Fracture processes in coal measures strata under liquid CO2 phase transition blasting

Abstract

Liquid carbon dioxide phase-transition blasting (LCO2-PB) has been applied to improve the permeability coal seam and enhanced coalbed methane recovery (ECBM). However, the mechanical behavior of crack generation and propagation in LCO2-PB under true triaxial stresses, has not been studied in detail. In this paper, five small-scale experiments of LCO2-PB were carried out with different restraint stresses on concrete blocks (150 × 150 × 150 mm3). Three uniaxial strain-gauges were used to monitor the surface strain of the blocks. The results show that the dynamic shock and expansion exists in the process of LCO2-PB. The release pressure markedly affected the relationship between shock and expansion. The expansion of the borehole and the fracture zone decrease with the increase in the confining pressure stress, and it cannot affect the number and direction generated of the blast cracks. The release pressure plays a decisive role in the number of fractures. The propagation process of LCO2-PB fracture was affected by the combination of LCO2 volume and surrounding rock stress field. The increase in the volume of LCO2, mainly influenced the length of cracks rather than the number of those. The propagation direction of fractures can be determinate by the effects of confining pressure. The research results will help enhance CBM recovery and reduce the occurrence of the gas disaster.