Study on the mechanism and microstructural testing of acid-induced hydrothermal phase-change fluid fracturing

Coalbed methane (CBM) has excellent prospects and value for extraction, and hydrothermal fracturing technology can produce a complex fracture network conducive to gas recovery. Hydrothermal treatment of coal dust specimens with different initial solutions was designed in this study, with slick water and fracturing fluid used as comparison group experiments, and microanalysis of the treated coal dust was carried out. The main research results were as follows: (1) The best pore expansion effect was achieved by using an acid solution as the hydrothermal initial fluid to treat the coal powder; the largest amount of minerals was removed, and the acidified hydrothermal treatment obtained the best CBM resolution effect. (2) The reason for mineral loss in coal was explained through the establishment of the acidified mineral dissolution model, and the concentration and diffusion rate of the acid solution affect the mineral decomposition effect. (3) The energy spectrum analysis reflects that the pore blockage was not only caused by mineral particles but also by coal particles, thus establishing the acid dissolution mineral model, which explains the principle of penetration enhancement by acidified hydrothermal impact. (4) The hydrothermal shock effect prompted the transformation of micropores into mesopores within coal, forming new flow pathways. This manifested as a systematic reduction in fractal dimension, revealing a synergistic enhancement of permeability alongside a diminishing complexity of the pore network.