Abstract:

Bedding shale is prone to forming secondary fractures along bedding weak planes during hydraulic fracturing, and the dynamic propagation behavior of these fractures plays an important role in fracture network complexity and reservoir stimulation effectiveness. However, conventional monitoring methods are unable to continuously and accurately capture the full process of secondary fracture initiation and propagation at high spatial resolution. To address this issue, an indoor experimental method for investigating the dynamic propagation of bedding-induced secondary fractures in shale was established based on optical frequency domain reflectometry (OFDR) distributed fiber-optic strain monitoring technology. Comparative constant-pressure fluid injection tests were conducted on two typical shale samples with bedding orientations parallel and perpendicular to the injection direction. The results show that OFDR technology can effectively identify the spatiotemporal evolution characteristics of strain during the initiation and propagation of bedding-induced secondary fractures in real time. Under the parallel-bedding condition, distinct bedding fractures were formed inside the sample, and the fiber-optic strain contour maps exhibited positive strain bands progressively propagating along the bedding direction, accompanied by negative strain zones around the fractures and local diffusion signals. Under the perpendicular-bedding condition, the sample was dominated by an overall negative strain response, with no obvious macroscopic fractures formed; only a weak positive strain diffusion zone appeared near the injection end. Significant differences were observed between the parallel- and perpendicular-bedding samples in terms of fracture morphology, strain field evolution, and pressure-flow response, indicating that bedding orientation exerts a strong control on the initiation and propagation of secondary fractures. These findings provide an experimental basis for the dynamic identification of secondary fractures and the analysis of their propagation mechanisms during hydraulic fracturing of bedded shale.

Key words: bedded shale, OFDR, distributed fiber optics, secondary fractures, dynamic propagation, hydraulic fracturing

摘要：

层理性页岩在水力压裂过程中易沿层理弱面形成次生裂缝，其动态响应对近井裂缝复杂化和压裂液能量分配具有重要影响。针对传统监测方法难以连续表征岩心尺度次生裂缝动态响应的问题，本文基于光频域反射计(OFDR)分布式光纤应变监测技术，建立了围压约束、恒压注液与分布式光纤应变监测相结合的室内试验方法。选取平行层理和垂直层理两类典型页岩样品，在相同围压与注入压力条件下开展对比试验，并结合压后裂缝形态、荧光示踪痕迹、压力—流量响应和光纤应变时空分布进行综合分析。结果表明：在本实验条件下，平行层理样品形成了较明显的层理裂缝，OFDR应变云图中表现为沿层理方向推进的连续正应变条带，并伴随局部负应变区和弱正应变扩散区；垂直层理样品未形成明显连续宏观裂缝，整体以负应变响应和注入端局部弱扰动为主。与垂直层理样品相比，平行层理样品表现出更强的累计吸液能力和更明显的应变响应范围。上述结果说明，OFDR分布式光纤应变监测可为层理性页岩次生裂缝动态响应识别提供有效实验手段，但表面应变信号与内部裂缝形态之间的定量对应关系仍需结合CT扫描、压后剖切或数值模拟进一步验证。

关键词: 层理性页岩, OFDR, 分布式光纤, 次生裂缝, 动态扩展, 水力压裂