煤岩水力压裂裂缝扩展规律及减冲效果评价

Fracture propagation behavior in coal hydraulic fracturing and impact mitigation effectiveness evaluation

  • 摘要: 水力压裂技术可使煤矿井下坚硬顶板产生裂缝、分层,及时垮落和卸压,但裂缝的几何扩展规律及影响因素研究仍然欠缺。采用理论分析方法,计算了实验室煤岩起裂压力值,并与试验数值进行比较,数据较吻合;采用真三轴水力压裂物理相似模拟装置,结合声发射特征,研究了不同应力条件和注水速率下水力压裂裂缝起裂、扩展规律及裂缝扩展范围的变化规律,采用ABAQUS软件的扩展有限元法,研究了不同应力条件和注水速率对水力裂缝扩展规律的影响,并与实验室试验进行对照分析,结果表明:垂向应力对水力裂缝起裂和扩展有显著影响。随着垂向应力的增加,试件所需起裂压力随之显著增大,裂缝扩展的稳定性和贯通性逐渐增强,扩展方向趋于垂向应力方向;而较低垂向应力下,裂缝扩展方向不稳定,形态更为分散,倾向于沿最大水平主应力方向发展。声发射特征显示,高垂向应力条件下,声发射事件主要集中于主裂缝区域,微破裂活动分布稳定;而在低垂向应力下,声发射事件分布较为离散,表明裂缝扩展过程中存在更多局部应力调整及次生破裂现象。最大水平主应力的降低削弱了岩体沿其方向张开裂缝的驱动力,裂缝起裂受到更强的侧向约束,导致岩体等效抗拉强度增加,水力裂缝需克服更大的应力障碍,才能完成起裂,从而使得试件的起裂压力随最大水平主应力的降低而显著升高,水压峰值也相应增大。最大水平主应力对裂缝扩展方向、稳定性及声发射特征具有重要影响。当最大水平主应力较高时,裂缝扩展方向更稳定,形态趋于笔直,贯通性增强,同时声发射事件主要集中在裂缝路径上,表明裂缝扩展过程中的应力释放更为集中。而当最大水平主应力较低时,裂缝扩展,裂缝形态更复杂,声发射事件分布较为离散,表明裂缝扩展不稳定。注水速率增大会显著提高试件的起裂压力。在低注水速率条件下,压裂液在孔内缓慢积聚,水分子渗入岩体微裂隙和孔隙,逐步削弱岩石局部强度,促进裂缝的平稳起裂。而高注水速率使水压在短时间内迅速聚集,裂缝尖端应力急剧集中,易超出岩石抗拉强度,需更高水压才能实现破裂。同时,高速注水诱发的强烈水力作用增加了裂缝起裂过程中的不稳定性,使裂缝路径更易受到扰动,扩展形式更加复杂。注水速率显著影响水力裂缝扩展范围、裂缝复杂程度及声发射定位点分布特征。低注水速率下,裂缝扩展稳定,破裂路径清晰,微破裂活动较少;中等注水速率时,流体动力作用引发局部裂缝偏折,声发射点分布集中但局部区域高密度聚集,表明裂缝受局部应力扰动影响;高注水速率下,导压裂缝扩展高度不稳定,裂缝网络复杂化,微破裂活动增强,能量释放剧烈。表明注水速率提高,水力裂缝扩展稳定性降低,扩展范围相对减小,局部破裂更强烈,会造成裂缝结构复杂化及能量损耗。此外,进行了水力压裂工业性试验,验证了裂缝扩展规律,消减了采煤工作面冲击危险性。研究结果可指导类似开采地质条件的煤矿井下分段水力压裂问题,丰富水力压裂理论基础。

     

    Abstract: Hydraulic fracturing technology can induce fractures and delamination in the hard roof of coal mine underground, facilitating timely collapse and pressure relief. However, research on the geometric propagation patterns of fractures and their influencing factors remains insufficient. Theoretical analysis methods were employed to calculate the initiation pressure of coal rock in laboratory settings, with the results compared against experimental data and showing good agreement. Using a true triaxial hydraulic fracturing physical simulation device combined with acoustic emission characteristics, the initiation and propagation patterns of hydraulic fractures under different stress conditions and injection rates, as well as the variations in fracture propagation range, were investigated. The extended finite element method (XFEM) in ABAQUS software was used to study the influence of different stress conditions and injection rates on hydraulic fracture propagation, and the results were compared with laboratory experiments. The findings indicate that vertical stress significantly affects the initiation and propagation of hydraulic fractures. As vertical stress increases, the initiation pressure required for the specimen increases significantly, while the stability and connectivity of fracture propagation gradually improve, with the propagation direction tending to align with the vertical stress direction. Under lower vertical stress, the fracture propagation direction becomes unstable, exhibiting more dispersed morphology and a tendency to develop along the maximum horizontal principal stress direction. Acoustic emission characteristics show that under high vertical stress conditions, acoustic emission events are mainly concentrated in the main fracture area, with stable distribution of micro-fracturing activities. In contrast, under low vertical stress, acoustic emission events are more dispersed, indicating more local stress adjustments and secondary fracturing during fracture propagation. A decrease in the maximum horizontal principal stress reduces the driving force for fracture opening along its direction, and fracture initiation is subjected to stronger lateral constraints, leading to an increase in the equivalent tensile strength of the rock mass. Hydraulic fractures need to overcome greater stress barriers to initiate, resulting in a significant increase in initiation pressure and corresponding peak water pressure as the maximum horizontal principal stress decreases. The maximum horizontal principal stress also significantly influences fracture propagation direction, stability, and acoustic emission characteristics. When the maximum horizontal principal stress is high, fracture propagation direction is more stable, morphology tends to be straighter, connectivity improves, and acoustic emission events are mainly concentrated along the fracture path, indicating more concentrated stress release during propagation. Conversely, when the maximum horizontal principal stress is low, fracture propagation becomes unstable, morphology more complex, and acoustic emission events more dispersed, indicating unstable fracture propagation. An increase in injection rate significantly raises the initiation pressure of the specimen. Under low injection rates, fracturing fluid accumulates slowly in the borehole, allowing water molecules to infiltrate micro-fractures and pores of the rock mass, gradually weakening local rock strength and promoting stable fracture initiation. In contrast, high injection rates cause water pressure to concentrate rapidly within a short time, leading to swift stress concentration at the fracture tip, which easily exceeds the tensile strength of the rock, requiring higher water pressure to achieve fracture. Simultaneously, the intense hydraulic action induced by high-speed injection increases the instability during fracture initiation, making the fracture path more susceptible to disturbances and resulting in more complex propagation forms. Injection rate significantly affects the propagation range of hydraulic fractures, fracture complexity, and the distribution characteristics of acoustic emission location points. Under low injection rates, fracture propagation is stable, the fracture path is clear, and micro-fracturing activities are limited. At medium injection rates, fluid dynamic effects cause local fracture deflections, with acoustic emission points concentrated but showing high-density clustering in local areas, indicating influence from local stress disturbances. High injection rates lead to highly unstable fracture propagation, complex fracture networks, enhanced micro-fracturing activities, and intense energy release. This demonstrates that increasing the injection rate reduces the stability of hydraulic fracture propagation, relatively decreases the propagation range, intensifies local fracturing, and results in complex fracture structures and energy dissipation. Industrial-scale hydraulic fracturing tests were conducted, validating the fracture propagation patterns and mitigating the risk of rockburst in coal mining faces. The research results can guide segmented hydraulic fracturing operations in coal mines with similar mining geological conditions and enrich the theoretical foundation of hydraulic fracturing.

     

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