三轴加载下煤岩脉冲水力压裂扩缝机制研究

Mechanisms of fracture extending in coal rock by pulse hydraulic fracturing under triaxial loading

为了研究煤岩脉冲水力压裂扩缝机制,利用三轴加载脉冲水力压裂试验系统,分别从改变脉冲频率和压裂液黏度两个方面对煤岩进行了水力压裂室内试验。试验结果表明:脉冲压力和声发射对压裂过程的响应情况可将煤岩脉冲水力压裂扩缝过程分为零散萌生、均匀扩展、突变贯通和破裂终止4个阶段,其中声发射定位点空间展布的平均速率在均匀扩展和突变贯通阶段较零散萌生阶段分别提高了4.6倍和9.6倍;声发射b值曲线的趋势显示,均匀扩展阶段煤岩内部以小尺度的微裂纹破裂为主,突变扩展阶段煤岩内部出现了较大尺度主裂缝扩展,并且部分b值曲线呈现出山脊线状与阶梯状相结合的特点;在达到最佳压裂脉冲频率前,高频脉冲压力作用的煤岩,扩缝过程的均匀扩展阶段和突变贯穿阶段的延续时间比低频对应的延续时间短;随着压裂液黏度增高,扩缝过程均匀扩展阶段的延续时间呈缩短趋势,但突变贯穿阶段的延续时间却有增长的趋势。

This study is to investigate the mechanisms of fracture extending in coal rock by pulse hydraulic fracturing. Experiments are performed on coal rock samples using a pulse hydraulic fracturing experimental system under triaxial loading by changing the pulse frequency and viscosity of fracturing fluid. Experimental results show that the fracturing response of pulse pressure and acoustic emission （AE） indicates the process of fracture extending induced by pulse hydraulic fracturing can be divided into four periods, i.e., scattered initiation period, uniform growth period, sudden coalescence period and failure termination period. The average rates of the spatial distribution of AE locations at the uniform growth period and sudden coalescence period are 4.6 and 9.6 times higher than the scattered initiation period respectively. The general tendency of b-value curves demonstrates that microcracks mainly propagate at the uniform growth period and large scale cracks primarily propagate at the sudden coalescence period. The partial b-value curves present the characteristics by combining the shape of ridge line with ladder shape. Before tile fracturing pulse frequency reaches the optimum state, the duration of uniform growth period and sudden coalescence period affected by high frequency pulse pressure is shorter than that by low frequency one. Under the same frequency, the duration of uniform growth period exhibits a shortening trend, as the viscosity of fracturing fluid increases, but the duration of sudden coalescence period shows the opposite way.