An Experimental Study on the Conductivity Changes in Coal during Methane Adsorption‐Desorption and their Influencing Factors

Abstract

During the processes of methane adsorption and desorption, the internal structure of coal changes, accordingly leading to changes in electrical conductivity. In this paper, using low rank coal seams of the Yan'an Formation in the Dafosi field as the research subject, the relationship between coal resistivity, methane adsorption quantity, and equilibrium pressure is analyzed through proximate analysis, mercury injection tests, low temperature liquid nitrogen adsorption tests, and coal resistivity measurements during methane adsorption and desorption. The results show that during the process of pressure rise and methane adsorption, the conductivity of coal increases, resulting from heat release from methane adsorption, coal matrix swelling and adsorbed water molecules replaced by methane, but the resistivity reduction gradually decreases. The relationship between coal resistivity and methane adsorption quantity and equilibrium pressure can be described by a quadratic function. During the processes of depressurization and desorption, the resistivity of coal rebounds slightly, due to decalescence of methane desorption, coal matrix shrinkage and water-gas displacement, and the relationship coincides with a linear function. Methane adsorption leads to irreversible changes in coal internal structure and enhances the coal conductivity, and resistivity cannot be restored to the initial level even after methane desorption. The resistivity and reduction rate of durain are higher than those of vitrain, with relatively greater homogeneous pore throat structure and fewer charged particles in the double electric layer. In addition, moisture can enhance the conductivity of coal and makes it change more complexly during methane adsorption and desorption.

Variation of adsorption effects in coals with different particle sizes induced by differences in microscopic adhesion

“…Thus, the adsorption capacity of QY is weaker than that of CZ. It is noteworthy that many closed pores are broken as the particle sizes of coal samples decrease [21]. This is also an influential factor that the adsorption capacity of the coal samples of 0.18-0.25 mm is stronger than that of 10-15 mm.…”

“…Regardless of CZ or QY, methane reaches the maximum adsorption volume when the residual adsorption potential is 0. As the pressure continues to increase, more desorption occurs at different sites of adsorbed phase [21], leading to the increase in the density of the free phase methane increasing the gap between AAV and EAV. This is an essential characteristic of supercritical methane, i.e., a downward trend in the EAV of supercritical methane in coal occurs when the pressure is very high.…”

“…Gou et al [20] tested the structural characteristics of pore-fracture at different scales using field emission scanning electron microscope (FE-SEM) and computerized tomography (CT), concluding that micropores and mesopores provided large adsorption space for adsorbed gases. Chen et al [21] showed that the internal structure of coal was changed after methane adsorption by low-temperature liquid nitrogen adsorption tests, resulting in higher electrical conductivity. Additionally, moisture altered the wettability of coal [22], leading to enhanced electrical conductivity.…”

Nano-CT measurement of pore-fracture evolution and diffusion transport induced by fracturing in medium-high rank coal

“…As shown in the literature, there are previous works that characterize the porefracture evolution on the micron scale or static physical structure (Chen et al, 2019;Kang et al, 2019;Tan et al, 2019;Zheng et al, 2018). Interestingly, the changes in thermal expansion during methane adsorption by pores in coal were evaluated quantitatively (Chen et al, 2021b;Kang et al, 2022;Zheng et al, 2019), which provided new ideas for a comprehensive understanding of adsorption effects (Jia et al, 2022b;Liu et al, 2022b;Xie et al, 2022).…”