Interactions and exchange of CO₂ and H₂O in coals: An investigation by low-field NMR relaxation

The mechanisms by which CO₂ and water interact in coal remain unclear and these are key questions for understanding ECBM processes and defining the long-term behaviour of injected CO₂. In our experiments, we injected helium/CO₂ to displace water in eight water-saturated samples. We used low-field NMR relaxation to investigate CO₂ and water interactions in these coals across a variety of time-scales. The injection of helium did not change the T₂ spectra of the coals. In contrast, the T₂ spectra peaks of micro-capillary water gradually decreased and those of macro-capillary and bulk water increased with time after the injection of CO₂. We assume that the CO₂ diffuses through and/or dissolves into the capillary water to access the coal matrix interior, which promotes desorption of water molecules from the surfaces of coal micropores and mesopores. The replaced water mass is mainly related to the Langmuir adsorption volume of CO₂ and increases as the CO₂ adsorption capacity increases. Other factors, such as mineral composition, temperature and pressure, also influence the effective exchange between water and CO₂. Finally, we built a quantified model to evaluate the efficiency of water replacement by CO₂ injection with respect to temperature and pressure.