Effects of acid-based fracturing fluids with variable hydrochloric acid contents on the microstructure of bituminous coal: An experimental study

Acid based fracturing fluid (AFF) with appropriate hydrochloric acid (HCl) content is not only conducive to the exploitation of coalbed methane (ECBM), but also can reduce the cost and the risk of environmental pollution. The optimum HCl content can be obtained by FTIR, MIP, and XRD test of the treated coal samples. The results show that the lag zone reaches the minimum value of 0.09 when the HCl content is about 3%, indicating that the connectivity of coal is the best. The effect of AFF on the adsorption pore structure is sensitive to HCl content. AFF with high HCl tends to increase the amount of adsorption pores by more than 9.7%, which enhances the gas adsorption capacity and is thus not conducive to ECBM. The functional group content initially increases with increasing HCl and then decreases. When the content of HCl is about 3%, they reache the maximum value, increasing by 26.5%, 16%, 38.5%, and 48.2%. The infrared indexes show the same trend of an initial increase and then decrease with increasing HCl content, whereas the aliphatic chain length shows a weakly positive trend. The results indicate that AFF with 3%–5% HCl is the most conducive for ECBM.