文摘
Unintended release of CO2 from carbon sequestration reservoirs poses a well-recognized risk to groundwater quality. Research has largely focused on in situ CO2-induced pH depression and subsequent trace metal mobilization. In this paper we focus on a second mechanism: upward intrusion of displaced brine or brackish-water into a shallow aquifer as a result of CO2 injection. Studies of two natural analog sites provide insights into physical and chemical mechanisms controlling both brackish water and CO2 intrusion into shallow aquifers along fault zones. At the Chimay贸, New Mexico site, shallow groundwater near the fault is enriched in CO2 and, in some places, salinity is significantly elevated. In contrast, at the Springerville, Arizona site CO2 is leaking upward through brine aquifers but does not appear to be increasing salinity in the shallow aquifer. Using multiphase transport simulations we show conditions under which significant CO2 can be transported through deep brine aquifers into shallow layers. Only a subset of these conditions favor entrainment of salinity into the shallow aquifer: high aspect-ratio leakage pathways and viscous coupling between the fluid phases. Recognition of the conditions under which salinity is favored to be cotransported with CO2 into shallow aquifers will be important in environmental risk assessments.