Abstract The effect of confinement pressure on fracture toughness may be significant in materials with poor tensile strength, as is the case of most rocks. In fracking wells for hydrocarbon exploitation, rock fracture is induced by a pressurized fluid. Indirect estimates of rock toughness based on injected pressure and volume records are much larger than laboratory-measured toughness values. This article discusses the design, fabrication and use of a linear elastic fracture mechanics method to assess the critical stress intensity factor (KI) of rocks under real well bottom conditions. Apparent Fracture Toughness (KIf) is determined using notched 1.5″ plugs machined from rocks. In this novel experimental set up, KI is applied by means of a hydraulic pressure inside the crack, so that fracturing fluid chemistry can be modified in order to optimize fracability. A secondary hydraulic system ensures variable confinement pressures, up to 80 MPa. Preliminary tests carried out in shale rocks at well bottom pressures show that rock toughness more than doubles those found at atmospheric pressure. It is shown that increasing triaxial pressure confinement allows to accurately model the conditions of rocks in oil & gas reservoir conditions.

中文翻译：

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油气藏条件下岩石断裂韧性测试装置

摘要 约束压力对断裂韧性的影响对于抗拉强度差的材料可能是显着的，大多数岩石就是这种情况。在用于油气开采的压裂井中，岩石破裂是由加压流体引起的。基于注入压力和体积记录的岩石韧性的间接估计比实验室测量的韧性值大得多。本文讨论了线弹性断裂力学方法的设计、制造和使用，以评估真实井底条件下岩石的临界应力强度因子 (KI)。表观断裂韧性 (KIf) 使用由岩石加工而成的带缺口的 1.5 英寸塞子确定。在这个新颖的实验装置中，KI 是通过裂缝内的液压施加的，从而可以修改压裂液的化学成分以优化可压裂性。辅助液压系统可确保可变的限制压力，最高可达 80 MPa。在井底压力下对页岩岩进行的初步测试表明，岩石韧性是在大气压下发现的韧性的两倍多。结果表明，增加三轴压力限制可以准确地模拟油气藏条件下的岩石条件。