Experimental Study of the Influence of Moisture Content on the Pore Structure and Permeability of Anthracite Treated by Liquid Nitrogen Freeze–Thaw

Abstract

The liquid nitrogen freeze–thaw (LN2-FT) method has been widely used to improve the coal permeability in the coalbed methane (CBM) production. However, the influence of moisture content on the permeability of coal treated by LN2-FT remains unclear, limiting the broad application of this technique. A novel seepage system was proposed to analyze the permeability evolution of anthracite coal samples treated by LN2-FT. Moreover, variations of the pore structure were analyzed using scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), and low-field NMR. The results showed that pores and fractures appeared on the coal surface after the LN2-FT treatment. As the moisture content of the coal increased, more pores and fractures tended to be formed during the LN2-FT treatment. The total pore volume, porosity, and average pore diameter of the anthracite coal after the treatment were 1.77, 2.44, and 5.58 times higher, respectively, than that of the raw coal. The change in the specific surface area exhibited three trends as the moisture content of the coal samples increased: a slow descent, a steady increase, and a rapid descent. Moreover, it was found that the LN2-FT treatment increased the connections between pores and fractures, improving gas migration in the coal. Furthermore, the LN2-FT treatment significantly increased the permeability of the anthracite coal samples. The higher the coal moisture, the higher the permeability of the coal samples after the LN2-FT treatment. Hence, the LN2-FT technique can substantially improve the permeability of coal reservoirs, providing essential information for the efficient utilization of CBM.

Study on Mechanical Properties and Crack Propagation of Raw Coal with Different Bedding Angles based on CT Scanning

“…As shown in Figure 5 b, it is known from Mohr stress circle theory that the normal stress σ and shear stress τ on the bedding plane of coal mass with the bedding dip angle can be calculated from the first- and third-principal stresses 29 …”

“…Lv et al 26 − 28 scanned the coal samples during compression by CT and, through processing the original images, revealed the crack evolution characteristics of coal samples at different loading stages. Li et al 29 , 30 processed the CT scanning images of coal using image processing software and established a three-dimensional (3D) model of coal pores and fissures, which can visually observe the distribution and shape of pores and fissures inside coal. Du et al.…”

“…Lv et al − scanned the coal samples during compression by CT and, through processing the original images, revealed the crack evolution characteristics of coal samples at different loading stages. Li et al , processed the CT scanning images of coal using image processing software and established a three-dimensional (3D) model of coal pores and fissures, which can visually observe the distribution and shape of pores and fissures inside coal. Du et al. − used CT scanning and 3D reconstruction technology to establish the 3D visualization model of the coal mineral structure and coal assemblage, carried out compression simulation analysis on it, and obtained the damage characteristics and energy evolution law of coal under different compression conditions.…”

Experimental Study of the Structural Damage to Coal Treated by a High-Voltage Electric Pulse Discharge in Water

“…Coalbed methane (CBM) is a principal cause of coal-mine disasters, but it is also a clean high-quality source of energy. − China has huge reserves of CBM-proven reserves, about 36.8 trillion m 3 , ranking it third in the world . The efficient extraction of CBM can not only improve China’s array of energy sources but also greatly reduce coal-mine gas disasters.…”

“…1−3 China has huge reserves of CBM-proven reserves, about 36.8 trillion m 3 , ranking it third in the world. 4 The efficient extraction of CBM can not only improve China's array of energy sources but also greatly reduce coal-mine gas disasters. Compared with other major coal-producing countries in the world (the United States, Australia, India, etc.…”