Deformation-Failure Characteristics of Coal During Liquid Co2 Cryogenic-Freezing Process: An Experimental and Digital Study

Abstract

Liquid CO2 (LCO2) fracturing technology is regarded as one of the anhydrous technics that can enhance coal permeability and support coalbed methane exploitation. In this paper, X-ray CT scanning technology is adopted to measure the microscopic-morphological parameters of coal with different metamorphic degree. Drawing the support from image processing and three-dimensional (3D) visualization functions of Avizo software, 3D spatial structure variation rules, as well as the deformation and permeability parameters are quantitatively calculated. Under the damage effect on LCO2 cryogenic-freezing, the macroscopic mechanical properties and deformation-failure degree of coal are thorough analyzed. The results show that fracture scale parameters of treated coal are significantly increased, resulting the spatial structure parameters including coal pillars total volume (Vt), fracture network volume (V0) and the proportion of fracture network (μ0) are increased clearly. A comparison analysis indicates that the coverage area of single value function from the percolation theoretical model for treated coal pillars become larger, and its percolation curves are more intensive, the quantitative coal permeability coefficients are increased to more than 40% on average, which further prove the permeability of coal by LCO2 cryogenic-freezing are significantly improved. Under the same uniaxial stress loading rate, the peak stress threshold value required by treated coal in the compaction and elastoplastic deformation stage is decreased. While the corresponding output acoustic emission energy is apparently increased, owing to the increased brittleness of coal, the deformation-failure of coal become more easily. Simultaneously, the fracture network and matrix surface of treated coal are more complex, and the corresponding fractal characteristic is obvious. It concluded that the coal pillars have a deformation-failure changes under the cryogenic-freezing by LCO2, which increasing the proportion of coal microstructure and enhancing coal permeability. Therefore, the capability of gas migration via coal microstructure becomes more easier, which is favorable for coalbed methane recovery.

Keywords: Deformation-failure characteristics, 3D reconstruction, Coal microstructure, Liquid CO2