Abstract Six hydraulic fracturing (HF) experiments were conducted in situ at the Grimsel Test Site (GTS), Switzerland, using two boreholes drilled in sparsely fractured crystalline rock. High spatial and temporal resolution monitoring of fracture fluid pressure and strain improve our understanding of fracturing dynamics during and directly following high-pressure fluid injection. In three out of the six experiments, a shear-thinning fluid with an initial static viscosity approximately 30 times higher than water was used to understand the importance of fracture leak-off better. Diagnostic analyses of the shut-in phases were used to determine the minimum principal stress magnitude for the fracture closure cycles, yielding an estimate of the effective instantaneous shut-in pressure (effective ISIP) 4.49 ± 0.22 MPa. The jacking pressure of the hydraulic fracture was measured during the pressure-controlled step-test. A new method was developed using the uniaxial Fibre-Bragg Grating strain signals to estimate the jacking pressure, which agrees with the traditional flow versus pressure method. The technique has the advantage of observing the behavior of natural fractures next to the injection interval. The experiments can be divided into two groups depending on the injection location (i.e., South or North to a brittle-ductile S3 shear zone). The experiments executed South of this zone have a jacking pressure above the effective ISIP. The proximity to the S3 shear zone and the complex geological structure led to near-wellbore tortuosity and heterogeneous stress effects masking the jacking pressure. In comparison, the experiments North of the S3 shear zone has a jacking pressure below the effective ISIP. This is an effect related to shear dislocation and fracture opening. Both processes can occur almost synchronously and provide new insights into the complicated mixed-mode deformation processes triggered by high-pressure injection.

中文翻译：

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结晶岩原位水力压裂裂缝开合的流体力学洞察

摘要 在瑞士格里姆塞尔试验场 (GTS) 进行了六次水力压裂 (HF) 实验，使用在稀疏裂缝结晶岩中钻出的两个钻孔。对压裂流体压力和应变的高时空分辨率监测提高了我们对高压流体注入期间和之后的压裂动力学的理解。在六个实验中的三个实验中，使用初始静态粘度比水高约 30 倍的剪切稀化流体来更好地理解裂缝渗漏的重要性。关井阶段的诊断分析用于确定裂缝闭合循环的最小主应力大小，得出有效瞬时关井压力（有效 ISIP）4.49 ± 0.22 MPa 的估计值。在压力控制阶跃试验期间测量水力压裂的顶进压力。开发了一种使用单轴光纤布拉格光栅应变信号来估计顶升压力的新方法，该方法与传统的流量与压力方法一致。该技术的优点是可以观察注入间隔附近天然裂缝的行为。根据注入位置（即，向南或向北到脆性-韧性 S3 剪切带），实验可分为两组。在该区域以南执行的实验具有高于有效 ISIP 的顶升压力。靠近 S3 剪切带和复杂的地质结构导致近井筒弯曲和不均匀应力效应掩盖了顶升压力。相比下，S3 剪切带以北的实验具有低于有效 ISIP 的顶升压力。这是与剪切位错和裂缝张开相关的效应。这两个过程几乎可以同时发生，并为高压注射引发的复杂混合模式变形过程提供了新的见解。