Study on the damage characteristics and damage model of organic rock oil shale under the temperature effect

Abstract

Thermal damage is of great importance to oil shale in situ mining. In this study, the relationship between the P-wave velocity, peak strength, and elastic modulus of samples from Nong’an County, Jilin Province, China, and temperature is analysed (at temperatures of 25–700 °C), and damage factors are defined to evaluate the damage degree. The effect of organic matter pyrolysis on the damage degree is investigated, and thermal damage is explained at the microscale according to scanning electron microscopy (SEM) images. Under the assumption that the strength of a micro-element obeys the power function distribution and micro-element failure follows the Mohr-Coulomb (M-C) strength criterion, a thermal damage constitutive model is established. It is found that when the temperature threshold is exceeded, organic matter pyrolysis becomes the main driver of the increase in thermal damage. The thermal damage rapidly increases, which is notably different from the damage process below the temperature threshold. This difference is clearly reflected in the physical and mechanical properties, damage factors, microstructure, and determined model parameters. The relationship between the mass loss rate, which can directly reflect the pyrolysis degree, and damage degree is notable. The research results have a certain theoretical significance that can help to better understand the thermal damage process of oil shale and the thermal damage difference between organic and inorganic rocks and can provide a reference for relevant engineering practices.