Monitoring CO2: The quest for a clean signal (examples from UT Austin BEG research)

The approach to CO2 monitoring is maturing as a result of knowledge gained from U.S. DOE NETL-funded, research-level CO2 injection demonstration projects, especially the Regional Carbon Sequestration Partnerships. These project have demonstrated successful storage of carbon dioxide (CO2) over the test durations. Here we present results from remediated and active oil and gas sites to get a better understanding of potential leakage signals, and to provide partial analogues for CO2 monitoring best practices. Geochemical sampling of the surface and shallow subsurface is attractive for monitoring CO2 storage for three reasons. First it is less expensive to sample the shallow subsurface (<1 m to 10s or 100s of meters) than it is to access the deep subsurface (100s of meters or kilometers). Second, protection of groundwater for drinking water supply is a key requirement of U.S. and international regulations and geochemical sampling is a well-known approach to documenting this protection. Third, the air/land surface interface is the horizon used for accounting for leakage from the subsurface to atmosphere in several reporting systems. Case studies show that natural processes create dynamic conditions that can mimic impacts from deep CO2 injection or obscure a subtle leakage response. Examples include biogenic degradation of methane (CH4) that produces CO2 and consumes oxygen (O2), water-rock interactions, and climate change. Additional complexity introduced by spatial and temporal changes at oilfield and other developed sites further complicate interpretation of shallow subsurface monitoring data. Other examples of induced change, which might suggest CO2 leakage, include seasonal variations in chemical analytes due to agricultural activities or increased chloride (Cl-) concentration in shallow groundwater that do not appear to be a result of CO2 injection operations. Experience with diverse types of monitoring in near surface environments provides a substantive but previously underutilized resource that can help guide effective monitoring for CO2 storage. Key lessons are to prepare and budget for the expected spatial and temporal complexity that will undoubtedly be encountered during shallow subsurface monitoring in order to extract an accurate and unambiguous (i.e. clean) signal for reporting to stakeholders.