Micro-structural Damage to Coal Induced by Liquid CO2 Phase Change Fracturing

The technology of liquid carbon dioxide phase change fracturing (LCPCF) was used to enhance the permeability of coal seams. The combination of mechanical tests, scanning electron microscopy (SEM) and high-pressure mercury intrusion porosimetry was adopted to study the damage characteristics of coal micro-structures. LCPCF had mechanical damage effects on coal micro-structures to varying degrees, and the maximum reduction in compressive strength reached approximately 25%. SEM results confirmed that surface morphology of coal was remarkably altered after conducting LCPCF. The fractal dimension (D) of coal subjected to LCPCF ranged from 1.5186 to 1.8794, demonstrating the three-stage changing trends. HP-MIP results showed that LCPCF mainly affected pores of > 100 nm within coal, and pores < 100 nm were hardly influenced. When 1.26 L of liquid CO2 was used to conduct physical blasting, at distance of < 1.5 m, the influence of LCPCF was strengthened. Affected by the high-energy gas and shock wave generated by LCPCF, meso-pores within coal were damaged and shifted to the larger pores, resulting in the increase in the number of macro-pores and micro-fractures. When distance was > 1.5 m, the obvious reduction in macro-pore and micro-fracture volumes implied that the fracturing effect was attenuated with the increase in distance. Once distance was > 6.0 m, pore and fracture structures within coal tended to be stable. Thus, in this study, the influence scope of LCPCF was around 6.0 m for a single fracturing borehole.

Automated summary

The technology of liquid carbon dioxide phase change fracturing (LCPCF) was used to enhance the permeability of coal seams. LCPCF had mechanical damage effects on coal micro-structures to varying degrees. HP-MIP results showed that LCPCF mainly affected pores of > 100 nm within coal and resulted in the increase in the number of macro-pores and micro-fractures.