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

Changxing Li; Baisheng Nie; Zhiwei Feng; Quanfei Wang; Huanying Yao; Chunlian Cheng

doi:10.1021/acsomega.1c06631

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

ACS Omega. 2022 Feb 22;7(9):7777-7790. doi: 10.1021/acsomega.1c06631. eCollection 2022 Mar 8.

Authors

Changxing Li^{1

2}, Baisheng Nie^{1

3}, Zhiwei Feng², Quanfei Wang⁴, Huanying Yao⁵, Chunlian Cheng^{1

2}

Affiliations

¹ School of Emergency Management and Safety Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China.
² School of Mining Engineering, Guizhou University of Engineering Science, Bijie 551700, China.
³ State Key Laboratory of Coal Mine Disaster Dynamics and Control, School of Resources and Safety Engineering, Chongqing University, Chongqing 400044, China.
⁴ School of Safety Science and Engineering, Henan Polytechnic University, Jiaozuo 454000, China.
⁵ School of Applied Chemistry and Engineering, University of Science and Technology of China, Hefei 230000, China.

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.