Infrared thermal image and heat transfer characteristics of coal injected with liquid nitrogen under triaxial loading for coalbed methane recovery

The liquid nitrogen (LN2) fracturing technology is a promising and effective means for coalbed methane stimulation. The flow and heat transfers in coal with LN2 injections directly influence the characteristics of fracture initiation and propagation which are being considered as determining factors of methane recovery efficiency of coal seams. The aim of this study is to probe the heat and mass transfer behaviors and fracturing mechanisms of LN2 stimulation. Using combined infrared thermal imaging, ultrasonic detection, and acoustic emission techniques, experimental work was carried out on simulated coal samples to study the heat and mass transfer behaviors under triaxial stress condition. Compared with a single injection, cyclic LN2 injections resulted in denser fracture pattern in the samples. And it was also found that temperature decreased and rose more quickly during the freezing and thawing processes for cyclic injections. With the same injection duration, the cooling range of the cyclic injection was much larger than that of the single injection. In the single injection for 10,000 s, the whole sample was mainly shrunk due to freezing, whereas freezing shrinkage-frost heave-freezing shrinkage occurs in the whole sample after 6 times of the cyclic LN2 injections for 7000 s. The maximum acoustic emission energy signal appeared in the 6th cycle, proving that fractures in the samples connected to form a network, which coincided with a decrease in the temperature gradient in the infrared thermal image. Based on analysis of the infrared temperature results, this study established a model for the temperature of the samples with LN2 injection time and demonstrated the changes in the cooling radius and temperature gradient during LN2 injections. (C) 2017 Elsevier Ltd. All rights reserved.