Investigations of CO2-water wettability of coal: NMR relaxation method

Carbon geo-sequestration (CGS) and recovery enhancement with carbon dioxide injection (CO2-ECBM) have brought increasing focus on the CO2 and water wettability of coal. The CO2 and water contact angles measured using existing conventional methods, such as the pendent drop tilting plate technique and the captive-bubble technique, show low reproducibility due to coal heterogeneity and operational complexity. In this study, a novel NMR-based approach was developed to evaluate the CO2 and water wettability of coal. The experimental results of nine bituminous and anthracite coals show that water wettability can be linearly correlated with the changes of the T2 spectra peak positions. Based on the measured contact angle from the profile of the water adhering to the coal powder disc and the T2 from the NMR of coal powder, we proposed a linear formulation to evaluate the contact angle using the change of T2g of P3. Using this method, we analyzed the CO2-water wettability characteristics of coal. The results show that CO2 reduces the water wettability of coal. Low temperature and/or high CO2 pressure can enhance the CO2 wettability of coal. The change of water-coal wetting behavior with injection of CO2, is resulted by three factors: change of CO2 adsorption capacity of coal, change of interfacial tension, and dissolution of CO2 in water. This study makes it possible to evaluate changes in the water and CO2 wettability of coal, which is essential for evaluating the fluid-interaction mechanisms during the process of carbon geo-sequestration and enhanced coalbed methane recovery with carbon dioxide injection.