Determining the crack initiation stress (Ci) for unconventional shale rocks is of critical importance in describing the entire failure process of unconventional shale reservoirs. We propose a new method to identify Ci values based on triaxial failure tests on four organic shale samples, attempting to improve the shortcomings of other methods. The new method is based on the relationship between crack development and strain energy evolution (SEE). Additionally, the proposed SEE method is compared with three widely used methods, including crack volumetric strain (CVS), moving point regression (MPR) and the lateral strain response (LSR), intending to examine the performance of different methods. The contrastive results indicate that the LSR method cannot determine Ci when the rock ruptures without volumetric dilatancy, which frequently occurs in the compression process of organic shales. Ci values obtained using the SEE method are consistent with those from the CVS and MPR methods. However, the proposed SEE method with a solid physical basis is more objective and stable than the CVS and MPR methods. The proposed method, from one aspect, compensates for the shortcomings of other methods when facing different failure modes in organic shales. From the other aspect, it provides a way to precisely determine Ci values for applications in wellbore stability evaluation and hydraulic fracturing design.

中文翻译：

---

基于应变能演化确定非常规页岩中的裂纹起始应力

确定非常规页岩的起裂应力（Ci）对于描述非常规页岩储层的整个破坏过程至关重要。我们提出了一种基于四个有机页岩样品的三轴破坏试验来识别 Ci 值的新方法，试图改善其他方法的缺点。新方法基于裂纹发展和应变能演化 (SEE) 之间的关系。此外，将所提出的 SEE 方法与三种广泛使用的方法进行了比较，包括裂纹体积应变 (CVS)、移动点回归 (MPR) 和横向应变响应 (LSR)，旨在检验不同方法的性能。对比结果表明，当岩石在没有体积剪胀的情况下破裂时，LSR方法无法确定Ci，这在有机质页岩的压缩过程中经常发生。使用 SEE 方法获得的 Ci 值与从 CVS 和 MPR 方法获得的值一致。然而，所提出的具有坚实物理基础的 SEE 方法比 CVS 和 MPR 方法更客观和稳定。该方法从一个方面弥补了其他方法在面对有机页岩不同破坏模式时的不足。另一方面，它为井筒稳定性评价和水力压裂设计中的应用提供了一种精确确定Ci值的方法。一方面弥补了其他方法在面对有机质页岩不同破坏模式时的不足。另一方面，它为井筒稳定性评价和水力压裂设计中的应用提供了一种精确确定Ci值的方法。一方面弥补了其他方法在面对有机质页岩不同破坏模式时的不足。另一方面，它为井筒稳定性评价和水力压裂设计中的应用提供了一种精确确定Ci值的方法。