The efficiency of the enhanced-coalbed-methane (ECBM) and CO2 sequestration process depends on the wettability behavior of the coal-water-CO2 system. It depends on the CO2 diffusion rate from the cleat network, through the micro-cleats, to the surface of the coal matrix. If the coal is hydrophobic, the gas will fill the smaller pores, which leads to faster diffusion of CO2 to the coal surface (diffusion coefficient of CO2 = 1.7*10−7 m2/s at 100 bar and 300 K). The diffusion coefficient decreases to 2*10−9 m2/s at the hydrophilic system.
This paper interoperates the published experimental outcomes of coal wettability at varies range of pressure, temperature, and gas composition. Wettability studies usually used contact angle measurements as a simple method to characterize the coal. Adsorption isotherm, Zeta potential measurements were used to interoperate the contact angle results and understand the wettability behavior. Recently, Fourier Transform Infrared Spectrometer (FTIR) technique was employed to investigate the functional chemical groups of the oxidized coals.
Coal has different ranks with different oxygen-containing polar groups such as carboxyl (COOH), hydroxyl (OH), and methoxyl (OCH3) groups. Coal becomes more water-wet with decreasing rank, carbon content and with increasing oxygen-containing groups. Regardless of the coal rank, the coal becomes more CO2-wet with increasing pressure up to the critical gas conditions where the coal wettability slightly increases. As the water-salt concentration increases, it compresses and destabilizes the double-layer surrounding the coal surface, causing a reduction in the absolute zeta potential value and making the coal surface more hydrophobic. The temperature has a negative effect on CO2 wettability to coal and the coal becomes more hydrophilic with increasing the temperature. In oxidized coal surfaces, FTIR technique is more sensitive than contact angle measurements.