Abstract:

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 m². 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 8×10⁴ 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.

Key words: thermal treatment, granite, three-dimensional micro-structure, permeability, porosity, modified Kozeny-Carman model