Quantitative analysis of fracture dynamic evolution in coal subjected to uniaxial and triaxial compression loads based on industrial CT and fractal theory

Abstract

The dynamic evolution characteristics of fractures in coal under different compressive loads are of great significance to the recovery of coalbed methane (CBM). In this paper, the X-ray computer tomography (CT) detections of coal samples under uniaxial and triaxial compressive loading conditions were performed by using a developed triaxial in situ rig and an industrial CT scanner. The image processing technique, three-dimensional reconstruction method and statistical principles were employed to describe two-dimensional and three-dimensional fracture evolution in coal under uniaxial and triaxial compressive loads. The results showed that the developed triaxial in situ rig has an enough scanning resolution and is suitable to carry out CT scanning inspections under uniaxial and triaxial compressive loads for porous materials of coal and rock. The two-dimensional cross-sectional CT images effectively reflect the dynamic evolution of two-dimensional fractures during the deformation of coal. Confining pressure and minerals have a significant effect on fracture propagation in coal. The evolution characteristics of three-dimensional fractures can be clearly revealed by the reconstructed three-dimensional digital fracture models of coal. Euler number effectively reflects the fracture connectivity in coal at different deformation stages. During the deformation of coal, the fracture volume, fracture rate, fracture density and fracture connectivity mainly undergo a slow decrease stage, a slow increase stage and a sharp increase stage, display a good linear correlation with fractal dimension, and quantitatively characterize the evolution process of the three-dimensional fractures in coal. The fractal dimension calculated by the box-counting method effectively mirrors fracture propagation and fracture geometric complexity and can be employed to uncover the fracture dynamic evolution in coal under different compressive loading conditions.