Geology And Trapping Mechanisms

The geology of the proposed injection sites is also very different. The Gorgon Carbon Dioxide Injection Project proposes to inject CO2 into the Dupuy Formation at a depth of some 2300 metres. The Dupuy formation is a sandstone saturated with brackish water. It is overlaid by several layers of shale and siltstone that should act as effective seals. Much nearer the surface is the Windalia Sandstone Member, which contains Australia’s largest onshore oil field and is itself sealed by thick layers of Gearle siltstone. That oil has been contained in the Windalia reservoir for millions of years indicates that the seal is effective.

The proposed injection strata in Harvey are the Lesueur Sandstone measures at a depth of between 1800 and 3,000 metres in the southern Perth Basin. The Perth Basin strata consist of basement Precambrian rock overlain by Permian Sue Group strata – mainly consolidated sandstone and siltstone with some coal measures. Overlying this is a thin belt of Sabina Sandstone and a much thicker (1600m) layer of Lesueur Sandstone. A coarse grained sandstone laid down during the Triassic period, the Lesueur Sandstone contains highly saline water.

Spectacle Haired Wallaby, Barrow Island

Overlying the Lesueur Sandstone is the Jurassic Cockleshell Gully Formation consisting of river-borne deposits of fine to coarse-grained sands interbedded with shale and silt. It is between 600m and 2000 metres thick.

The Yarragadee Formation, which is an important fresh-water aquifer in the south-west of Western Australia, salinity less that 1000mg/L, is present in parts of the southern Perth Basin but is absent in the target area. Avoiding any potential impacts on the Yarragadee is extremely important as potable water supplies are a major community and political issue.

Groundwater salinity typically rises rapidly with depth. In the storage target area the formation salinity rises to in excess of 15000mg/L.

The Lesueur Measures are deep saline formations, which may be one of the best potential options for CO2 geological storage. However it does not have a proven continuous effective seal, so retention of CO2 will depend on other trapping mechanisms.

Carbon dioxide can be trapped within permeable strata by a range of different mechanisms including structural seals, solubility, residual gas and mineral trapping.

Solubility trapping occurs because CO2 is weakly soluble in water. When it is exposed to the ground water it will gradually dissolve and be held in solution within the aquifer.

In residual gas trapping, tiny bubbles of CO2 become trapped in the pore spaces by capillary pressure. CO2 may move through the reservoir rock, as a non-wetting phase, while the saline formation water acts as a wetting phase. Once the main CO2 ‘cloud’ has passed through the rock, a small droplet of CO2 remains trapped in the centre of the pores. This droplet of CO2 is immobilised by the capillary pressure exerted by the wetting phase across the pore throats.

Solubility and residual gas trapping should occur at both Barrow Island and Lesueur. However, unlike the Dupuy Formation, the Lesueur lacks the assurance of a continuous impermeable seal. The discontinuous layers of shale in the Cockleshell Gully formation act as baffles, increasing the length of the passage the CO2 plume must take towards the surface. It is calculated that this process, together with the trapping mechanisms outlined, will ensure that CO2 is permanently retained in the Lesueur Formation.

Modelling for the South West Hub project indicates that approximately 60 percent of CO2 will be trapped in the Lesueur sandstone, 30 percent will be dissolved in the saline water and 10 percent will percolate to the shale baffles.

The Lesueur reservoir is calculated to be capable of receiving six million tonnes of CO2 annually for 40 years.

Injection operations at the Gorgon Carbon Dioxide Injection Project are anticipated to occur for approximately 40 years, but in this case termination will be due to depletion of the natural gas reserves, the source of the carbon dioxide.

Existing wells that penetrate into the reservoir formations are possible sources of leakage. Only four wells are known to penetrate the Lesueur formation and they are at a considerable distance from the proposed injection site. On Barrow Island only 28 of more than 1000 existing wells penetrate the Dupuy formation. It is planned that those that are anticipated to come into contact with the injected carbon dioxide will be studied and if required remediated to ensure they do not provide potential leakage pathways.

None of the known geological faults at either Barrow Island or the Southern Perth Basin are active. However they have been active in the past and could become active again. Geophysical tests carried out on core samples from wells in the South Perth Basin showed faults there were able to withstand a pressure increase of up 10000 kPa without leakage. Modelling of the Barrow Island proposal showed that the CO2 plume is unlikely to come in contact with the main Barrow Island Fault.

Perentie, Barrow Island