Water produced during coal seam gas (CSG) production can be safely injected hundreds of metres underground, according to new CSIRO research.
The research shows that injecting large volumes of treated CSG water at suitable locations within the Surat Basin is unlikely to cause any harm to groundwater quality. Injection water has to be treated adequately to eliminate the risk of polluting groundwater with arsenic that occurs naturally in some rock formations.
CSG water and injection in Queensland
In Queensland, the state government encourages the beneficial use of CSG water in a way that protects the environment and maximises its productive use as a valuable resource. Although injection has been considered as a water management option by CSG proponents, it is not routine practice in Australia with only 22% of the total volume of CSG water injected.
The Reedy Creek injection scheme began in 2015 and Origin Energy has injected over 10 gigalitres of treated CSG into the Precipice aquifer.
The research project
Research from injection projects around the world has shown that injecting clean water into aquifers has the potential to mobilise naturally occurring contaminants such as arsenic.
CSIRO used modelling to simulate groundwater flow and quality from injection trials at Reedy Creek and Condabri, both located in the Surat Basin in Queensland. During the research elevated arsenic levels were found during field experiments, and computer modelling suggest that arsenic release was triggered by the injected water.
The analysis shows that mobilisation of arsenic is dependent on the chemical composition of the injected water. The research concluded that adjusting the pH and removing oxygen from the water prior to injection are the most important factors in preventing release of arsenic.
Experiments conducted at the Condabri injection site showed that, when oxygen was not removed from the water to be injected, dissolution of naturally-abundant mineral pyrite, or "fool's gold" occurred. Arsenic is often embedded in trace amounts in this mineral.
Computer modelling of results from the Reedy Creek injection site demonstrated that the form of arsenic mobilization that was observed could have been completely prevented by adjusting the pH of the injected water to match the pH of the naturally-residing groundwater.
Where does the arsenic come from?
Arsenic is a generally immobile toxic element that occurs naturally in some rock formations.
When rainwater seeps underground and becomes groundwater it changes its composition. During the subsurface passage that can often take thousands of years the groundwater composition changes slowly to successively take on the characteristics of the rocks.
When water with a non-compatible composition is directly injected into deep aquifers, the injected water will also react with the rocks and therefore change its characteristics to one that is compatible with the new host rock. This occurs through the release of elements from the rocks, a process called mineral dissolution.
In addition, arsenic mobilisation can also occur by a process called desorption, in which case loosely attached ions are released from mineral surfaces. Both processes may proceed until a new balance or "geochemical equilibrium" is established and both have the potential to mobilise toxic elements such as arsenic.