Pressure Dependence of the CO2 Contact Angle on Bituminous Coal and Semi- Anthracite in Water

Carbon dioxide (CO2) injection into coal layers serves the dual purpose to enhance coal bed methane production (ECBM) and to store CO2. The efficiency of this process is expected to be much higher if water is the non-wetting phase in the coal-water-gas system. Therefore, carbon dioxide contact angles in two coal-water-CO2 systems have been measured. The captive bubble technique was used within a pressure range from atmospheric pressure and 140 bar at a constant temperature of 45 ° C. Two sets of measurements have been performed, one on a polished semi-anthracite, the second set on a polished high volatile bituminous B coal sample. For the anthracite the following observations have been made. At atmospheric pressure, the contact angle of a CO2 droplet increases with time, but stays below 90°. At higher pressures (>3 bar) the contact angle increases beyond 90°. This shows that the semi-anthracite coal sample behaves CO2-wet at system pressures above 3 bar. CO2 contact angles on the bituminous coal sample show a different behavior. In the pressures range from atmospheric to 85 bar, CO2 contact angles are independent of pressure and exhibit values of around 85°. At pressures beyond 100 bar, the coal surface becomes CO2 wet. The bituminous coal sample is water wet up to much higher pressures than the anthracite. This behavior is related to the difference in stability of the water film between the coal surface and the CO2. The hydrophobicity of coal increases with rank due to a loss of functional groups which are responsible for hydrogen bonds between coal molecules and water. For the relevant pressures during CO2 storage, the anthracitic coal behaves CO2-wet, whereas in bituminous coal the injection pressure has to exceed a pressure of 100 bar in order to wet the coal surface. It can be concluded that the efficiency of carbon dioxide injection and retention in coal strongly depends on surface properties of the prevailing coal