Determining crack initiation stress in unconventional shales based on strain energy evolution

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.