Effects of effective stress changes on permeability of latrobe valley brown coal D. Jasinge a , P.G. Ranjith a,⇑ , S.K. Choi b a Department of Civil Engineering, Monash University, VIC 3800, Australia b CSIRO, Division of Earth Science and Resource Engineering, Clayton, Australia article info Article history: Received 3 June 2010 Received in revised form 27 October 2010 Accepted 28 October 2010 Available online 12 November 2010 Keywords: Natural brown coal Reconstituted brown coal Permeability Effective stress Coal swelling abstract One of the key issues with geological sequestration of carbon dioxide in coal seams is change of perme- ability caused by carbon dioxide (CO 2 ) 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 CO 2 . 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 recon- stituted 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 CO 2 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. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction The addition of a considerable amount of greenhouse gases (GHGs) to the atmosphere occurs through the burning of fossil fuels such as coal to meet increasing energy demands. With coal production expected to increase, it is imperative to find a satisfac- tory solution to minimize future risks. Carbon capture and storage (CCS) has the potential to counter global climate change to some extent and will enable the world to cope with increasing energy demands while achieving reductions in GHG emissions. The geological sequestration of CO 2 in unminable coal seams is considered a promising potential option. The injected CO 2 tends to displace the adsorbed methane in methane-rich coal-beds. The permeability of a coal-bed is identified as one of the most critical parameters that affect both CO 2 injectivity and methane produc- tion. The permeability of a medium denotes the resistance of that porous medium to the flow of a fluid through the connected pores. It is assumed to be independent of the fluid. However, as perme- ability depends on the structure of the connected pore space, any factor that would contribute to a change in the pore structure will likely affect the permeability. Coal behaves differently to different gases. For example, nitrogen gas permeability in coal is higher compared to that of CO 2 , primarily due to the matrix swelling ef- fect. Coal-bed permeability is influenced by a number of factors such as coal rank, in situ stress, ground temperature, gas content and gas composition. During the experimental programme reported here we con- ducted studies on gas permeability in brown coal. CO 2 gas was used for the permeability measurements. The main aim of this lab- oratory-scale experimental study is to find out whether any corre- lation exists between effective stress and permeability, taking into account gas injection pressure. Any empirical relationship identi- fied will be used for future laboratory-scale numerical simulation studies based on the present experimental data. However, brown coal is highly heterogeneous and possesses highly variable proper- ties. It has a high moisture content and high adhesion, thus making it a difficult material to work with, when compared to other bulk solids. Its highly unpredictable variations, including moisture con- tent, mechanical properties, coal lithology and chemical composi- tion make the interpretation of laboratory results very difficult. Therefore, in addition to the tests conducted using natural brown coal specimens, we used reconstituted brown coal specimen made out of coal particles. The procedure for manufacturing the reconsti- tuted coal specimen was selected after a series of uniaxial com- pression test studies, and the procedure that produced specimens that best represented the mechanical properties of natural brown coal was adopted to produce the specimen for this study. 0016-2361/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2010.10.053 ⇑ Corresponding author. Tel.: +61 399 054982; fax: +61 399 54944. E-mail address: Ranjith.PG@eng.monash.edu.au (P.G. Ranjith). Fuel 90 (2011) 1292–1300 Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel