In this work, thermal cracking inside an oil shale sample under different temperatures was tested using a micro-CT system. The critical temperature was determined at which sharp thermal cracking in the oil shale sample occurs. It was found that the critical temperature of Chinese Fushun oil shale is 350 °C. After reaching 350 °C, all sites of the sample reveal fissures whose number, length and width increase dramatically, resulting in the formation of an enormous network of fissures. A high-temperature rock-permeability testing machine was used to test the changes in permeability at different temperatures. The results similar to those of thermal cracking were obtained. Therefore, thermal cracking can be regarded as the decisive factor to affect the changes of permeability in oil shale particle.
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 °C and 0.1–15 MPa. Further, both
X-ray microcomputed tomography (μCT) 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 °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 μCT
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 °C and further extend
along the bedding plane or pass through it at 600 °C.
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