Effects of liquid-nitrogen cooling on pore characteristics of high-temperature limestone based on NMR

Liquid nitrogen (LN₂) ultra-low temperature fracturing presents a broad application range for geothermal energy development. Therefore, studying the effect of LN₂ cooling on the pore characteristics of high-temperature limestone is critical. In this study, Rizhao limestone obtained from Shandong Province was selected as the test object and subjected to heat treatment at different temperatures (25–600 °C) to investigate the changes in the pore characteristics of limestone after rapid LN₂ cooling based on nuclear magnetic resonance. The total porosity of limestone continuously increases due to the continuous expansion of original fractures in the limestone and the generation of new fractures caused by thermal stress resulting from high temperatures and the rapid LN₂ cooling. For heat treatment of 600 °C, no high amplitude is observed in the damage process. The fractal dimension of permeable pores (D_P) initially decreases and then increases with an increase in the quenching temperature difference, and the fractal dimension of total pores (D_T) shows a general decreasing trend. The compressive strength increases with an increase in D_T. Acoustic emission (AE) technology was used to monitor the deformation and failure of rocks. The low amplitude of AE is mainly distributed in the low-frequency band.