Electrical response and pore structure evolution affected by cyclical plasma breakdown

Artificial improvement of coal seam permeability is the key to solve the low gas drainage efficiency and prevent gas dynamic disaster. Plasma based on physical discharge is one of the effective means of coal seam antireflection. However, previous studies focus on the characterization of pore-fissure structure of single breakdown, ignore the polarization effect of plasma on coal, and lack of in-depth research on the electrical properties and the evolution characteristics of pore-fissure structure under the action of cyclical plasma of loaded coal. In this paper, the cyclical plasma breakdown experiments of coal samples were carried out. The voltage and current waveforms in the process of coal breakdown were monitored thorough combining with high-voltage attenuation rod and the Roche coil, and the variation of electrical parameters such as pre-breakdown period, plasma breakdown period, number of wave peaks, peak voltage, peak current and energy conversion efficiency were analyzed. The evolution of small pores, medium pores, large pores and micro cracks under the conditions of 1, 5, 10, 15 and 20 times of breakdown was tested by NMR. The fractal dimension of seepage pore was also discussed through geometric fractal theory. The results show that the pre-breakdown period is at the level of thousands of microseconds only for the first time, and then it will drop to tens of microseconds, while the plasma breakdown period shows a “ladder” growth with the increase of breakdown times. The energy conversion efficiency is maintained at 28.7%−55.9%, showing a trend of rapid increase at first and then stable, indicating that the polarization effect of plasma on the electrical properties of coal is limited. The growth rate of medium and large pores is the most obvious, and some micro fracture structures will achieve a “0” breakthrough, indicating that plasma will significantly improve the seepage of gas. The fractal dimension shows a decreasing trend after breakdown, which proves that the originally isolated pore structure will be connected by the fracture from the perspective of pore fracture space dimension. The similarity between the change trend of porosity and energy conversion efficiency shows that plasma has an impact on the electrical properties of coal, and the change of coal electrical properties will also have an impact on the distribution of plasma channels.