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不同孔界距煤岩复合体水力压裂裂缝扩展试验研究

Experimental study on hydraulic fracturing crack propagation of coal-rock complex with different hole-interface distance

  • 摘要: 为揭示煤层顶板水力压裂过程中压裂孔底部与煤岩界面距离对压裂裂缝扩展的影响规律,开展了不同孔界距的水力压裂特性试验。选用孔界距分别为2、5、7、10、12、15 mm的煤岩组合相似试件,利用RTR-4600煤岩力学参数高精度测量试验机与DS5-8B全信息声发射信号分析仪,模拟水力压裂过程并获取声发射特征。结果表明:随着孔界距的减小,起裂压力呈现下降趋势,但并非完全线性;最优孔界距为5 mm,裂缝扩展速率与范围达到平衡,声发射点分布密度和损伤覆盖率分别为5.2 events/mm3和83%,该条件下裂缝网络的复杂性与渗透性提升最为显著;孔界距通过调控流体—应力—损伤多场耦合关系主导损伤演化路径,较大孔界距条件下界面主导的裂纹扩展受限于压力场弥散效应,临界孔界距实现应力集中能与结构弱面分布的最佳匹配,超小孔界距因近场应力超限导致裂缝网络失稳退化。

     

    Abstract: To reveal the influence law of the distance between the bottom of fracturing borehole and the coal-rock interface on the crack propagation during the hydraulic fracturing of the coal seam roof,hydraulic fracturing characteristic experiments with varying hole-interface distances were conducted. Similar composite specimens with hole-interface distances of 2 mm,5 mm, 7 mm,10 mm,12 mm,and 15 mm were prepared. The hydraulic fracturing process was simulated using an RTR- 4600 highprecision measurement testing machine for coal and rock mechanical parameters combined with a DS5-8B full-information acoustic emission signal analyzer to capture acoustic emission characteristics. Experimental results show that the fracture initiation pressure decreases with increasing hole-interface distance,but it is not completely linear. The optimal hole-interface distance is 5 mm. The crack propagation rate and range are balanced. The distribution density of acoustic emission points and the damage coverage rate are 5. 2 events/mm3 and 83% respectively. Under this condition,the complexity and permeability of the fracture network increase most significantly. The hole-interface distance dominates the damage evolution path by regulating the multi-field coupling relationship of fluid-stress-damage. Under the condition of larger hole -interface distance,the crack propagation dominated by the interface is limited by the diffusion effect of the pressure field. The critical hole-interface distance can achieve the best match between stress concentration and the distribution of weak surfaces in the structure. The ultra-small hole-interface distance leads to the instability and degradation of the crack network due to the over-limit near-field stress.

     

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