Studies on mechanical properties and failure characteristics of anisotropic shale under true triaxial loading at real-time high temperature

Abstract

The efficient extraction of deep shale gas (burial depth > 3500 m) in challenging high-temperature and high-stress environments plays a pivotal role in advancing natural gas development. This study investigates how real-time high temperatures and bedding plane inclinations (BPI) affect the mechanical properties of shale, including strength, deformation, and brittleness, under true triaxial loading conditions. Experiments on Longmaxi Formation shale reveal that the true triaxial compressive strength (TCS) and elastic modulus (TEM) exhibit significant temperature thresholds between 120 ℃ and 160 ℃, attributed to the dissipation of adsorbed water in clay minerals speculatively. Failure modes are significantly affected by temperature and bedding plane inclination (BPI). Notably, for samples with a 45° BPI, fracture strike changes occur at higher temperatures (160 ℃ and 200 ℃) due to reduced shear strength along bedding planes. Numerical simulations confirm that this is driven by differing thermal expansion coefficients between bedding planes and the matrix. Additionally, brittleness, evaluated through stress–strain based indices, is affected by temperature, BPI, and loading direction relative to bedding planes. These findings significantly advance the understanding of anisotropic shale behavior under high-temperature conditions, offering insights for optimizing engineering strategies in deep shale gas extraction.