A post-combustion captured greenhouse gas (GHG) stream, derived from the combustion of Walloon Coal, may contain up to 5-30 ppm SOx, 100 ppm NOx and 30 ppm O2 along with other gases including N2 and Ar.
When this fluid is injected into the quartz-rich Precipice Sandstone and interacts with the clay-rich Evergreen Formation seal at the Surat CCS Project, preliminary geochemical studies suggest that some heavy metals and metalloids may be released.
Site specific information about the processes that control trace element behaviour is essential. It is necessary to accurately predict the likely concentrations of heavy metals and metalloids present in formation water, as a result of reactions with rock and water at the site.
The results from this project will help understand the type, amount and fate of heavy metals and metalloids, both before and after GHG stream injection, and provide a baseline and calibration data for the site’s environmental and groundwater impact assessment.
The project uses geochemical modelling, together with detailed geochemical and mineralogical analysis of Precipice Sandstone and Evergreen Formation core from the West Wandoan 1 well, and from laboratory batch experiments. This approach allows comparisons between simulations of water chemistry and measurements from experiments, providing confidence in the validity of model predictions of long term water chemistry impacts. The project also evaluates the application of partial and sequential extraction methods as a rapid, cost effective approach for investigating labile trace metal abundances in reservoir and seal rocks at carbon storage sites.
There is limited information on sources and sinks of trace metals and non-metals, or mechanisms of release, even with pure CO2 reaction under low salinity carbon storage conditions. Natural analogue studies of CO2 leakage sites have provided valuable data on the behaviour and fate of specific metals. Several initially elevated metals, including Ca, Fe and Mn, were repeatedly observed in previous field trials and experiments. Nevertheless, the sources of metals and non-metals are site specific and depend on mineralogy, water chemistry and the composition of the GHG stream.
Recent results suggest four potential processes may occur (highlighted by natural analogues, injection trials and experimental studies). They are:
- Dissolution/precipitation reactions are affected by pH and redox but are also important in controlling these parameters and may drive metal sorption and/ or desorption on iron oxides, hydroxides and clay surfaces.
- Ion exchange reactions of reactive clays in response to fluid migration and mixing or elevated concentrations of Ca from calcite or plagioclase dissolution liberate major ions including Na.
- Sorption of anions on Fe-oxides and hydroxides including bicarbonate, sulphate (and potentially nitrate) from the GHG stream may cause desorption of trace metals and metalloids.
- Oxidation/reduction of the system as a result of injected CO2, O2, NOx and SOx (oxidizing) or H2S and CH4 (reducing) makes some metals more or less soluble and will alter major and trace element behaviour. The precipitation of Fe-oxides under oxidising conditions can act as a sink for metals through co-precipitation and adsorption.