Experimental Study on the Fractal Features and Permeability Characteristics of Low Metamorphic Coal Pore Structure under Thermal Damage

Zhen Liu,Shijian Yu,Mingrui Zhang,Lin Xin,Liutao Sun,Gang Wang

doi:10.1155/2020/8864571

Abstract

Underground coal gasification and exploitation of geothermal mine resources can effectively improve coal conversion and utilization efficiency, and the basic theory of the above technologies generally relies on the change law of the coal pore structure under thermal damage. Therefore, the influence mechanism of the development of the coal pore structure under thermal damage is analyzed by the nuclear magnetic resonance experiment, and the temperature-permeability fractal model is created. The results show that compared with microtransitional pores, the volume of meso-macropores in the coal body is more susceptible to an increase in temperature, which was most obvious at 200-300°C. During the heating process, the measured fractal dimension based on the T2 spectral distribution is between 2 and 3, indicating that the fractal characteristics did not disappear upon a change in external temperature. The temperature has a certain negative correlation with DmNMR, DMNMR, and DNMR, indicating that the complexity of the pore structure of the coal body decreased gradually with the increase of the temperature. Compared with the permeability calculated based on the theoretical permeability fractal model, the permeability obtained from the temperature-permeability fractal model has a similar increasing trend as the permeability measured by the NMR experiment when the temperature increases. The experimental study on pore structure and permeability characteristics of the low metamorphic coal under thermal damage provides a scientific theory for underground coal gasification and geothermal exploitation.

Highlights

In recent years, there have been many means to achieve coal conversion and comprehensive utilization, including underground coal gasification and geothermal mining [1, 2]
With an increase in the temperature, the amplitudes of two peaks of the T2 spectrum obviously increase, indicating that the amounts of water entering the Xiagou Mine (XG) coal samples increase with the increase of the temperature
With the increase of the temperature, the left peak amplitude of the T2 spectrum decreases, and the right peak amplitude of the T2 spectrum increases, indicating that the amounts of water entering the smaller pore structure of the HS coal samples decrease with the increase of the temperature, and the amounts of water entering the larger pore structure of the HS coal samples increase with the increase of the temperature

Summary

Introduction

There have been many means to achieve coal conversion and comprehensive utilization, including underground coal gasification and geothermal mining [1, 2]. Underground coal gasification is a kind of clean coal comprehensive utilization technology that is converted into combustible gas in situ by a thermochemical reaction [3, 4]. This technology is one of the important ways to achieve coal conversion, which improves the energy structure, enhances the safety of coal mine production, and considerably improves the resource recovery rate [5, 6]. The heat released from the combustion center bakes the coal rock, which causes the mechanical properties of the coal rock and its pore fracture to change, affecting the seepage characteristics of the gas in the pore fracture of the coal and determines whether the gasification reaction can continue. This study conducts a series of analyses and studies on the influence mechanism on the development of pore structure and permeability characteristics of the low metamorphic coal under thermal damage

Methods

Results

Conclusion