Wettability determination by contact angle measurements: hvbB coal-water system with injection of synthetic flue gas and CO2.

Department of Geotechnology, Delft University of Technology, Delft 2628CN, The Netherlands.
Journal of Colloid and Interface Science (Impact Factor: 3.55). 08/2011; 364(1):237-47. DOI: 10.1016/j.jcis.2011.07.091
Source: PubMed

ABSTRACT Geological sequestration of pure carbon dioxide (CO(2)) in coal is one of the methods to sequester CO(2). In addition, injection of CO(2) or flue gas into coal enhances coal bed methane production (ECBM). The success of this combined process depends strongly on the wetting behavior of the coal, which is function of coal rank, ash content, heterogeneity of the coal surface, pressure, temperature and composition of the gas. The wetting behavior can be evaluated from the contact angle of a gas bubble, CO(2) or flue gas, on a coal surface. In this study, contact angles of a synthetic flue gas, i.e. a 80/20 (mol%) N(2)/CO(2) mixture, and pure CO(2) on a Warndt Luisenthal (WL) coal have been determined using a modified pendant drop cell in a pressure range from atmospheric to 16 MPa and a constant temperature of 318 K. It was found that the contact angles of flue gas on WL coal were generally smaller than those of CO(2). The contact angle of CO(2) changes from water-wet to gas-wet by increasing pressure above 8.5 MPa while the one for the flue gas changes from water-wet to intermediate-wet by increasing pressure above 10 MPa.

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    ABSTRACT: To quantify and rank gas wettability of coal as a key parameter affecting the extent of CO2 sequestration in coal and CH4 recovery from coal, we developed a contact angle measuring system based on a captive gas bubble technique. We used this system to study the gas wetting properties of an Australian coal from the Sydney Basin. Gas bubbles were generated and captivated beneath a coal sample within a distilled water-filled (pH 5.7) pressurised cell. Because of the use of distilled water, and the continuous dissolution and shrinkage of the gas bubble in water during measurement, the contact angles measured correspond to a ‘transient receding’ contact angle. To take into account the mixed-gas nature (CO2, CH4, and to a lesser extent N2) of coal seam gas in the basin, we evaluated the relative wettability of coal by CH4, CO2 and N2 gases in the presence of water. Measurements were taken at various pressures of up to 15 MPa for CH4 and N2, and up to 6 MPa for CO2 at a constant temperature of 22°C. Overall, our results show that CO2 wets coal more extensively than CH4, which in turn wets coal slightly more than N2. Moreover, the contact angle reduces as the pressure increases, and becomes < 90° at various pressures depending on the gas type. In other words, all three gases wet coal better than water under sufficiently high pressure.
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