In recent years, liquid nitrogen (LN2) fracturing technology has been applied in coalbed methane (CBM) development. However, the impact of the liquid nitrogen freeze‐thaw process on the pore structure of anthracite coal has not yet been systematically investigated. In this study, nuclear magnetic resonance (NMR) analysis and scanning electron microscopy (SEM) imaging of coal samples after treatment with liquid nitrogen for different freezing times and freeze‐thaw cycles were performed to study the pore structure damage of anthracite subjected to the liquid nitrogen freeze‐thaw process as well as the variation in the porosity, fracture evolution, and permeability. The results show that the LN2 freeze‐thaw process can enlarge the pore size, enhance the pore connectivity, and form a fracture network on the surface of coal samples, which increases the total porosity, residual porosity, effective porosity, and permeability. This study provides a theoretical value for LN2 fracturing in the development of CBM.
To further more deeply understand the influence of cyclic loading‐unloading on coal seams during mining and reduce the rock burst and coal and gas outburst disasters and ensure safety during coal mining, triaxial cyclic loading‐unloading tests were performed on coal samples. The permeability recovery rate, residual deformation, and dissipated energy ratio were defined to analyze the evolution in the characteristics of permeability, deformation, and energy observed during the triaxial cyclic loading‐unloading test. The results show as following: (a) The absolute permeability recovery rate first gradually decreases and then increases before failure, while the relative permeability recovery rate decreases sharply during the compaction stage, remains stable during the elastic phase, and gradually increases before failure; (b) The cumulative residual deformation increases with increase number of cyclic loading‐unloading, while relative residual deformation first decreases gradually, then stabilities, and then rises sharply before failure; (c) With increases in deviatoric stress, the total energy of coal samples increased as an exponential function. The dissipated energy ratio gradually decreased in the initial stages of cyclic loading‐unloading, then tended to stabilize in the elastic stage and then increase gradually before failure. This study provides precursor conditions for the early warning of rock burst and coal and gas outburst disasters, which has great importance for reducing coal mine dynamics disasters.
The waterless fracturing method with liquid nitrogen (LN 2 ) 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 LN 2 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 LN 2 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 T 2 spectra variation showed that LN 2 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 LN 2 freeze-thaw cycles. The NMR imaging directly reflects the change characteristics of the internal pore structure before and after LN 2 freeze-thaw, which provide a new way to reveal the pore evolution law of anthracite. These results show that LN 2 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 LN 2 fracturing.
K E Y W O R D Sinfrared thermal imaging, liquid nitrogen freeze-thaw cycles, nuclear magnetic resonance, pore evolution law | 3345 CHU and ZHanG
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