Hydrogeological Analysis in Support of Identifying Suitable CO₂ Storage Sites and ECBM in the Sydney Basin, Australia

Karsten Michael¹, Mohinudeen Faiz¹, and Mario Werner^2
1CO₂CRC, CSIRO, Kensington, WA, Australia
²CO₂CRC, Australian School of Petroleum/University of Adelaide, Adelaide, SA, Australia

The Sydney Basin contains the largest concentration of industrial stationary CO₂ emission sources in Australia. However, no actual sites for CO₂ geological storage have been identified to date. The Sydney Basin comprises an up to 6000 m thick succession of mainly Permo-Triassic clastic sediments with large coal resources and an emerging coalbed methane industry. Coal seams are an attractive target for CO₂ sequestration in the Sydney Basin due to their proximity to power plants and their potential for enhanced coalbed methane production (ECBM). However, the impact on existing coal resources and containment of the injected CO₂ at relatively shallow depth might prove to be a liability. Geological storage in deep saline aquifers may provide a lower-risk alternative.

The main challenge of finding a suitable CO₂ storage site in saline aquifers in the Sydney Basin is the generally low permeability, particularly at depths below 800 m. Although a large part of the sedimentary succession consists of sandstones potentially forming aquifers, the majority of the original porosity is plugged with diagenetic cements. Testing of deep potential reservoirs produced some gas; however rates were generally uneconomic with little water production. Preliminary analysis of the fluid pressures and compositions suggests that the deeper parts of the Sydney Basin are gas-saturated and underpressured.

Exploration for suitable reservoirs or CO₂ storage sites should therefore focus on identifying “sweet spots” in the otherwise low-permeability environment by combining detailed litho-stratigraphic mapping with the analysis of the hydrogeology and the stress regime. A comprehensive analysis of flow patterns in the Sydney Basin would also help the CBM industry to identify areas of increased CBM producibility, and to asses the quality and quantity of produced water.

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