Evolution of Pore and Fracture Structure of Oil Shale under High Temperature and High Pressure

In order to study the coupled effect of the temperature and pressure on pyrolysis characteristics and pore and fracture structures of oil shale, a total of 25 groups of pyrolytic reaction experiments have been conducted on 14 mm long and 7 mm in diameter cylindrical oil shale specimens under different temperature and pressure conditions ranging from 20 to 600 degrees C and 0.1-15 MPa. Further, both X-ray microcomputed tomography (mu CT) and mercury intrusion porosimetry (MIP) have been used to comprehensively investigate the network structure, interconnectivity, and evolution of pore and fractures. The results show that the temperature significantly affects the pyrolysis characteristics of oil shale. With rising temperature, both the mass loss and the porosity increase gradually, the number and the maximum aperture of fractures also increase, and the pyrolytic degree intensifies progressively. The increase is most significant from 300 to 500 degrees C. The maximum mass loss ratio is 20.84%, the largest porosity is 13.52 times larger than that under the room temperature, and the total number and the maximum aperture of the fractures are 813 and 0.383 mm, respectively. Moreover, the pressure has a significant effect on the pore and fracture structures of oil shale. As the pressure increases, both the pore volume and the fracture distributions first decreased and then increased. With the continuous increase of pressure, the porosity and the total number of fractures reach a maximum-at the pressure of 15 MPa. Under the coupled effect of temperature and pressure, with both the temperature and pressure increasing, the pores and fractures in the oil shale specimens developed increasingly. Furthermore, using the mu CT scan technology, the distribution laws and the connectivity characteristics of the pores and fractures have been investigated. The connected fractures appear when the temperature reaches 300 degrees C and further extend along the bedding plane or pass through it at 600 degrees C.