Fracture toughness property is often ignored but has a significant impact on the hydraulic fracture design, i.e. length and height. It represents the resistance of the rock to hydraulic fracturing and is a direct indicator of rock fracability. It is difficult to measure fracture toughness in shales using current experimental procedures designed for conventional rocks largely due to mechanical and chemical instability in shales. This study measures fracture toughness in shales using nano-indentation. Nano-indentation can, in fact, be used to derive both tensile and shear dominated fracture toughness. The observations in the study show that while crack-length method derived fracture toughness is close to Mode I (tensile) fracture toughness, energy-method based fracture toughness is closer to Mode II (shear) fracture toughness.

Fracture toughness was calculated for 67 samples in different shale formations namely Marcellus, Wolfcamp, Woodford and Eagle Ford. The results show that mineralogy had a strong control on fracture toughness. Fracture toughness decreases with increasing porosity, TOC (Total Organic Carbon) and clay content. The fracture toughness also shows strong anisotropy with fracture toughness measured parallel to the bedding planes being generally 33% higher than the corresponding values measured normal to the bedding planes. The fracture toughness shows a direct correlation with other mechanical properties like Young's modulus, hardness and brittleness. Typically, samples with fracture toughness values greater than 3 MPa m^0.5 were found to be brittle based on Young's modulus and Poisson's ratio cross plots. Evaluation of fluid effects on fracture toughness shows that it decreases by 10–37% due to spontaneous imbibition with low salinity brines. This could help understand the impact of hydraulic fracturing fluid on fracture propagation and stimulated reservoir volume (SRV) creation.

断裂韧性通常被忽略，但对水力压裂设计（即长度和高度）产生重大影响。它代表了岩石对水力压裂的抵抗力，是岩石易碎性的直接指标。在很大程度上，由于页岩的机械和化学不稳定性，使用针对常规岩石设计的当前实验程序很难测量页岩的断裂韧性。这项研究使用纳米压痕技术测量了页岩的断裂韧性。实际上，纳米压痕可用于导出拉伸和剪切为主的断裂韧性。研究中的观察结果表明，虽然裂缝长度法得出的断裂韧度接近于I型（拉伸）断裂韧度，但基于能量法的断裂韧度却更接近II型（剪切）断裂韧度。

计算了不同页岩地层中的67个样品的断裂韧度，即Marcellus，Wolfcamp，Woodford和Eagle Ford。结果表明，矿物学对断裂韧性有很强的控制作用。断裂韧性随着孔隙率，TOC（总有机碳）和粘土含量的增加而降低。断裂韧性还表现出强的各向异性，平行于顺层平面测得的断裂韧性通常比垂直于顺层平面测得的相应值高33％。断裂韧性与其他机械性能如杨氏模量，硬度和脆性直接相关。通常，断裂韧性值大于3 MPa m ^0.5的样品根据杨氏模量和泊松比交叉图，发现它们是脆性的。流体对断裂韧性的影响评估表明，由于低盐度盐水的自发吸收，流体的断裂韧性降低了10-37％。这可能有助于了解水力压裂液对裂缝扩展和增产油藏（SRV）的影响。