An enhanced coalbed methane recovery technique based on CO2 phase transition jet coal-breaking behavior

In recent years, CO2 phase transition jet (CPTJ) coal-breaking technology has been reported to increase coal seam permeability. However, the structural evolution effect of coal subjected to CPTJ technology is unclear, which restricts widespread CPTJ technology application. In this study, a laboratory experimental system and field technical equipment were developed, and coal-breaking experiments under different CPTJ pressure conditions were conducted. We investigated pore structure changes by combining scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) to better understand the coal pore structure evolution characteristics under different CPTJ pressures. Furthermore, an enhanced coalbed methane (ECBM) recovery experiment using CPTJ coal-breaking technology was conducted with the self-developed technical equipment. The results show that notable damage occurs in the coal body influenced by CPTJ, and the damage area increases with increasing jet pressure. The SEM results revealed that more pores and cracks are produced due to liquid CO2 CPTJ, and the porosity and crack size increase with increasing jet pressure. MIP analysis indicates that the pore structure of the breaking coal samples mainly includes macropores, and coal sample macroporosity increases significantly with increasing jet pressure. The field ECBM experiment showed that CPTJ technology can reduce the coalbed methane (CBM) drainage decay coefficient and increase the CBM extraction pure flow rate and recovery efficiency 8.3-10.4 and 20.4 times, respectively.