基于声发射能量分析的周期注水应力改造下煤系页岩裂缝扩展规律试验研究

周期注水作为一种有效开采页岩气的应力改造技术,其对页岩裂缝起裂扩展影响机制尚不明确。基于页岩储层真实三维应力环境,研制了真三轴水力压裂试验系统,以黑龙江双鸭山矿区煤系页岩为研究对象,制作了煤系页岩相似材料模型,进行了先周期注水应力改造、后水力压裂的页岩裂缝扩展模拟试验。利用声发射技术监测了应力改造阶段和压裂阶段裂缝起裂扩展过程,根据声发射能量变化和频谱特征,分析了应力改造和水力压裂阶段的裂缝特征、起裂扩展规律,提出了基于声发射能量分析的水力压裂裂缝由起裂阶段进入扩展阶段的判别指标和判定方法。试验结果表明:应力改造阶段裂缝尺寸以微裂缝为主,随周期注水压力增加,张拉型裂缝占比减少,剪切型裂缝占比增加;压裂阶段裂缝尺寸以宏观裂缝为主,随周期注水压力增加,张拉型裂缝占比增加,剪切型裂缝占比减少,当周期注水压力为1.6MPa时为最优,易形成缝网。提出以平均声发射能量能率k作为裂缝起裂判据,发现当k降幅超过26.87%时,裂缝由起裂阶段进入扩展阶段。周期注水应力改造可以产生微裂缝,沟通水力裂缝降低压裂时裂缝起裂难度,从而提高页岩气开采率。所得结论可为水力压裂应力改造效果评价与裂缝控制提供参考。

Experimental study on crack propagation of coal shale under stress transformation of cyclic water injection based on acoustic emission energy

Cyclic water injection can be served as a new stress transformation technology of mining shale. The influence mechanism of cyclic water injection on crack initiation and propagation of the shale is not clear. A true triaxial hydraulic fracturing testing system is developed based on true three-dimensional stress conditions of coal shale. A similar simulation model of the coal shale based on the Shuangyashan coal mine in Heilongjiang province is developed, and the cyclic water pre-flooding stress transformation and hydraulic fracturing of laboratory simulation tests are carried out. The acoustic emission technology is used to monitor the whole process of crack initiation and propagation in stress transformation stage and the fracturing stage. Based on the energy variation of the acoustic emission and the spectrum characteristics, the effect of the water injection pressure on crack size, the degree of crack development and the crack type of stress transformation stage are analyzed, and a discrimination method and the parameter index of hydraulic fracturing from the initiation stage to the propagation stage are presented. The experimental results show that the size of the cracks in the stress transformation stage is mainly determined by micro-cracks. As the water injection pressure increases, the proportion of tension cracks decreases and that of shear cracks increases. The size of the cracks in the fracturing stage is mainly determined by macro-cracks. As the water injection pressure increases, the proportion of shear cracks decreases and that of tension cracks increases. The optimized situation happens when the water injection pressure is about 1.6 MPa, in which cracks can be easily formed. The average of the acoustic emission energy rate k is used to determine the crack initiation. When the decreasing amplitude of k is more than 26.87%, the cracks change from the initiation stage into the propagation stage. Cycle water injection for stress transformation can produce micro-cracks. The difficulty of initiation decreases with stress transformation, so the mining rate of the shale gas will increase. The conclusions provide an important reference for design of volume fracturing transformation.