Interactions and exchange of CO(2) and H(2)O in coals: an investigation by low-field NMR relaxation

The mechanisms by which CO(2) and water interact in coal remain unclear and these are key questions for understanding ECBM processes and defining the long-term behaviour of injected CO(2). In our experiments, we injected helium/CO(2) to displace water in eight water-saturated samples. We used low-field NMR relaxation to investigate CO(2) and water interactions in these coals across a variety of time-scales. The injection of helium did not change the T(2) spectra of the coals. In contrast, the T(2) spectra peaks of micro-capillary water gradually decreased and those of macro-capillary and bulk water increased with time after the injection of CO(2). We assume that the CO(2) 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 CO(2) and increases as the CO(2) adsorption capacity increases. Other factors, such as mineral composition, temperature and pressure, also influence the effective exchange between water and CO(2). Finally, we built a quantified model to evaluate the efficiency of water replacement by CO(2) injection with respect to temperature and pressure.