Thermal fracture behavior and strength evolution of oil shale under high-temperature steam treatment: A dual-stage analysis of mechanical response

Abstract

This study investigates the mechanical and fracturing behavior of oil shale (OS) under high-temperature steam to understand the stability of surrounding rock in main fracture channels and the mechanisms of hardening and enhanced pyrolysis. Using Balikun OS, experiments were performed with a high-temperature uniaxial testing machine, examining mechanical properties under different bedding orientations in steam conditions. Combined with CT scanning and microcomponent analysis, the evolution of pore-fracture structures and their influence on mechanical properties were comprehensively studied. Results show significant strength softening of OS in range of 25℃ to 400℃, while strength recovery occurs between 400℃ and 550℃. A deterioration index reveals 400℃ as the critical temperature where softening transitions to hardening. Microscopic analysis shows that fracture volume and porosity increase with temperature, following a “rapid growth–slow growth–exponential rise” trend. At the same temperature, oil shale with parallel bedding forms complex pore networks, while perpendicular bedding samples exhibit more dispersed fractures. Below 400℃, increased porosity and fracture network expansion lead to softening, with parallel bedding samples showing greater damage. Above 400℃, matrix strengthening due to pyrite decomposition and lattice changes in kaolinite and chlorite drive strength recovery. The mechanical response of oil shale transitions from “high strength-low ductility” to “low strength-high ductility” as the temperature increases, and gradually begins to recover its “high strength-low ductility” characteristics around 400℃. From 400℃ to 550℃, friction between fractures and the matrix weakens, and pore pressure from pyrolysis gases induces brittle failure. Engineering applications suggest multi-stage high-temperature steam fracturing with proppants to avoid the “low strength-high ductility” stage in main fracture channels, enhancing stability and enabling efficient pyrolysis extraction.