Study on microstructural changes of coal after methane adsorption

•Based on the Electron microscope scanning results, in accordance with the degree of pore fracture development and its filling condition, the target areas of coal can be divided into four categories: no-development pore area, not filled pore area, mineral-filled pore structure area, and mixing area of pore fissure structure and clay minerals.•Based on the CT scanning results, found that after methane adsorption, coal exhibits non-uniform expansion deformation, leading to a reduction in the low-density area and an increase in the high-density area.•Methane adsorption and expansion deformation of coal are related to the pore structures of coal. Adsorption and expansion mainly occur in the unfilled area or the pore structure area filled with clay minerals.•During adsorption expansion, the opposite effects of mutual extrusion and expansion deformation occur within the coal structure.

This study analyzes the microstructure and deformation rule after methane adsorption on coal by scanning electron microscopy (SEM) and computed tomography (CT) scanning of microscopic coal samples. Studies have shown that coal is a natural rock composed of vitrinite coal matrix and clay mineral. After methane adsorption, coal undergoes non-uniform expansion deformation. This occurrence prompts coal density to decrease and then increase, causing the density distribution of coal to become highly concentrated. During swelling after adsorption, the effects of deformation and expansion on coal structures become stronger than that of mutual squeezing. Under low adsorption pressure, coal expansion deformation is more likely to crack the pore structure of the original coal to acquire space for expansion. When the adsorption pressure increases, compression becomes mainly concentrated in the low-density region; as adsorption pressure continues to increase, expansion deformation occurs from high-density to low-density regions. The methane adsorption properties of coal are related to its pore structure. Adsorption and swelling mainly occur in the region where the pore structure is unfilled or filled with clay minerals. Expansion deformation conforms to the Langmuir equation; the region without pore structure development exhibits no swelling; the deformation degree and range of the pore structure and clay mineral mixing zone exhibit increase volatility. Overall, results reveal a microstructural change after methane adsorption.