Effects of effective stress changes on permeability of latrobe valley brown coal

One of the key issues with geological sequestration of carbon dioxide in coal seams is change of permeability caused by carbon dioxide (CO₂) injection, and especially any resulting reduction in injectivity. Injection causes changes in pressure and effective stress, with further changes caused by coal matrix swelling associated with adsorption of CO₂. In this paper we aim to study how the change in effective stress and coal swelling may influence the gas permeability in brown coal using natural coal and reconstituted coal specimens. Tests were conducted at different confining pressures to represent conditions at different depths. Different gas injection pressures were also employed at each confining stress stage. The test results clearly depicted an exponential reduction of coal permeability to CO₂ gas when effective stress increases. Based on the experimental results, an empirical correlation to represent the effect of stress on permeability was developed. The results also showed that increase in pore pressure can induce further swelling of the coal specimens, and this can lead to further decrease in permeability which can have important impact on field injectivity. Test results for natural brown coal specimens were compared with results of tests on reconstituted coal specimens made from compaction of coal particles obtained from crushing of blocks of natural coal. Observed permeability behaviour of gas in reconstituted coal was similar to the natural coal specimen permeability trend, when effective stress increases.