Evolutionary Mechanisms of Deep Coal Rock Chemical Structures Under Various Pre-Fracturing Fluids

Hydraulic fracturing is an effective method for enhancing coalbed methane (CBM) recovery. The injected fluids affect the chemical and physical structures of coal, resulting in diverse stimulation effects. While most current research primarily focuses on alterations in pore-fracture structures, few studies have examined or compared the changes in chemical structures during the fracturing process. This study presents a comparative analysis of the effects of five types of pre-fracturing fluids—slick water, acid solutions, and oxidant solutions—on coal, with the aim of identifying the similarities and differences in how these fluids modify the chemical structure of coal. The results indicate that, after being treated by five pre-fracturing fluids, the aromaticity index (I) increased, and the degree of aromatic condensation polymerization (DOC) decreased. The length of the aliphatic chain (L) increased after being treated by PAM but decreased after being treated by acids and oxidizers. Additionally, the graphitization (g) of all coal samples increased. Among the treatments, the combined acid system of hydrochloric acid (HCl) and hydrofluoric acid (HF) demonstrated a more pronounced effect on enhancing aromaticity and graphitization of the microcrystalline structure compared to HCl alone. Sodium hypochlorite (NaClO) had the most significant impact on the ordering of the macromolecular structure, while hydrogen peroxide (H2O2) exerted the most pronounced effect on the graphitization of the microcrystalline structure. These findings contribute to a deeper understanding of the interaction mechanisms between fracturing fluids and coal, providing theoretical support for the efficient development of CBM.