Investigation of the influence of coal swelling on permeability characteristics using natural brown coal and reconstituted brown coal specimens

Permeability has been identified as one of the foremost factors that determine the effectiveness of CO2 sequestration in deep coalbeds. Coal matrix swelling can cause a remarkable decrease in the permeability of a coalbed reservoir during the carbon dioxide injection process. The main aim of this paper is to correlate the diametrical swelling percentage with coal permeability. This experimental program further attempts to investigate the effect of gas flow properties in coal due to CO2 sorption. Furthermore, the applicability of using a laboratory-made reconstituted coal specimen for permeability testing has been comprehensively investigated. A laboratory-based experimental program has been carried out using a high pressure triaxial experimental set-up. In this study, the effect of swelling on coal permeability has been investigated using two types of specimens. An intact brown coal specimen cored from a natural coal block and a reconstituted brown coal specimen made by compacting coal dust in a mould has been subjected to testing. Tests have been carried out under different gas injection and confining pressures. CO2 and N-2 gases have been used as injection media. Gas injection has been carried out with two stages of N-2 injection, prior to and after CO2 injection. Diametrical swelling of the two specimens has been compared with the permeability data. A good correlation was observed between permeability and coal swelling. The coal swelling percentage increased on exposure to carbon dioxide compared to exposure to N-2. This effect increased as gas injection pressure increased. Exposure of the coal specimens to CO2 has contributed to a detrimental effect on permeability. This was clearly evident during the second injection of N-2 gas. (c) 2012 Elsevier Ltd. All rights reserved.