High temperature can cause the
thermal fracture of rock, which affects the permeability of rock. In order to
investigate the effect of temperature on the microstructure and permeability of
rock, the ultrasonic velocity, density and gas permeability of granite after thermal treatment at 50−800 ℃ were measured. With the aid of CT
scanning technique, the microstructure of granite was extracted and
reconstructed. The influence of variation of microstructure after thermal
treatment on permeability was also discussed in detail. According to this, the
applicability of the K-C model and its improved model at high temperatures was
discussed and verified. Finally, combined with the pore fractal model, a
temperature-permeability model of granite after thermal treatment was proposed.
The results show that: 1) The change of the
internal microstructure of granite can be observed obviously as temperature
increases. Uneven thermal expansibility
of minerals before 400 ℃ can contribute to generation of many small pores
between rock particles, which constitutes the pore structure. As
temperature increases, these pores expand rapidly and connect with each other,
forming the pore-fracture network. 2) When
the temperature of heat treatment is not higher than 600 ℃, the probability
distribution curve of the shape factor of the internal granite
microstructure is basically consistent. When the temperature is higher than 600
℃, the peak value of the shape factor shifts to the left obviously, and the average shape factor decreases greatly. 3)
In the stage of pores, the permeability of granite doesn’t change too much, which is around the value of 10^{−18} m^{2}.
In the stage of the pore-fracture network, the permeability of granite
increases exponentially with the
connection of fracture.
The
permeability of granite under 800 ℃ is 8×10^{4} times of the normal
atmospheric temperature condition. 4) According to the fitting results of porosity and
permeability at 50−800 ℃ using four
porosity-permeability models, Bayles model and Costa model are more reasonable
to describe granite after thermal treatment, and the fitting results are higher
than K-C model and S-R model. 5) When the
temperature is lower than 600 ℃, there is basically no change in the shape of
the internal microstructure of the granite,
and the shape coefficient of K-C can be considered as a constant. This
indicates that Bayles model and Costa model are applicable for granite when the temperature is lower than
600 ℃. 6) On the basis of Costa model, the
temperature-permeability model of granite after thermal treatment is obtained
by combining the pore fractal characteristic of high-temperature heated
granite. The experimental data within the temperature range of 50−600 ℃ are used to verify the new model and the
coefficient of determination of fitting result reaches 0.99.