Summary
Reservoir acidizing stimulation is among the commonly adopted methods for increasing natural gas production. With the effect of acid, the structural properties of coal change, which directly affects the gas storage and transportation behaviors within the reservoir. Understanding these structural evolutions and their corresponding control mechanisms is crucial. This work determined the optimal acid concentration through acidification experiments and characterized the structural evolution laws of samples before and after optimal acid treatment using Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), mercury intrusion porosimetry (MIP), low-temperature N2 (LTG-N2), and low-temperature CO2 (LTG-CO2) methods. The results showed that acid treatment altered the physical and chemical structure of coal, reducing the effective gas adsorption sites. This effect also increases average pore size and changes pore proportions, with micropores and mesopores evolving towards macropores, improving the gas transport properties. The control mechanism of pore structure evolution and migration enhancement is clarified from the micro level. A full-range characterization method for pore FD (fractal dimension) is proposed. The decrease in sample FD indicates that the pore network transforms into a simpler form after acidification, and the connectivity of pores and gas fluidity is improved. In engineering practice, it is necessary to comprehensively consider the marginal effect of mining, cost, environmental issues, etc., and to pay attention to matching the reservoir properties when optimizing the acidizing process parameters. The research results can provide theoretical guidance for the selection of acid stimulation reservoirs and enhanced coalbed methane (ECBM) process parameters.
