On modeling the potential impacts of CO2 sequestration on shallow groundwater: Transport of organics and co-injected H2S by supercritical CO2 to shallow aquifers

Abstract

Proper site selection for CO2 geologic storage requires assessing the impact of potential leakage of CO2 from deep subsurface reservoirs to overlying drinking water aquifers. Although recent studies have largely focused on the mobilization of trace elements in response to the intrusion of CO2 into such aquifers, in this paper we investigate two other leakage issues and potential effects on groundwater quality: the transport of organic compounds by supercritical CO2 from deep storage reservoirs and the upward migration of CO2 with co-injected H2S. Numerical simulations show that organic compounds that may be present at depth, such as benzene, could be mobilized by supercritical CO2 and migrate with the leaking CO2. Modeling results also show that upon the transport of CO2+H2S mixtures through a hypothetical leakage pathway, H2S arrival in the shallower aquifer is delayed in comparison with that of CO2 due to the preferential dissolution of H2S into the aqueous phase. The potentially adverse impacts of leakage on shallow groundwater quality may be exacerbated for cases of leaking CO2+H2S, compared to intrusion of pure CO2, possibly leading to the mobilization of thiophilic elements such as arsenic. Geochemical reactions included in the simulations involve adsorption/desorption, reductive dissolution of goethite, precipitation of pyrite, siderite, and arsenic sulfide phases. The models presented are generic in nature, exploring important processes regarding organic compounds and co-injected H2S, and calling attention to the need for more site-specific studies taking into account the variability and uncertainty of key hydrogeologic and geochemical parameters.