压裂是油气、地热等流态矿产储层增渗改造以及井工矿围岩控制的重要手段，缝尖断裂过程区（微裂缝区）及裂缝面两侧微裂缝带的发育使压裂裂缝呈非线性断裂，因此，刻画压裂裂缝的非线性断裂是深化压裂理论与有效监测控制裂缝扩展的基础。以岩石中形成（微）裂缝会释放声发射（弹性波）的物理机制为切入点，自主开发了岩石非线性断裂的声发射全波形多参量监测分析程序，优化了高质量波形拾取、波形同步与起振时间计算以及定位算法等系列环节，较现有商业声发射定位程序，定位结果与缝面形态的一致性提升，声发射事件空间分布的离散性降低，进而可通过声发射特征参数的空间展布刻画压裂裂缝断裂过程区与微裂带内能量耗散、损伤程度、多尺寸破裂及拉-剪-压缩（塌陷）断裂机制等空间演化特征，丰富压裂裂缝非线性断裂特性的表征参量。选用四川白砂岩开展了声发射监测下的真三轴压裂物理模拟试验，通过声发射全波形多参量分析方法刻画了压裂裂缝的非线性断裂特征：① 通过声发射能量空间分布识别了断裂过程区（31 mm长、12 mm宽）与水力微裂缝带，在压裂裂缝横截面呈条带状的断裂过程区具有非等向发育的特征，断裂过程区是压裂裂缝的优势扩展路径，宏观裂缝面在断裂过程区内部形成并近乎失稳扩展。耗散能在断裂过程区与微裂缝带宽度方向呈“中高两低”的对称分布，累积耗散能沿压裂裂缝扩展方向呈线性递减分布，线性相关系数达0.9。② 声发射波速越低表明震源周围损伤程度越高，在断裂过程区与微裂缝带宽度与长度方向，波速均呈起裂点低外侧高的分布特征，表明微裂缝带内损伤程度由起裂点向外侧递减。③ 在压裂裂缝扩展不同阶段，断裂过程区与水力微裂缝带内的声发射频率呈高低频交错的随机分布特征，表明断裂过程区发育与宏观裂缝的扩展均服从微裂缝萌生与融合同时进行的演化规律。④ 声发射断裂机制揭示了平面水力裂缝起裂与扩展均以拉张微破裂为主（51.7%～65.3%）的断裂特征，随压裂裂缝扩展，拉张微破裂占比由65.3%降低至51.7%，而剪切微破裂占比由19.3%升高至234%，可推断裂缝扩展对裂缝周围岩石压剪效应增强，增渗性提高。

Hydraulic fracturing is an effective approach in geo-energy extraction,such as enhancing the reservoir per-[LM]meability of the flow mineral resource and controlling the surrounding rock in the mine.Due to the development of the fracture process zone(FPZ; microcrack zone) at the fracture tip and the hydraulic microcrack band(HMB) at both sides of the fracture surface,hydraulic fracture(HF) extension presents remarkable nonlinear fracturing characteristics.Therefore,characterizing the HF nonlinear fracturing is fundamental to deepening HF nonlinear fracturing theory and controlling HF propagation.Due to the physical mechanism that cracks generation releases elastic waves(i.e.,the acoustic emission(AE)),AE can be employed to characterize the nonlinear fracturing of hydraulic fracture.In this work,the AE full-waveform and multi-parameter analysis method was proposed,optimizing nonlinear fracturing characterization with a newly developed analysis program.The newly developed program involves high-quality waveform pick-up,waveform synchronization and vibration time calculation,and AE source positioning algorithm.Compared with the widely used commercial AE positioning program,the distribution of AE events obtained by the newly developed program is more consistent with the real fracture surface.Besides,the distribution dispersion of AE events is reduced.Then the spatial distribution of AE characteristic parameters can characterize the evolution of dissipated energy,damage,multi-size fracturing and tension-shear-collapsed fracture mechanism in FPZ and HMB,enriching the characterization parameters of the nonlinear fracture characteristics.To validate the AE full-waveform and multi-parameter analysis method,the true-triaxial fracturing experiment was conducted on the Sichuan white sandstone.AE was employed to precisely characterize the HF nonlinear fracturing.Several new findings were obtained.① The FPZ(31 mm long and 12 mm wide) and HMB are identified via the spatial distribution of AE energy.The strip-shaped FPZ on the HF cross-section presents non-isotropic development.FPZ is the HF dominant propagation path,and the real fracture surface is formed inside the FPZ and almost propagates unsteadily.The dissipated energy is symmetrically distributed along the width of FPZ and HMB,with a distribution pattern of high in the middle and low on both sides.The cumulative dissipated energy is linearly decreasing along the fracture propagation direction,with a linear correlation coefficient of 0.9.② The low AE wave velocity represents the high damage degree surrounding the seismic source.Along both the width and length of the FPZ and HMB,the AE wave velocity is low surrounding the fracture initiation point and high outside.This phenomenon indicates that damage degree in FPZ and HMB decreases from the fracture initiation point to the outside.③ The AE frequencies in FPZ and HMB presents high and low random distribution characteristics.These results indicate that both the FPZ development and the HMB extension obey coalescence and nucleation of microcracks at the same time.④ AE fracture mechanism reveals that micro-fractures are mainly tensile(51.7%-65.3%) during the initiation and propagation of plane hydraulic fractures.With the HF propagation,the proportion of tensile micro-fractures decreases from 65.3% to 51.7%.In contrast,the proportion of shear micro-fracture increases from 19.3% to 23.4%.We can infer that the compressive-shear effect will be enhanced surrounding the HF.Then the permeability surrounding the HF will be improved due to the increasing proportion of shear micro-fractures.

1 声发射刻画压裂裂缝非线性断裂特征的物理机制

2 刻画岩石非线性断裂的全波形多参量分析方法

   2.1 基于全波形分析的声发射定位方法

   2.2 声发射特征参数与非线性断裂特征的相关关系

3 压裂物理模拟的声发射监测试验

   3.1 试样准备、实验系统

   3.2 试验方案

   3.3 压裂缝几何形态与声发射定位结果

4 基于声发射特征参数空间演化的压裂裂缝非线性断裂特征

   4.1 声发射能量——裂缝扩展的能量耗散演化特征

   4.2 声发射波速——震源周围损伤演化评价

   4.3 声发射频率——微裂缝区破裂尺寸演化评价

   4.4 声发射断裂机制——拉张、剪切、压缩(塌陷)型微破裂演化

5 结论