Water chemistry and isotopic characteristics to monitor fluid-rock interactions, during a small scale CO_2 injection

In order to investigate the extent of in-situ CO2-fluid-rock reactions relevant to the effectiveness of CO2 sequestration two single well push-pull tests were performed in a basaltic rock aquifer at the Lamont-Doherty Earth Observatory test well site, New York:\ud - Blank test: a non reactive test, without CO2 equilibration of the injection fluid,\ud - CO2 test: a reactive test, with CO2 equilibration (CO2 pressure ≈ 1 bar) of the injection fluid.\ud For both tests, conservative chemical and isotopic tracers (NaCl, D, 18O) were added to the injection fluid, in order to evaluate mixing between the injected water and the background water in the aquifer. The injection interval, located at the contact zone between the chilled dolerite and the underlying metamorphosed sedimentary rocks, was hydraulically isolated with a straddle packer system.\ud For the blank test, post-injection chemical and isotopic characteristics of retrieved water samples (major ions, DIC, δ13CDIC) remain unchanged, confirming the non reactivity of the system (in the absence of CO2). For the CO2 test, the variations of these characteristics underline the reactivity of the CO2 in the aquifer and allow to identify reactions of the dissolved CO2 with the surrounding rocks, mainly the dissolution of carbonate minerals.\ud Results provided by the CO2 test show that, within the context of this study (incubation period of 3 weeks, injection fluid equilibrated with CO2 pressure of 1 bar); the CO2 reacted with the rock minerals to form carbon-bearing ionic species (HCO3-). Thus ionic trapping was the main trapping mechanism