基于巴西劈裂试验的页岩声发射与能量分布特征研究

Acoustic emission characteristics and the energy distribution of the shale in Brazilian splitting testing

  • 摘要: 为探究不同角度层理下页岩力学特性和声发射特征,通过对不同角度层理龙马溪组页岩的巴西劈裂试验、声发射测试以及3DEC数值模拟研究,对页岩破坏模式进行了对比分析。研究结果表明:由于页岩具有明显的层理构造,其抗拉强度具有明显的各向异性。页岩巴西劈裂应力-时间曲线可划分为3个阶段,对低角度层理,受载的累计声发射曲线表现为“平缓—陡增”增长模式;对高层理角度,累计声发射曲线表现为“台阶式”增长模式。研究物理实验与3DEC数值模拟结果均表明页岩破裂模式与层理角度密切相关,页岩声发射能量概率密度P(E)均较好的满足幂定律,层理效应并不改变声发射能量的尺度不变性。层理角度为90°时,能量概率密度曲线的临界参数最大,在低能量区间的声发射信号所占比例更大,利用临界参数可以对巴西劈裂试验过程中页岩损伤程度进行表征。研究成果可对页岩气压裂工程的设计提供借鉴。

     

    Abstract: To explore the mechanical properties and the acoustic emission(AE)characteristics of shale with different bedding angles, the failure mode of shale with different bedding angles was analyzed by using Brazilian splitting testing, AE technique and 3DEC numerical simulation.The results show that the tensile strength of shale exhibits obvious anisotropic behavior due to the obvious bedding structures in shale.The stress-time curves obtained from Brazilian splitting testing can be divided into three main stages.At the lower bedding angles, the cumulative AE events-time curves exhibit a flat to sharply rising trend; while at the higher bedding angles, the cumulative AE events-time curves exhibit "multiple steps" characteristics.The failure modes of shales are closely related to the bedding angle, and the failure patterns obtained from the Brazilian splitting tests are consistent with the results of 3DEC numerical simulation.The AE energy probability density P(E) distribution curves of all shale specimens were fitted well with the power law function.Bedding angles in shale did not change the scale invariance of AE energy in the process of Brazilian splitting testing.The critical parameters of the AE energy probability density distribution curve at bedding angle of 90° is the largest, and the proportion of AE signals is getting larger at the low energy range.The critical parameters of AE energy probability density distribution curve can be used to characterize the damage degree of the shale in the process of Brazilian splitting testing.The results are of some significance for the reservoir stimulation design in shale gas development.

     

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