Investigation of the impact of heating and liquid nitrogen (LN2) cooling on the mechanical and acoustic emission (AE) properties of coal

Liquid nitrogen has emerged as a promising fracturing medium for unconventional natural gas extraction, particularly in the context of coalbed methane extraction, generating significant interest in recent times. In this paper, we analyze the acoustic emission (AE) characteristics of coal samples treated with liquid nitrogen under three-point bending load conditions using indoor experiments and field investigations. We investigate the impact of liquid nitrogen on coal samples by analyzing AE data. The mechanical properties of coal samples with varying initial temperatures were tested under three-point bending load conditions after treatment with liquid nitrogen. The load-displacement curve was analyzed to study the mechanical properties of the coal samples after treatment with liquid nitrogen. Through indoor three-point bending experiments, data analysis, and knowledge of mechanics, we systematically investigated the changes in mechanical properties caused by liquid nitrogen treatment under different conditions. The results demonstrate that the application of liquid nitrogen has significant impacts on the mechanical and AE characteristics of coal samples. The load-displacement curve indicated that the mechanical properties of the coal samples changed after treatment with liquid nitrogen. The AE experiment revealed regular changes in vibration and energy parameters of coal samples after liquid nitrogen treatment under different conditions. We further investigated the changes in mechanical properties and deformation characteristics of coal samples after undergoing liquid nitrogen freezing and thawing in differing environments. Graphs such as load-displacement curves, load-time curves, cumulative vibration count-energy count graphs, and RA-AF density distribution diagrams visually demonstrated the changes in mechanical properties and deformation characteristics. Through an integrated approach of indoor experiments, data analysis, and knowledge of mechanics, our study provides a better understanding of the behavior of coal samples under different conditions. This understanding can contribute to the development of safer and more efficient methods for extracting coalbed methane.