Study on Coal Wettability under Different Gas Environments Based on the Adsorption Energy

Abstract

Coal seam water injection is a kind of comprehensive prevention and control measure to avoid gas outburst and coal dust disasters. However, the gas adsorbed in the coal seriously influence the coal-water wetting effect. With the deepening of coal seam mining, the gas pressure also gradually increases, but there is still a lack of in-depth understanding of the coal-water wetting characteristics under the high-pressure adsorbed gas environment. Therefore, the mechanism of coal-water contact angle under different gas environments was experimentally investigated. The coal-water adsorption mechanism in pre-absorbed gas environment was analyzed by molecular dynamics simulation combined with FTIR, XRD, and 13C NMR. The results showed that the contact angle in the CO2 environment increased most significantly, with the contact angle increasing by 17.62° from 63.29° to 80.91°, followed by the contact angle increasing by 10.21° in the N2 environment. The increase of coal-water contact angle in the He environment is the smallest, which is 8.89°. At the same time, the adsorption capacity of water molecules decreases gradually with increasing gas pressure, and the total system energy decreases after the coal adsorbs gas molecules, leading to a decrease in the coal surface free energy. Therefore, the coal surface structure tends to be stable with rising gas pressure. With the increase in environmental pressure, the interaction between coal and gas molecules enhances. In addition, the adsorptive gas will be adsorbed in the pores of coal in advance, occupying the primary adsorption sites and thus competing with the subsequent water molecules, resulting in a decline of coal wettability. Moreover, the stronger the adsorption capacity of gas, the more obvious the competitive adsorption of gas and liquid, which further weakens the wetting capacity of coal. The research results can provide a theoretical support for improving the wetting effect in coal seam water injection.

Wetting Behavior of CO2–H2O–coal with Different Metamorphic Degrees under In Situ Thermophysical Conditions

“…Hydrogen bonds, as the main driving force behind the adsorption behavior of water molecules on coal surfaces, are formed by the electrostatic interaction of a hydrogen atom bonded to one atom of the pair having a high affinity for electrons (typically F, N, or O) . Polar oxygen-containing functional groups on the surface (especially hydroxyl groups) bond with hydrogen atoms in water molecules (polar) in the form of hydrogen bonds through dipole forces to minimize the Gibbs energy of the system, and then Lewis acid–base interaction forces stronger than van der Waals forces are formed between polar molecules . Notably, the hydrogen bonds formed between oxygen-containing functional groups and water molecules are stronger than those between water molecules .…”

The Wetting Characteristics and Microscopic Wetting Mechanism of Coal under High-Pressure Nitrogen Environment

“…Although the contact angle has been studied under pressured-gas conditions for a long time, the mathematical expression in Equation ( 1) is addressed for the first time in this study [24,30,44].…”

“…Arif et al [27] tested the contact angles of different coal rank samples under CO 2 pressures of 0-20 MPa and found that the contact angles of the different coal ranks increased with an increasing CO 2 pressure. A research group tested the coal-water contact angles of different-rank coal under CH 4 , N 2 , CO 2 , and He pressures of 0-2 MPa [28][29][30]. They found that, as the gas pressure increased, the contact angles all increased, the higher the coal rank, the stronger the gas adsorption and the greater the increase in contact angle.…”