Influence of high-temperature CO2 in hot flue gas on the wettability of coal surface: Experimental and molecular simulation study

Competitive wetting is a key factor influencing capillary forces, CO₂, and water adsorption, and gas desorption and migration. Aiming at uncovering the patterns and mechanisms of water wettability changes with coal pores and fractures under high-temperature and high-pressure CO₂ injection (333–413 K, 4 MPa), this paper presents experiments on the T₂ spectra of moisture within coal pores and fractures, the adsorption characteristics of CO₂ in coal, and the coal-water-gas interfacial property parameters, an analysis on Scanning Electron Microscope (SEM) images of coal surfaces treated with CO₂ at different temperatures, as well as molecular dynamics simulations on CO₂ injection into slits of water-bearing rough coal. The experimental and simulation results disclose that, under high-temperature CO₂ injection, the water wettability within coal pores changes in three stages from the injection well toward the interior of the coal seam, characterized by a “decrease–increase–decrease” pattern. These three stages are governed by different mechanisms: initially by strong activation arising from the temperature field, followed by moderate activation, and finally by CO₂-moisture competitive wetting. Additionally, a water film of appropriate thickness on the coal surface is beneficial for CO₂ adsorption. The three-phase contact angle (θ) cannot serve as a sole basis for accurately judging changes in water wettability within the flow channels of micropores and fractures in coal. Instead, it only effectively reflects changes in droplet wettability caused by competitive adsorption of CO₂ molecules on the coal surface. This study provides guidance on the optimal injection temperature for hot flue gas, which is beneficial for CO₂ capture and improves the extraction efficiency of CBM.