Effects of Liquid CO2 Phase Transition Fracturing on Methane Adsorption of Coal

Liquid CO2 phase transition fracturing (LCPTF) is a kind of novel waterless fracturing technology for enhancing the coalbed methane (CBM) recovery. Relevant current studies are focused on exploring the transformed effect of LCPTF on the nanopore structure in coal. However, its influence on gas adsorption capacity has been rarely reported. This study addresses coal induced with LCPTF. The structure alterations of mesopores (2-50 nm) and macropores (>50 nm) and their effects on the gas adsorption capacity are evaluated with comprehensive measure-ments of a mercury intrusion porosimetry, low-temperature N2 adsorption, and isothermal adsorption. The results indicate that LCPTF has an enlarged effect on macropores, resulting in an increase in pore size and volume and a reduction of the pore specific surface area. The pore-enlarged-transformed effects of LCPTF cause an increase in pore size and a reduction of the pore volume and pore specific surface area of mesopores. The variations of the pore structure after LCPTF cause a reduction of the adsorption constant a and the increase in the adsorption constant b, indicating LCPTF's reductive effect on adsorption capacity as well as its enhanced effect on desorption capacity. A novel effect evaluation method of LCPTF for improving CBM recoverability is proposed based on the variations of gas saturation and critical desorption pressure. This study examines, from the perspective of adsorption and desorption, the mechanism of LCPTF for enhancing CBM recovery, which provides theoretical guidance for LCPTF's technical improvement and optimization of field application so as to secure a more reliable and efficient CBM recovery.