Response characteristics of coal microstructure with liquid CO2 acidizing treatment: An experimental study

To study the response characteristics of coal microstructure, especially pores and fractures structure variations with liquid CO2 (LCO2) acidizing treatment, the weak acid attributes and plugging removal effect of coal microstructure are studied by the combination methods of diffraction of X-rays, X-ray fluorescence spectrometer, scanning electron microscope and Fourier transform infrared spectrometer, before and after the coal is treated by acidizing. The results showed that the mineral content in the coal decreased after LCO2 acidification, and the carbonate content decreased by an average rate of 0.79%. Meanwhile, most elements in the coal migrated that performed a pressure sensitive tendency, resulting in the maximum migration rate of Calcium (Ca) is 13.66%, the contents of o-trisubstituted aromatic hydrocarbons (3H) and monosubstituted benzene rings (5H) of organic matter in the coal increased, the contents of benzene ring pentasubstituted (1H) adversely and tetrasubstituted m-phenylene rings (2H) decreased. Furthermore, the contents of aliphatic ethers, CC, CO and C–O functional groups decreased as a whole, while the content of –COOH structure increased significantly. The contents of symmetric CH2 and CH3 and asymmetric CH2 and CH3 decreased as a whole, and the hydroxyl functional groups in components increased significantly. The response characteristics of coal microstructure with LCO2 acidizing dissolution treatment mainly include dissolution, transformation and precipitation, hydration swelling of minerals. Meanwhile, the chemical reactions, such as addition reaction, substitution reaction and bond breaking effect, occurs in the coal microstructure of organic matter. This paper provides theoretical basis for the study of acid plugging removal effect of coal microstructure during LCO2 injection to enhance coalbed methane recovery.