Variations in surface fractal characteristics of coal subjected to liquid CO2 phase change fracturing

Enhancement of coal permeability plays the key role in methane extraction underground coalmine. As the physical blasting method, liquid CO2 phase change fracturing (LCPCF) technique can effectively stimulate the coal reservoirs, further enhance the coal permeability. In this work, the combination of scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) was adopted to quantitatively evaluate the variations in the pore/fracture structure and fractal characteristics of coal subjected to LCPCF. Fracture fractal dimension (D-F) and pore fractal dimensions (D-P1, D-P2) are calculated based on the SEM images and MIP data, respectively. After conducting LCPCF, a large number of microfracture and macropore clusters are generated and observed in SEM images. The maximum fracture ratio in treated coal is enhanced by approximately 200% compared with that in raw coal. MIP results show that pore distribution within coal exhibits a nonlinear alteration with four stages and the pore connectivity of coal is improved after LCPCF. The impact of LCPCF on pore/microfracture characteristics of coal is closely correlated with the distance from the fracturing borehole. The maximum values of D-F and D-P1 are found at the distance of around 2.0 m, indicating the best fracturing effect at such distance. It has been found that the effective influence scope of LCPCF on coal is within 7.0 m. The evolution of coal pore/fracture structure during LCPCF is attributed to the new pore/fracture generation and the conversion of original micropores into larger ones. Knowledge of these properties is essential for the application LCPCF in methane drainage.