A FRACTAL PERSPECTIVE ON STRUCTURAL DAMAGE AND FRACTURE CHARACTERISTICS OF COAL SUBJECTED TO LIQUID NITROGEN COOLING AT LABORATORY-SCALE

Abstract

To explore the influence of liquid nitrogen (LN[Formula: see text] cooling on the structural damage and failure characteristics of coal, a series of structural experiments, uniaxial compression tests, and crack morphology characterization experiments were carried out in this study. Based on the fractal theory, the pore structure damage and failure characteristics of the coal before and after the LN2 cooling treatment were compared. After the LN2 treatment, the structural damage inside the coal increases. Compared to the untreated coal, the fractal dimensions of the total pores, seepage pores, and adsorption pores in the LN2-treated coal decrease by 0.88%, 0.24%, and 5.16%, respectively. The results indicate that the LN2 cooling treatment can increase the connectivity of pores, especially adsorption pores. The internal structure damage of the coal induced by the LN2 treatment reduces its compressive strength and elastic modulus by 27.86% and 32.62%, respectively. Simultaneously, it can be found that the fragments fractal dimension of the coal decreases by 9.53%, and the fractal dimension of the induced fracture surface increases from 2.134 to 2.151 after the LN2 treatment. By the LN2 treatment, the distribution of fragments becomes more uniform, and the complexity of the generated fracture surface increases. Therefore, the LN2 cooling treatment can increase the effective seepage path and the contact area between the crack and the coal matrix to promote the gas flow and enhance the gas desorption in the coal matrix. The experimental results also show that the damage of the single LN2 treatment on the coal is limited. LN2 cycling treatment is suggested to use to improve the efficiency of the coalbed methane extraction because that it can give full play to the multiple cracking effects of LN2 on the coal.