Arsenic and Selenium in Formation Waters: Environmental Implications for Carbon Dioxide Storage in Geological Media

Abstract

In an attempt to mitigate climate warming, CO2 is being stored in geological media, including deep strata in sedimentary basins where formation waters commonly have high salinity. While there are numerous studies of reactions between injected CO2 and formation waters, little attention has been given to problems which might arise in areas affected by the pressure build-up generated by CO2 injection, which may extend for tens to hundreds of kilometres from the injection site. This paper draws on information from the Alberta Basin (Canada), and considers possible contamination consequences should formation water leak into shallow protected groundwater, with particular reference to As and Se. Arsenic and selenium were determined in 300 formation waters from strata 800 m or deeper: i.e., at depths and locations potentially suitable for CO2 storage. Although in about 72% of samples both elements were below detection (0.1 mg L ) and Se (44 mg L -1 ) are far above those reported in other sedimentary basins. Most samples were from drill stem tests, so contamination by As-bearing chemicals was unlikely. The most likely source for both elements is pyrite present in the aquifer, with chloride being the main removal control. More than 20,000 oil and gas wells in Alberta have lost their integrity, resulting in gas migration and surface casing vent-flow, which, if within the area of pressure build-up, could be conduits for contaminating local groundwater. Dilution up to several thousand times would be required to render these waters safe in the groundwater regime -possible, though with a low probability. These results suggest the need for more attention to the effect of pressure build-up and formation water composition in CO2 storage projects.