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

摘要

Permeability has been identified as one of the foremost factors that determine the effectiveness of CO_2 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 CO_2 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. CO_2 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 CO_2 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 CO_2 has contributed to a detrimental effect on permeability. This was clearly evident during the second injection of N_2 gas.