Energy response of shale during triaxial compression following thermal damage

As shale gas extraction extends into deep, high-temperature reservoirs, thermal damage induced by temperature variations has emerged as a critical scientific issue affecting reservoir stimulation and wellbore stability. In this study, the influence of temperature on the failure mode and energy evolution of shale was systematically investigated through conventional triaxial compression tests on heat-treated shale samples at different temperatures, combined with acoustic emission (AE) monitoring and energy analysis. The results show that as the temperature increases from 25 °C to 450 °C, the failure mode of shale gradually transitions from brittle fracture to brittle–ductile behavior. Concurrently, AE events shift from high-energy, abrupt signals to low-energy, dispersed activity, with the maximum AE energy and total AE energy decreasing by 68.52% and 41.22%, respectively. In addition, the heat treatment temperature significantly alters the energy allocation pathway in shale. With rising temperature, the pre-failure stage evolves from being dominated by elastic energy storage to being dominated by energy dissipation. At peak stress, both total energy and elastic energy decay exponentially with increasing temperature, whereas dissipated energy grows exponentially. Correspondingly, the proportion of elastic energy drops sharply from 73.08% to 34.60%, while that of dissipated energy increases from 26.92% to 65.40%.