title: Interactions and exchange of CO2 and H2O in coals: an investigation by low-field NMR relaxation.
abstract: 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.
authors.affiliation: Coal Reservoir Laboratory of National Engineering Research Center of CBM Development &Utilization, China University of Geosciences, Beijing 100083, PR China.
authors.forename: Xiaoxiao
authors.initials: X
authors.lastname: Sun
authors.affiliation: Coal Reservoir Laboratory of National Engineering Research Center of CBM Development &Utilization, China University of Geosciences, Beijing 100083, PR China.
authors.forename: Yanbin
authors.initials: Y
authors.lastname: Yao
authors.affiliation: Coal Reservoir Laboratory of National Engineering Research Center of CBM Development &Utilization, China University of Geosciences, Beijing 100083, PR China.
authors.forename: Dameng
authors.initials: D
authors.lastname: Liu
authors.affiliation: Department of Energy and Mineral Engineering, Pennsylvania State University, University Park, PA 16802, USA.
authors.forename: Derek
authors.initials: D
authors.lastname: Elsworth
authors.affiliation: CSIRO Energy Flagship, Private Bag 10, Clayton South, VIC 3169, Australia.
authors.forename: Zhejun
authors.initials: Z
authors.lastname: Pan
references.citation: Environ Sci Technol. 2014 Apr 1;48(7):3908-15
references.pmid: 24625081
references.citation: J Colloid Interface Sci. 2006 May 15;297(2):755-61
references.pmid: 16376358