Characteristics Evolution of Multiscale Structures in Deep Coal under Liquid Nitrogen Freeze-Thaw Cycles

Yingfeng Sun,Yulin Li,Nima Noraei Danesh,Yixin Zhao,Zetian Zhang,Yong Liu

doi:10.1155/2021/8921533

Yingfeng Sun, Yulin Li + Show 4 more

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https://doi.org/10.1155/2021/8921533

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Abstract

Liquid nitrogen freeze-thaw fracturing has attracted more and more attention in improving the coal reservoir permeability. In order to reveal the impact of liquid nitrogen freeze-thaw on the multiscale structure of deep coal, the multiscale structure evolution law of deep and shallow coal samples from the same seam in the Qinshui coalfield during the liquid nitrogen freeze-thaw cycling was investigated using NMR T 2 spectrum, NMRI, and SEM. The connectivity between mesopores and macropores in deep and shallow coal is improved after liquid nitrogen freeze-thaw cycles. The influence of liquid nitrogen freeze-thaw cycles on the structure evolution of deep and shallow coal is the formation and expansion of microscopic fractures. The initial NMR porosity of deep coal is lower than that of shallow coal from the same coalfield and coal seam. The NMR porosity of both the deep and shallow coal samples increases with the increase of the number of freeze-thaw cycles, and the NMR porosity growth rate of the deep sample is lower than that of the shallow sample.

Highlights

Global reserves of deep coalbed methane (CBM) are approximately 47:6 × 1012 m3 [1], and the development of deep CBM in the United States started early and has been gradually commercialized [2,3,4,5,6,7]
Xu et al analyzed the pore structure of medium-high rank coal samples from depths of 1001800 m in Yichuan of Shaanxi Province and Shizhuang of Shanxi Province by mercury intrusion, low-temperature liquid nitrogen adsorption, and nuclear magnetic resonance (NMR)
The HL sample was collected from coal seam no. 15 in the Hengling block, and the SJZ sample was collected from coal seam no. 15 in the Sijiazhuang coal mine, with sampling depths of 1436 and 420 meters, respectively

Summary

Introduction

Global reserves of deep coalbed methane (CBM) are approximately 47:6 × 1012 m3 [1], and the development of deep CBM in the United States started early and has been gradually commercialized [2,3,4,5,6,7]. Qin et al investigated the pore-fracture characteristics of deep coal with a depth of more than 1000 m in the Daning-Jixian area using nuclear magnetic resonance (NMR) and found that the development of macropores and fractures is superior to that of micropores [12]. Xu et al analyzed the pore structure of medium-high rank coal samples from depths of 1001800 m in Yichuan of Shaanxi Province and Shizhuang of Shanxi Province by mercury intrusion, low-temperature liquid nitrogen adsorption, and NMR. Their results show an increase in micropores and a decrease in mesopores and macropores with depth [14]. Lu et al analyzed the porefracture structure of coal samples from different depths of 14 coal mines in the south of Qinshui Basin and found that

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