CO₂-wettability of low to high rank coal seams: Implications for carbon sequestration and enhanced methane recovery

Coal seams offer tremendous potential for carbon geo-sequestration with the dual benefit of enhanced methane recovery. In this context, it is essential to characterize the wettability of the coal-CO₂-water system as it significantly impacts CO₂ storage capacity and methane recovery efficiency. Technically, wettability is influenced by reservoir pressure, coal seam temperature, water salinity and coal rank. Thus a comprehensive investigation of the impact of the aforementioned parameters on CO₂-wettability is crucial in terms of storage site selection and predicting the injectivity behaviour and associated fluid dynamics. To accomplish this, we measured advancing and receding water contact angles using the pendent drop tilted plate technique for coals of low, medium and high ranks as a function of pressure, temperature and salinity and systematically investigated the associated trends. We found that high rank coals are strongly CO₂-wet, medium rank coals are weakly CO₂-wet, and low rank coals are intermediate-wet at typical storage conditions. Further, we found that CO₂-wettability of coal increased with pressure and salinity and decreased with temperature irrespective of coal rank. We conclude that at a given reservoir pressure, high rank coal seams existing at low temperature are potentially more efficient with respect to CO₂-storage and enhanced methane recovery due to increased CO₂-wettability and thus increased adsorption trapping.