Atomic force microscopy investigation of nano-scale roughness and wettability in middle to high rank coals, with samples from Qinshui Basin, China

Coal wettability plays important role in multiphase fluids flow in coal formation including pore scale distribution of gas and water, the fluid flow velocity and drainage efficiency, which thus has a significant effect on the coalbed methane
(CBM) production. In this work, we collected 17 coals with different ranks from
Qinshui basin, North China and Junggar basin, Northwest China; the pore structures, mechanical properties and wettability (macroscopic and microscopic contact angle) were studied with nanodrops vapor condensation measurements and atomic force microscopy (AFM). The results show there is a negative relationship between the contact angle and roughness due to the change of adsorption sites, which is consistent with Wenzel model. The negative correlation trend between adhesive force and wettability of coal should be related to the minerals and oxygen-containing groups, the more hydrophilic of coal, the high content of –OOH and hydrophilic mineral in coal. Contact angle is largely determined by nanodrop size based on the AFM technology. Furthermore, the line tension on the surface of coal is 2.52×10-11 J/m, drop size less than 10 nm are most heavily affected by line tension. Line tension is
negligible when research the imbibition process in micropores and mesopores, and fluid flow behavior in fractures scarcely influenced by line tension. This work could provide new insights into the interactions between water molecules and coal pore surface, and provide a basis for the optimization of favorable areas in CBM reservoirs.