Experimental Study of the Structural Damage to Coal Treated by a High-Voltage Electric Pulse Discharge in Water

Abstract

High-voltage electric pulse (HVEP) technology has been gradually adopted as an effective method in the drainage and utilization of coalbed methane (CBM) in low-permeability coal reservoirs. An experimental device was established to study the structural variation characteristics of bituminous coal samples treated by an HVEP. The structure parameters of coal samples before and after the HVEP treatment were measured using a scanning electron microscope and low-temperature nitrogen adsorption. Additionally, the current waveforms in the process of HVEP discharge were analyzed by a digital oscilloscope. The results showed that the pressure of the shockwave produced by an HVEP increased with an increase in discharge energy, which intensifies the damage effect on the coal body. Some newly generated pores and cracks appeared in the bituminous coal samples after the HVEP treatment. As the discharge energy increased, the number of pores and fractures increased, and the diameters of the pores and the widths of the fractures also gradually increased. The average pore diameter, specific surface area, and most probable pore diameter of bituminous coal samples treated by an HVEP were greater than those of the untreated coal. Furthermore, after the HVEP treatment, the fractal dimension D 1 was larger than that of raw coal, while the fractal dimension D 2 was smaller than that of raw coal. This indicates that although the pore structure in coal became more complex after an HVEP, the surfaces of the pores became smoother. These changes were unfavorable to the adsorption of gas in the coal but conducive to the diffusion and flow of gas. The combined action of the shockwave cracking effect, the cavitation vibration effect, and the effect of thermal expansion promoted the expansion and connection of micro-and macropores and fractures in the coal, improving the permeability of coal seams. The research results demonstrate that the HVEP technology can significantly alter the structural characteristics of coal deposits in a way favorable for the development of CBM.

Research advances in enhanced coal seam gas extraction by controllable shock wave fracturing

“…After being treated, it promotes the expansion and penetration of micro and macro pores and fractures in coal, which improves the permeability of the coal seam and verifies the effectiveness of the CSW on the coal fracturing (Li et al 2022a;Yan et al 2019). Chen et al ( 2012) conducted an experiment of pulse discharge in water in a rectangular tank, and the experimental device is shown in Fig.…”

“…8a, the Institute of Electrical Industry of the Chinese Academy of Sciences adopted the pulse discharge fracturing experiment of the specimen in the pressure kettle to apply the stress to the coal rock specimen, so as to better simulate the underground engineering implementation conditions of the borehole. Researchers in France have developed a set of experimental device for pulse discharge fracturing and permeability enhancement, in which the principle stress is applied to the top hydraulic cylinder and confining pressure is loaded with lateral constraints (Li 2011;Chen et al 2012), as shown in Fig. 8b.…”

Modelling of enhanced gas extraction in low permeability coal seam by controllable shock wave fracturing

“…The CSW fracturing coal is preliminarily explored. Compared with the traditional hydraulic fracturing method, the CSW technology is water-saving with high efficiency, good fracture connectivity, complex fracture network, and repeatable fracturing [18,19] . At present, the effectiveness of CSW in fracturing coal has been verified by experiments [20] .…”

“…At present, the effectiveness of CSW in fracturing coal has been verified by experiments [20] . Many scholars have studied the initiation and propagation mechanism of coal cracks under the impact load [21] . It is found that the initiation coal cracks follows the maximum tensile stress criterion [22] , and the damage propagation in coal is related to the times of impact [23] .…”

Study on the Parameters of Strengthening Soft Surrounding Rock by Electric Pulse Grouting in the Mining Face

“…In order to achieve the desired effect, grouting reinforcement technology is often combined with fracturing, antireflection and other technologies. In recent years, electric pulse technology has gradually been popularized and applied with increasing permeability [14][15][16][17]. The principle of electric pulse antireflection is that the capacitor stores a certain high voltage and uses two electrodes placed in water to generate pulse discharge in a very short time.…”