Study on the pore evolution law of anthracite coal under liquid nitrogen freeze‐thaw cycles based on infrared thermal imaging and nuclear magnetic resonance

Abstract

AbstractThe waterless fracturing method with liquid nitrogen (LN2) as the fracturing fluid has been proposed and successfully applied in coalbed methane (CBM) production in recent years. The temperature of the coal reservoir sharply decreases, causing damage to the pore structure of the coal reservoir due to the ultralow temperature of LN2 during fracturing. Thus, in this paper, infrared thermal imaging (ITI) and nuclear magnetic resonance (NMR) were used to measure the temperature distribution and pore evolution law of anthracite coal. The results demonstrate that the temperature of the coal sample after being frozen by LN2 was far less than 0°C, which causes the internal water of the coal sample to freeze and turn into ice and produce a frost‐heave force. In addition, the temperature of the coal samples was not fixed but fluctuated, which led to the formation of a temperature gradient and induced thermal stress. The T2 spectra variation showed that LN2 freeze‐thaw cycles can promote the development of pores in coal samples and enhance the connectivity of pores. Some of the micropores gradually connect and expand to form a large number of mesopores and macropores under the influence of frost‐heave force and thermal stress. The total porosity, residual porosity, and effective porosity increase with the number of LN2 freeze‐thaw cycles. The NMR imaging directly reflects the change characteristics of the internal pore structure before and after LN2 freeze‐thaw, which provide a new way to reveal the pore evolution law of anthracite. These results show that LN2 freeze‐thaw cycles can damage the pore structure of anthracite coal, and a large number of mesopores and macropores are formed to provide channels for CBM migration, which improves the efficiency of LN2 fracturing.