Abstract
The liquid nitrogen freeze-thaw (LN(2)-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 LN(2)-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 LN(2)-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 LN(2)-FT treatment. As the moisture content of the coal increased, more pores and fractures tended to be formed during the LN(2)-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 LN(2)-FT treatment increased the connections between pores and fractures, improving gas migration in the coal. Furthermore, the LN(2)-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 LN(2)-FT treatment. Hence, the LN(2)-FT technique can substantially improve the permeability of coal reservoirs, providing essential information for the efficient utilization of CBM.