Experimental study and finite element simulations for LN₂ fracturing in coal from Karaganda Basin

This study looked at the effectiveness of liquid nitrogen (LN₂) in two distinct coal fracturing processes, freezing time (FT) and freezing–thawing cycle (FTC), for specimens that were saturated with water. The effectiveness of cryogenic treatment for coal rocks was compared using uniaxial compressive strength (UCS) and Brazilian tests. The results indicated in both USC and Brazilian tests, FTC experiments showed better results compared to FT experiments, leading to the conclusion that it may be possible to attain a higher desired permeability improvement by using more LN₂ injections in real-world LN₂ fracturing applications. In brittleness analysis, FT experiments showed higher values than FTC in the B21, B22, B24, and B25 indexes, while FTC experiments showed higher values in the B19 and B20 indexes. A scanning electron microscope (SEM) was used for fracture evolution analysis. Further, numerical simulations detailed the stress distribution patterns within coal samples under cryogenic treatment, utilizing von Mises stress analysis to compare the progression over time. The simulation results, particularly at peak stress periods, strongly correlate with experimental data, validating the model's effectiveness. These findings offer substantial evidence for the feasibility of LN₂-based treatments in enhancing coal bed permeability, with implications for improving the efficiency and safety of coal extraction processes.