Enhanced coal bed methane recovery finalized to carbon dioxide storage

Abstract

The recovery of coal bed methane can be enhanced by injecting carbon dioxide (CO2) in the coal seam at supercritical conditions. Through an in situ adsorption/desorption process the displaced methane (CH4) is produced and the adsorbed CO2 is permanently stored. This process is called Enhanced Coal Bed Methane recovery (ECBM) and it is a technique under investigation as a possible approach to the geological storage of CO2 in a carbon dioxide capture and storage (CCS) system. ECBM recovery is not yet a mature technology, in spite of the growing number of pilot and field tests worldwide that have shown its potential and highlighted its difficulties. The problems encountered are largely due to the heterogeneous nature of coal and its complex interaction with gases. These issues, which represent the motivation of this research work, need to be addressed both at laboratory and field test scales. The aim of this thesis is therefore to develop experimental and modeling tools that are able to provide a comprehensive characterization of coal required first to understand the mechanisms acting during the process of injection and storage and secondly to assess its potential for an ECBM operation. In particular, sorption data of CO2, CH4 and N2 on several coals from different coal mines worldwide have been measured at conditions typically encountered in coal seams. CO2 maximum sorption capacity per unit mass of dry coal has been found to range between 5% and 14% weight and to depend on coal rank in a non-monotonic way. Moreover, for a specific coal, competitive sorption isotherms of the binary and ternary mixtures of these gases have been obtained, showing that CO2 adsorbs always more than CH4, and CH4 more than N2. This property is of key importance for ECBM application. In order to investigate the coal volumetric behavior upon exposure to an adsorbing gas, two approaches have been followed. In the first, the utilization of a visualization technique allowed to measure the unconstrained expansion of a coal disc, confirming that indeed coal swells when exposed to a gas that is able to adsorb on its surface and penetrate into its structure, whereas exposure