Research on the pressure variation law and enhancing CBM extraction application effect of CO₂ phase transition jet coal seam fracturing technology

Abstract

Due to the limited permeability and high methane content of the majority of China’s coal seams, significant coal mining gas disasters frequently occur. There is an urgent need to artificially improve the permeability of coalbed methane (CBM) reservoirs, enhance the recovery efficiency of CBM and prevent mine gas accidents. As a novel coal rock fracture technology, the CO₂ phase transition jet (CPTJ) has been widely used due to its advantages of safety and high fragmentation efficiency. In this study, to ascertain the effects of the pressure of CPTJ fracturing, the influence of its jet pressure on cracked coal rock was revealed, and its effect on CBM extraction was clarified. In this research, the law of CPTJ pressure decay with time was investigated using experimental and theoretical methods. Based on the results, the displacement and discrete fracture network law of CPTJ fracturing coal rock under different jet pressure conditions were studied using particle flow code numerical simulation. Finally, field experiments were conducted at the Shamushu coal mine to assess the efficiency of CPTJ in enhancing CBM drainage. The results showed that the pressure of the CPTJ decreased exponentially with time and significantly influenced the number and expansion size of cracks that broke coal rock but not their direction of development. CPTJ technology can effectively increase the number of connected microscopic pores and fractures in CBM reservoirs, strongly increase the CBM drainage flow rate by between 5.2 and 9.8 times, and significantly reduce the CBM drainage decay coefficient by between 73.58% and 88.24%.

Graphical abstract

Keywords

coalbed methane (CBM) / CO₂ phase transition jet / pressure evolution / damage of coal / CBM drainage

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