Feasibility investigation of cryogenic effect from liquid carbon dioxide multi cycle fracturing technology in coalbed methane recovery

Liquid carbon dioxide (LCO₂) fracturing technology has been applied in the enhanced coalbed methane recovery (ECBM), and it has made some progress in the physical experiments and field applications. However, the freeze phenomenon during the injection process might induce coal matrix shrinkage, hindering the fracturing efficiency. A multiple cycle LCO₂ fracturing technology is proposed, and the feasibility of the cryogenic effect from LCO₂ on the crack evolution of five different coal cores under the loading state was investigated by using an innovative cryogenic loading experimental system. Nuclear magnetic resonance (NMR) and infrared thermal imaging (ITI) were used to measure the pore changes and temperature distribution, respectively. After 25 injection cycles, some cracks on the side and lateral surfaces of five cores were generated, and a low temperature distribution was formed. The temperature values were almost less than −18 °C, which could cause the saturated water to freeze into ice with a 9% volume increase; thus, the stress analysis diagram during one cycle injection was analyzed, and the initiation criterion was deduced. The T₂ spectra variation showed that the various pore sizes changed with the increased number of cycles. The peaks increased in amplitude and shifted to the right under saturated conditions, while they decreased and shifted to the left under centrifuged conditions, causing the amplitude increment ΔA in the post-test stage to be greater than that in the pre-test stage, which indicated that the cryogenic effect of LCO₂ could significantly improve the connectivity of pores. The total porosity φ_t and effective porosity φ_e of all five cores increased with the number of cycles. A quadratic function described the relationship between incremental ratio of φ_e (D_c) and cycle number, the fitting coefficients for which all exceeded 0.99, which indicated that the cryogenic effect of LCO₂ could improve the permeability observably.