Field demonstration of CO2 leakage detection and potential impacts on groundwater quality at Brackenridge Field Laboratory

Abstract:

One concern related to GCS is possible risk of unintended CO₂ leakage from the storage formations into overlying potable aquifers on underground sources of drinking water (USDW). Here we present a series of field tests conducted in an alluvial aquifer which is on a river terrace at The University of Texas Brackenridge Field Laboratory. Several shallow groundwater wells were completed to the limestone bedrock at a depth of 6 m and screened in the lower 3 m. Core sediments recovered from the shallow aquifer show that the sediments vary in grain size from clay-rich layers to coarse sandy gravels.

Two main types of field tests were conducted at the BFL: single- (or double-) well push-pull test and pulse-like CO₂ release test. A single- (or double-) well push-pull test includes three phases: the injection phase, the resting phase and pulling phase. During the injection phase, groundwater pumped from the shallow aquifer was stored in a tank, equilibrated with CO₂ gasand then injected into the shallow aquifer to mimic CO₂ leakage. During the resting phase, the groundwater charged with CO₂ reacted with minerals in the aquifer sediments. During the pulling phase, groundwater was pumped from the injection well and groundwater samples were collected continuously for groundwater chemistry analysis. In such tests, large volume of groundwater which was charged with CO₂ can be injected into the shallow aquifer and thus maximize contact of groundwater charged with CO₂. Different than a single- (or double-) well push-pull test, a pulse-like CO₂ release test for validating chemical sensors for CO₂ leakage detection involves a CO₂ release phase that CO₂ gas was directly bubbled into the testing well and a post monitoring phase that groundwater chemistry was continuously monitored through sensors and/or grounder sampling.

Results of the single- (or double-) well push-pull tests conducted in the shallow aquifer shows that the unintended CO₂ leakage could lead to dissolution of carbonates and some silicates and mobilization of heavy metals from the aquifer sediments to groundwater, however, such mobilization posed no risks on groundwater quality at this site. The pulse-like tests have demonstrated it is plausible to use chemical sensors for CO₂ leakage detection in groundwater.