Changes in pore structure and permeability of anthracite coal before and after high-voltage electrical pulses treatment

Abstract

High-voltage electrical pulses (HVEP) can potentially be used to increase the yield of low-permeability coalbed methane (CBM) reservoirs. This study highlights the pore change and permeability evolution characteristics of anthracite coal samples using HVEP treatment. An experimental system was developed to study the variation of coal core permeability by subjecting the coal samples to HVEP treatment under confining conditions. Scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) analyses were adopted to gain a clearer insight into the changes in pore structure. SEM images revealed that the surface morphologys of the raw coal samples were complete and smooth, whereas those of the coal samples subjected to HVEP developed many pores and cracks. The pores and cracks generated enhanced the permeability of HVEP-treated coal samples. MIP test results showed that the cumulative pore volume of the coal samples subjected to HVEP was significantly greater than that of raw coal samples. Importantly, the HVEP technology had a significant effect on mesopores and macropores, compared with micropores. Because the mesopores and macropores are critical for gas transport, the technology will be helpful for enhancing CBM recovery. Seepage experiments results showed that with the variation of CH4 injection pressure from 0.4 MPa to 1.2 MPa, the permeability of both raw coal samples and coal samples subjected to HVEP showed a decline at a particular confining stress. The permeability of Yangzhuang (YZ) coal sample subjected to HVEP was 1.6–2.2 times that of YZ raw coal under a constant confining pressure, and the permeability of Guhanshan (GHS) coal sample subjected to HVEP is 1.8–2.4 times that of the GHS raw coal sample. At the same confining pressure and CH4 injection pressure, the permeability of the coal samples subjected to HVEP was obviously higher than that of the raw coal samples.