Abstract The effect of penetrating and non-penetrating fluids, and of opening shape, on hydraulic fracturing has been extensively studied, both theoretically and experimentally. These factors are important because of their effect on fracture initiation and breakdown pressure. This paper revisits these problems, but with a novel experimental procedure that allows one to see details of fluid penetration and fracturing. Prismatic gypsum specimens were tested under external biaxial loads and with hydraulically pressurized openings. Unlike previous experimental research, that used different materials, fluid viscosities and/or pressurization rates to vary the fluid penetration, this study keeps these testing conditions constant and only modifies the permeability of the material around the opening by sealing or not sealing its interior faces with wax. Most importantly, while previously fluid penetration and fracture initiation and propagation patterns could only be visually observed after completing the experiments, the imaging techniques employed in this paper enable us to see the entire fracturing process in detail through high-speed video. Four types of geometries were tested, namely a 5 mm circular flaw, a single vertical ovaloid flaw, and 5 mm circular openings with short and long notches (2.5 mm and 5 mm length, respectively). The first two shapes were analyzed with both, penetrating and non-penetrating fluids. The results confirm that fluid penetration lowers breakdown pressure, does not affect the fracturing pattern, and influences the difference between fracture initiation and breakdown. Also, the high-speed imaging makes it possible to see a distinct fracturing behavior (stable, unstable) within the sealed and not sealed zones. The main contribution is that these effects are not only inferred but are clearly observed during the tests.

中文翻译：

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水力压裂流体渗透及开孔几何实验研究

摘要 渗透流体和非渗透流体以及开口形状对水力压裂的影响已在理论和实验上得到广泛研究。这些因素很重要，因为它们对断裂起始和破裂压力有影响。本文重新审视了这些问题，但采用了一种新颖的实验程序，可以让人们看到流体渗透和压裂的细节。棱柱状石膏试样在外部双轴载荷和液压加压开口下进行测试。与之前使用不同材料、流体粘度和/或加压速率来改变流体渗透的实验研究不同，本研究保持这些测试条件不变，并且仅通过密封或不密封其内表面来改变开口周围材料的渗透性蜡。最重要的是，虽然以前的流体渗透和裂缝起裂和扩展模式只能在完成实验后才能直观地观察到，但本文采用的成像技术使我们能够通过高速视频详细了解整个压裂过程。测试了四种类型的几何形状，即 5 毫米圆形缺陷、单个垂直椭圆形缺陷和 5 毫米具有短和长切口（分别为 2.5 毫米和 5 毫米长）的圆形开口。前两种形状用渗透流体和非渗透流体进行分析。结果证实，流体渗透降低了破裂压力，不影响压裂模式，并影响裂缝起裂和破裂之间的差异。此外，高速成像可以看到明显的压裂行为（稳定、不稳定）在密封和非密封区域内。主要贡献是这些影响不仅可以推断出来，而且在测试过程中可以清楚地观察到。