Experimental Investigation of the Anisotropic Evolution of Tensile Strength of Oil Shale Under Real-Time High-Temperature Conditions

Under natural conditions, oil shale with abundant kerogen and bedding planes exhibits obvious mechanical anisotropy. High temperature affects significantly the tensile strength and microstructure of oil shale; therefore, studying the tensile strength of oil shale at different temperatures is necessary. In this study, the tensile strengths σ t-par , σ t-per , and σ t-orth in the temperature range 20–600 °C were obtained by loading parallel to bedding, perpendicular to bedding, and orthogonal to bedding, respectively, and the changes in the morphology of the oil shale at the same position were obtained as well. The results show that before the temperature reached 400 °C, the σ t-par , σ t-per , and σ t-orth decreased rapidly, except in the temperature range 100–200 °C; however, the σ t-par did not change much and the values of σ t-per and σ t-orth increased. When temperature exceeded 400 °C, the values of σ t-par and σ t-orth decreased slowly, while that of σ t-per increased rapidly. The change in surface morphology indicates that thermal action led to the closure of primary pores and fractures and the formation of new pores at less than 300 °C. When temperature exceeded 300 °C, several cracks appeared, and the form of thermal cracking was inter-granular fracture. The thermal effect also affected the fracture types of the oil shale after failure. In the temperature range 20–300 °C, the fracture type was a multiple fracture type, in which the main crack was a through central fracture. When temperature exceeded 300 °C, the central crack changed into a non-central crack with increasing temperature. These results are important to determine the fracturing parameters in the process of in situ oil shale exploitation.