Author(s)

BRIAN HITCHON, STEFAN BACHU

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^–1), the maximum values for as (86 mg L^–1) 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.

Keywords

arsenic, selenium, formation water, Alberta Basin, CO₂ storage, pressure build-up