Exploration of the law of pore structure and permeability transformation of coal under acidification: Comparative analysis of formic acid and hydrofluoric acid

Acidizing fracturing is an important means to increase coalbed methane production in the coal industry, so research on acidizing fracturing fluid is also of great significance. In this paper, formic acid and hydrofluoric acid are used to dissolve coal samples to explore the reform law of organic and inorganic acids on the pore structure and permeability of coal. The liquid nitrogen adsorption experiment and mercury injection experiment are combined to analyze the damage law of the pores in each range of the coal body. Finally, the change law of the permeability coefficient of the coal body is studied by the triaxial seepage experiment to reveal the reform mechanism of the permeability property. The results indicate that there are notable differences in the conversion effects of coal when subjected to organic and inorganic acids. Specifically, these two types of acids exert distinct influences on the coal matrix. Through comparative analysis, it becomes evident that organic and inorganic acids possess unique properties and characteristics that facilitate coal transformation. Treatment of the coal sample with hydrofluoric acid leads to the formation of a highly porous structure with multiple directionalities, primarily serving to enlarge pore spaces. Formic acid treatment generates an interconnected mesoporous-microporous network in coal, primarily functioning as a pore-forming agent. The permeability of coal samples treated with hydrofluoric acid generally surpassed that of samples treated with formic acid.