Impact of CO2 injection on wettability of coal at elevated pressure and temperature

•With gas CO2, the wettability of coal is insensitive to temperature and only decreases slightly with pressure after reaction.•Contact angles increase greatly with increasing temperature and pressure when reaction with liquid and supercritical CO2.•Effect of temperature and pressure on coal wettability is well correlated to the CO2 density. Injection of carbon dioxide (CO2) into coal seams has been demonstrated as an effective technology for enhanced methane recovery and CO2 storage. However, the impacts of the geochemical reactions between CO2 and coal on the wettability of coal pore surfaces, which influences immiscible multiphase displacement, are not yet well understood. We studied wettability alterations of coal surfaces resulting from reactions with gas, liquid and supercritical (sc)CO2 under varied pressure (1–141 bar) and temperatures (˜25 - 60 °C) through measuring static and dynamic contact angles with anthracite coal plates. We found that reactions with gas CO2 only slightly changed the wettability of coal surfaces from water-wet to intermediate-wet with static contact angles from ˜60˚ to 70°-90°. However, reactions with liquid and scCO2 altered the coal surfaces to strongly CO2-wet, with the contact angles up to 115-180°. We also found that both static and dynamic contact angles increase significantly with increasing pressure. Temperature affects the contact angles reversely especially under supercritical pressure conditions. These relationships of contact angles with pressure and temperature may be explained by the CO2 density dependence on pressure and temperature.