Analysis of fractal dimension of coal subjected to electrical breakdown based on nuclear magnetic resonance

The relationship between breakdown energy and the porosity growth rate were quantitatively investigated through nuclear magnetic resonance (NMR) and fractal dimension analysis. It can be found that the porosity of electrically broken coal sample increased, and the growth rate of macro-pores is much greater than that of meso-pores and micro-pores, which can promote the extraction of coalbed methane. Furthermore, the porosity growth rate of macro-pores change obviously with the increase of electrical breakdown energy, representing that larger energy can lead to more fractures. Micro-pores and meso-pores can be easily changed by small energy, while larger energy is conducive to the formation of cracks. Some meso-pores and micro-pores will be transformed into macro-pores, resulting in negative growth rates of meso-pores and micro-pores, indicating that electrical breakdown helps to expand primary fractures and generate new fractures. According to the pore size, there are two types of fractal dimensions, namely, adsorption pores and seepage pores. The results indicate that the value of fractal dimensions of adsorption pores is smaller than 1, verifying that adsorption pores do not follow the characteristics of fractal. However, the value of seepage pores lies within the range of 2–3 and boasts high fitting degrees, suggesting that seepage pores follow the characteristics of fractal. The value of DA increases while that value of DS decreases, which represents that electrical breakdown is more conducive to enhancing connectivity between macro-pores and tiny fractures. Moreover, there is no obvious positive correlation or negative correlation between fractal dimension and electrical breakdown energy, indicating that fractal dimension is the intrinsic property of the pore surface.