Analysis of the impact of carbon dioxide on coal wettability and its primary mineral factors

The geological sequestration of carbon dioxide in depleted coalbed methane wells is a key area of exploration in carbon dioxide utilization. However, not all coal-bearing strata are suitable for carbon dioxide geological sequestration, and the mineral composition of coal is an important factor in selecting appropriate sequestration zones. Currently, the effects of coal sample mineral composition, industrial components, gas pressure, and the changes in wettability and permeability after carbon dioxide-water solution interaction are not well understood. Further research is needed to enhance the theoretical framework for selecting and evaluating sites for carbon dioxide geological sequestration. To investigate the impact of carbon dioxide gas pressure on the wettability of coal and the role of mineral factors, this study immersed coal samples in a carbon dioxide-water solution at pressures of 1, 3, 5, and 7 MPa. Using maceral and industrial analysis, the study examined how carbon dioxide affect coal wettability and identified the dominant component factors influencing this effect. The results showed that the wettability of coal samples is influenced by their industrial and maceral components. The wetting contact angle is positively correlated with the contents of volatile matter, fixed carbon, vitrinite, inertinite, and the maximum vitrinite reflectance, but negatively correlated with the contents of ash and minerals. The hydrophilicity of coal samples treated with carbon dioxide deteriorates as pressure increases from 1, 3, 5, and 7 MPa, with the wetting contact angles of the five samples increasing from 60°-85° to 63°-78°, 68°-91°, 72°-95°, and 74°-103°, respectively. The greater the CO2 gas pressure, the stronger the wettability modification effect on coal. These findings suggest that while coal wettability modification hinder carbon dioxide injection, it enhances carbon dioxide storage. Therefore, coal reservoirs with higher permeability, as well as elevated volatile matter and fixed carbon content, should be prioritized for carbon dioxide geological sequestration. This strategy will not only ensure efficient injection but also promote effective storage. These research findings provide valuable insights for selecting suitable coal seams for carbon dioxide storage.