Characterization of Physical and Chemical Structures of Calcite-Bearing Coal under Coupled Acidization and Water Intrusion

Mineral filling is very common in coal seams and significantly affects the pore structure and permeability of coal. Calcite is the main component of the contained minerals. According to chemical theory, calcite can be dissolved in acetic acid, which may be applied as an environmentally friendly additive in the fracturing fluid to improve the permeability of coal seams. However, it has not been well accepted in the industry, especially in coalbed methane (CBM) recovery. To investigate the effects of acetic acid on coal, this work provides an experimental investigation of the physical and chemical structure of acidized coal based on the measurements of X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and low-temperature N2 adsorption. The results indicate that acetic acid can effectively dissolve mineral particles embedded in coal, clearing blockages and expanding microstructure, making the coal matrix more prone to microcrack formation. Nitrogen adsorption experiments demonstrate that acetic acid cycling treatment increases the specific surface area and pore volume of coal samples, diversifying pore size distribution and complicating the pore structure. FTIR analysis reveals that acetic acid has a solubilizing effect on certain functional groups in coal, with the effect intensifying as the number of treatment cycles increases, significantly affecting aromatic hydrocarbons and oxygen-containing functional groups. XRD results show that acetic acid disrupts the crystalline structure of coal, leading to recrystallization. The newly formed crystals are larger in size but fewer in number. Repeated treatment further increases the expansion and decreases the stacking degree, resulting in a more loosened and fragile crystal structure.