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 sigma(t-par), sigma(t-per), and sigma(t-orth) in the temperature range 20-600 degrees 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 degrees C, the sigma(t-par), sigma(t-per), and sigma(t-orth) decreased rapidly, except in the temperature range 100-200 degrees C; however, the sigma(t-par) did not change much and the values of sigma(t-per) and sigma(t-orth) increased. When temperature exceeded 400 degrees C, the values of sigma(t-par) and sigma(t-orth) decreased slowly, while that of sigma(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 degrees C. When temperature exceeded 300 degrees 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 degrees C, the fracture type was a multiple fracture type, in which the main crack was a through central fracture. When temperature exceeded 300 degrees 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.

Automated summary

This study investigates the mechanical properties and morphology changes of oil shale at different temperatures through tensile experiments. It reveals that temperature has a significant effect on tensile strength and fracture types of oil shale. Results show a decrease in tensile strength below 400 degrees C, with a multiple fracture type observed in this temperature range. Above 400 degrees C, a shift in tensile strength values and fracture types is observed.