Deep/ultra-deep shale strength criterion: A case study of southern Sichuan Basin shale

Abstract

Deep/ultra-deep shale gas reservoirs are a vital area for future shale gas development, which mainly relies on the stimulation of network fracturing. In such a reservoir, engineering technologies such as wellbore stability evaluation and network fracturing design are all closely related to the shale mechanical characteristics. Currently, most studies focus on the elastic-plastic, brittle-ductile properties of shale, which are characterized based on shale strength. However, there is still a lack of in-depth research on their failure strength and strength criteria specifically for deep/ultra-deep shale. To accurately characterize shale failure strength, numerous scholars have researched shale strength criteria. Nevertheless, due to the high geo-stress of deep/ultra-deep reservoirs, shale properties tend to transition from brittle to ductile, resulting in poor fitting of classical rock strength criteria to test data under high confining stresses. This study focused on the outcrops of the Longmaxi Formation shale, and high-confining-stress triaxial compression tests were conducted to investigate the strength variation characteristics of shale. Based on the Mohr-Coulomb criterion, a power-form MC criterion was proposed by introducing a power function of confining stress as a correction term, and its reliability, sensitivity, and applicability were evaluated. The results indicate that the non-linear characteristics of the power-form MC criterion curve better match the actual situation, allowing for a more accurate prediction of shale strength under high confining stress. Furthermore, the power-form MC criterion has low sensitivity to changes in confining stresses, indicating that the required amount of test data for fitting can be appropriately reduced during application. Additionally, the power-form MC criterion is applicable to some extent to other types of high-strength rocks buried at greater depths. The results of this study contribute to understanding the strength characteristics of deep/ultra-deep shale under high confining stress, providing important references and theoretical basis for drilling, completion, and stimulation practices in deep shale gas reservoirs.