The Effect of Coal Rank on the Wettability

The injection of carbon dioxide (CO2) or flue gas into coal layers enhances the coal bed methane production (ECBM) and is also an option for CO2-storage. The success of this combined process depends strongly on the wetting behavior of the coal, which is a function of coal rank, ash content, pressure, temperature and composition of the gas. Two coal samples have been used for this study representing different ranks: hvBb and semi-anthracite rank. The wettability behaviour of the wet coal samples upon injection of synthetic flue gas and pure CO2 was investigated in a modified pendant drop cell at a constant temperature of 318 K and pressures varying between 0.1- 16 MPa. For the hvBb sample, the wettability of the coal surface changed from intermediate-wet to CO2-wet at pressures above 8.5 MPa. When injecting synthetic flue gas, only a change from water-wet to intermediate-wet was observed. For the semi-anthracite rank Selar Cornish sample and CO2 injection this alteration was observed at about 5.3 MPa. Experimental results with synthetic flue gas revealed that the wettability of Selar Cornish coal is intermediate wet at all pressures and the contact angle only slightly increases with increasing pressure