煤岩脉动水力压裂过程中声发射特征的试验研究

Experimental study on acoustic emission characteristics in coal seam pulse hydraulic fracturing

利用真三轴加载脉动水力压裂试验系统和PAC声发射仪,开展不同注液频率和不同压裂液黏度条件下煤岩脉动水力压裂过程的声发射(AE)试验研究,揭示煤岩脉动水力压裂过程中声发射参数、损伤演化规律及震级-频度关系参数b的动态特征。试验结果表明:煤岩在脉动水力压裂过程中,声发射行为演化过程可划分为平静期、提速期、加速期和稳定期4个阶段,其中提速期和加速期累积能量均值较平静期分别提高40.9和52.7倍;煤岩损伤演化过程可分为初始阶段、损伤发展阶段及损伤加速发展阶段,煤岩的声发射特征能较好地描述其损伤演化特性;脉动频率较低时煤样的AE活动水平和能量释放越强,裂隙发育程度越高;当压裂液黏度较高时,煤样的AE活动水平和能量释放越弱,裂隙发育程度较低;在煤岩脉动压裂整个过程中,当脉动频率或压裂液黏度较低时,煤岩AE振幅分布变化幅度较大,且其b更大,大尺度AE事件比例更低。

Experiments on acoustic emission characteristics of the process of pulse hydraulic fracturing at different pulse frequencies and different fracturing fluid viscosities were carried out by the true tri-axial pulse hydraulic fracturing test system and the PAC AE instrument. The acoustic emission behavior evolution characteristics, the damage evolutionary rule and the dynamic feature of b were analyzed. The results show that the evolution of acoustic emission behavior stimulated by pulse hydraulic fracturing in coal bed can be divided into four phases with time, including gentle period, acceleration period, boosting period and steady period. The accumulated average energy of acceleration period and boosting period increases respectively by 40.9 and 52.7 times compared with those of the gentle period. The damage evolution process can be divided into three phases, including initial damage period, damage development period and damage accelerating development period, and AE information can fairly reflect the damage evolution character of coal. Besides, a lower frequency corresponds to stronger AE energy release and higher fracture growth level, higher fracturing fluid viscosity corresponds to a smoother AE energy release and relatively lower fracture growth level, and also the variation of AE amplitude distribution is larger, b is greater and proportion of larger AE events is smaller when the frequency or the fracturing fluid viscosity is lower during the fracturing process.