Damage Law of Liquid Nitrogen Freeze–Thaw Action on Deep Coal Seam with Different Angle Joints

Abstract

Due to the significant pollution caused by the current extraction technology for deep coalbed methane (CBM), we propose the application of anhydrous cracking technology—liquid nitrogen cyclic freeze–thaw technology—to crack deep coal seams to achieve the release and extraction of coal bed methane. The self-made system for applying confining pressure is employed to simulate the deep coal seam in coal sample testing. Subsequently, the effect of joint structure with different angles of coal surface and bedding on the liquid nitrogen cyclic freeze–thaw process is evaluated. The investigation study identifies the damage patterns of the coal sample against the sole factor and coupling effect of confining pressure and joint angle. The findings reveal a clear impact of the cyclic freeze–thaw of liquid nitrogen on the expansion of deep coal seams. The application of appropriate inclined joints, combined with confining pressure, significantly minimizes the crushing time of coal samples, with the maximum expansion rate occurring at joints with a bedding angle of 45°±5°. This study provides vital theoretical support for the green and efficient exploitation of deep CBM. The results of this study indicate that the waterless hydraulic fracturing technology—liquid nitrogen cyclic freeze–thaw technology—is highly suitable for coalbed methane extraction in deep, low-permeability coal seams, leading to increased gas recovery. The deeper the coal seam, the more pronounced the effects. Moreover, this technology does not cause environmental pollution. It specifically targets the pores, fractures, and joint structures within the coal, preserving the overall framework of the coal and preventing tunnel instability, thereby enhancing coal mining safety. Additionally, under the influence of liquid nitrogen cyclic freeze–thaw, joints at different angles exhibit varying rates of expansion. Joints extending at a 45° angle to the coal’s bedding planes show the most rapid expansion, indicating that this orientation stores the maximum amount of coalbed methane postliquid nitrogen freeze–thaw. This pattern can be scientifically applied in the actual engineering design of extraction wells. The experimental results can provide a theoretical basis for guiding the on-site application of deep coal seam permeability enhancement and green coalbed methane extraction technology, thereby reducing environmental pollution.