Application of a CWFS model-based brittleness index for evaluating anisotropic brittleness in terrestrial shale under triaxial stress

For shale reservoir operations, assessing the brittleness of anisotropic shales is vital of optimizing wellbore stability analysis and fracturing design. Current brittleness indices lack effectiveness in characterizing shale brittleness anisotropy. Consequently, the experimental results reported in this paper derived from triaxial tests conducted on shale core samples from the Qingshankou Formation of the Songliao Basin. This study investigates the anisotropy of shale failure patterns and mechanical properties with respect to the bedding plane dip angle (θ), and quantifies the effect of confining pressure. Building on the cohesion weakening and friction strengthening (CWFS) theory, we established a novel triaxial brittleness index (B_t). This index uniquely combines the uniaxial brittleness index (B_u), reflecting inherent brittleness, with the brittleness weakening coefficient (B_w), quantifying the effect of confining pressure. Assessment of the anisotropic brittleness of shale based on B_t under varying confining pressures reveals that B_t first increases but then decreases with increasing θ. The brittleness peaks at θ = 0° and reaches its lowest point at θ = 60°, a trend that aligns closely with the observed variations in the failure patterns of shale. Furthermore, the ability of the confining pressure to decrease shale brittleness varies with θ. At θ = 0°, the uniaxial brittleness is the highest, but the confining pressure has the strongest weakening effect on shale brittleness. In contrast, the uniaxial brittleness at θ = 90° is second only to that at 0°, but the brittleness in this direction is least affected by the confining pressure. Compared with the five existing brittleness indices, the proposed index accounts for both inherent and apparent brittleness. It is more sensitive to internal lithological characteristics and external stress conditions and has strong potential for integration with geophysical data. This study provides valuable guidance for sweet spot identification, wellbore stability assessment, and fracturing scheme optimization in shale oil and gas exploration.