Evolution law and mechanisms of permeability of organic rocks under the action of thermal-mechanical coupling

In in-situ modified fluidized mining, the use of high temperatures to treat ore deposits is an effective method. Exploring the evolution law of pyrolysis characteristics, microstructure, permeability, deformation, etc. with the temperature variation of organic mineral deposits under in-situ conditions is of great significance to the efficient exploitation of resources. Using a high-temperature and high-pressure triaxial testing machine system combined with the stress-containing micro-CT scanning technology, this study investigated the evolution law of permeability, pyrolysis gas production, microstructure and axial deformation with the temperature variation of three organic rocks (oil shale, bituminous coal and shale) under the action of thermal-mechanical coupling. The permeability evolution mechanism was also explored based on pyrolysis gas production, axial deformation and microstructure. The study found that under the action of thermal-mechanical coupling, there is a threshold temperature for the permeability evolution of organic rocks. Under the stress conditions of axial pressure of 10 MPa and confining pressure of 7 MPa, the threshold temperatures of oil shale, bituminous coal and shale are 400 °C, 450 °C and 500 °C respectively. When the pyrolysis temperature is less than the threshold temperature, the permeability of organic rocks is dominated by rock strength, the pores and cracks close as the temperature increases, and the permeability changes in a negative correlation with temperature. When the pyrolysis temperature is greater than the threshold temperature, the permeability of organic rocks is dominated by the degree of pyrolysis of the rock, the pores and cracks gradually develop and communicate with the increase of temperature, and the permeability changes in a positive correlation with the temperature. Compared with that before and after the threshold temperature, the increases of pyrolysis degree and rock strength of oil shale are the largest among three types of organic rocks. The massive loss of organic matter after pyrolysis promotes the creation and communication of pores and fissures, and the increase in rock strength weakens the compression and closing effect of stress on pores and fractures, the permeability of oil shale has been greatly improved.