Microscopic wettability of medium rank coals involved pore features and functional groups

Abstract

Wettability affects the immiscible flow process of the gas-water phases in coalbed methane reservoirs, which has an important effect on coalbed methane production. The sessile drop method, infrared spectroscopy and atomic force microscopy (AFM) were used to study the wettability, functional groups and pore structure of 8 coal rocks with middle ranks from the Qinshui Basin, North China, and the Junggar Basin, Northwest China. The pore structure of the coal sample, including the porosity, number of pores, pore area, pore diameter, percentage of different types of pores and roughness under the scale of 50 μm, was determined using the Nanoscope and Gwyddion analysis methods. The relationships between the pore structure, functional groups, coal surface roughness and wettability were studied. The results show that the larger the proportion of macropores on the coal surface is, the stronger the hydrophilicity of coal. The more oxygen-containing functional groups there are, the better the wettability of coal and rock. The contact angle has a sinusoidal relationship with the roughness due to the change in the adsorption point. For coal with very low or very high roughness, the contact angle is negatively correlated with the roughness, while for coal with low to average roughness, the contact angle is significantly positively correlated with the roughness. This work provides new insights into the interactions between water molecules and the coal pore surface.