stress of the roof must be greater than that of the coal seam. The interlayer stress difference of 1-3 MPa can not only ensure the effect of through-layer propagation of fractures, but also avoid excessive fracture initiation and extension pressure. When the stress profile of “overlying rock-roof-coal seam” is “low-high-medium” type, the distance between the horizontal well and the top surface of the coal seam has a great influence on fracture propagation. It is recommended that the distance between the horizontal section of the horizontal well and the top surface of the coal seam should be less than 2.0 m. (3) The larger the angle between the horizontal well and the minimum horizontal stress, the greater the fracture turning radius and turning distance. With the same fracturing spacing, the fracture interference is stronger at an angle of 45° than at an angle of 0°, which is not conducive to the propagation of subsequent fractures. It is recommended that the angle between the horizontal well and the minimum horizontal stress should be within ± 15°. (4) For staged multi-cluster fracturing, the fracture stress interference, fracturing fluid flow friction, and perforation friction jointly lead to non-uniform propagation of fractures for each perforation cluster, which can be controlled by using limited entry fracturing, temporary plugging, etc., to promote the uniform propagation of fractures. (5) The pilot test has achieved good gas production, and the fracture extension characteristics are in good agreement with the theoretical research. The research results can provide reference for the optimization design and application of operational parameters of horizontal wells in the coal seam roof.
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图 1 煤层顶板分段压裂水平井技术原理
Fig. 1 Schematic diagram of the staged fracturing horizontal well in coal seam roof
图 2 煤层顶板水平井裂缝网络及渗流模式
Fig. 2 Schematic diagram of the fracture network and seepage pattern of the horizontal well in the coal seam roof
图 3 穿层压裂有限元数值模拟模型
Fig. 3 Finite element numerical simulation model of through-layer fracturing
图 4 不同垂向应力差异系数裂缝扩展形态
Fig. 4 Fracture propagation morphology under different vertical stress coefficients
图 5 不同层间应力差条件下裂缝扩展形态
Fig. 5 Fracture propagation morphology under different interlayer stresses between roof and coal seam
图 6 不同层间应力差条件下注入压力随时间的变化
Fig. 6 Variation of injection pressure with time under different interlayer stresses between roof and coal seam
图 7 不同层间应力差条件下起裂压力的变化
Fig. 7 Variation of fracture initiation pressure under different interlayer stresses between roof and coal seam
图 8 不同地应力剖面条件下裂缝穿层延伸形态
Fig. 8 Fracture propagation morphology in different in-situ stress profiles
图 9 不同地应力剖面裂缝穿层延伸形态
Fig. 9 Fracture propagation morphology in different in-situ stress profiles
图 10 不同布井方位角条件下裂缝延伸形态
Fig. 10 Fracture propagation morphology with different well placement azimuths
图 11 不同布井方位角双裂缝延伸形态
Fig. 11 Fracture propagation morphology for double fractures with different well placement azimuths
图 12 分段多簇压裂有限元数值模拟模型
Fig. 12 Finite element numerical simulation model of staged multi-cluster fracturing
图 13 2簇射孔条件下裂缝延伸形态
Fig. 13 Fracture propagation morphology for two-cluster perforation
图 14 2簇射孔条件下注入点压力变化
Fig. 14 Variation of injection pressure at the injection point for two-cluster perforation
图 15 三簇射孔条件下裂缝延伸形态
Fig. 15 Fracture propagation morphology for three-cluster perforation
图 16 3簇射孔条件下注入点注入压力变化
Fig. 16 Variation of injection pressure at the injection point for three-cluster perforation
表 1 模型计算参数
Table 1 Simulation model parameters
参数 上覆岩层 顶板 煤层 抗拉强度/MPa 1.50 1.00 0.50 弹性模量/GPa 4.50 3.00 1.00 泊松比 0.25 0.30 0.35 渗透率/10−3 μm2 0.01 0.01 0.10 地层孔隙率/% 2 2 9 煤层孔隙压力/MPa 6 6 6 垂向地应力/MPa 16 16 16 最小水平主应力/MPa 14 13 12 最大水平主应力/MPa 12 11 10 
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表 2 水平井压裂裂缝微地震监测结果
Table 2 Microseismic monitoring results of horizontal well hydraulic fracturing
裂缝参数 第3段 第7段 1357~1360 m 1317~1320 m 947~950 m 裂缝长度/m 左翼长度 43 46 100 右翼长度 未检测到有效裂缝 68 74 裂缝高度/m 11 16 12 裂缝方位 SW 70° NE 49° NE 45° 裂缝产状 垂直 垂直 垂直
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