Energy & Fuels, Vol.35, No.1, 226-236, 2021
Changes in Pore Structure and Permeability of Middle-High Rank Coal Subjected to Liquid Nitrogen Freeze-Thaw
The low permeability of coal reservoirs makes the exploitation and utilization of coalbed methane difficult. In recent years, the liquid nitrogen freeze-thaw (LN2-FT) fracturing technique has been widely adopted in the field, improving the permeability of coal reservoirs. A self-developed experimental device was established to investigate the permeability characteristics of middle-high rank coal treated by LN2-FT under different stress conditions. The pore structure parameters of middle-high rank coal before and after LN2-FT treatment were measured by mercury intrusion porosimetry and nuclear magnetic resonance imaging. The results showed that the cumulative volume of pores in coal treated by LN2-FT was significantly larger than that of raw coal and the increased amplitude of fat coal was larger than that of anthracite coal. After the LN2-FT treatment, the connectivity of pores and the porosity increased and the total specific surface area of pores decreased. The fractal dimensions D-1 and D-2 of pores in coal treated by LN2-FT were smaller than those of raw coal, which suggested that the structures of pores in coal were simpler. The seepage experiment results show that with the change of CH4 injection pressure from 0.5 to 1.5 MPa, the permeability of both raw coal samples and coal samples treated by LN2-FT decreased and then increased at a specific confining stress, demonstrating the Klinkenberg effect. The permeability of coal samples treated by LN2-FT was higher than that of raw coal. The permeabilities of fat coal samples and anthracite coal samples treated by LN2-FT were 1.75-2.1 and 1.18-1.86 times those of raw coal samples, respectively. The research results demonstrated that the LN2-FT technique can effectively improve the pore structure characteristics and permeability of middle-high rank coal. The study provides support for applying the LN2-FT technique to improve the permeability of coal reservoirs.