Modification and enhancement of permeability of coal seam using mined CO2 foam fracturing fluid

Fracturing is an effective method for enhancing the permeability of coal seam, whereas CO2 foam fracturing is a new fracturing method having enormous potential. This study focused on the anthracites in Chengzhuang Coal Mine, Shanxi, China, and examined the structures and properties of raw coal samples, water-soaked samples and the samples soaked in CO2 foam fracturing fluid (CFFF) using Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) and methane adsorption methods. According to the experimental results, the mean pore diameters of these three types of coal samples were found to be 14.6489 nm (raw coal sample), 28.9397 nm (water-soaked coal sample) and 27.5610 nm (coal soaked in CFFF), and the specific surface areas of these three samples were 1.9114 m(2)/g (raw coal sample), 1.2025 m(2)/g (water-soaked coal sample) and 1.4223 m(2)/g (coal soaked in CFFF). After the immersion in water and CFFF, the adsorption peaks of some functional groups, such as -OH, -CH2- and -C-O- in the coal decreased. According to the SEM results, the fractures and secondary pores appeared in the coal sample because of the dissolution and corrosion, which were conducive to the migration of gas. Meanwhile, under the action of dodecyl trimethyl ammonium bromide (DTAB) in CFFF, the hydrophilicity of coal surface weakened, which in turn reduced the capillary resistance for the migration of CFFF in the fractures. Finally, the methane adsorption capacities of different coal samples were also analyzed, which validated the conclusion that the wettability alternation in coal samples can promote the desorption of adsorbed gas. The present study can provide a new insight regarding the coalbed methane extraction and safety of mines.