Assessment of geophysical monitoring methods for detection of brine and CO2 leakage in drinking water aquifers

We evaluated and compared the effectiveness of four surface-based geophysical monitoring methods to down-hole pressure and chemical methods to detect brine and CO2 leakage in underground sources of drinking water overlying a CO2 storage reservoir. This assessment uses synthetic monitoring data generated from 400 simulated aquifer impact data sets. The six monitoring techniques can detect impacted groundwater once 20,000 tonnes of CO2 have leaked into the drinking water resource. Geophysical methods are most effective at detecting shallow plumes. Although downhole monitoring methods outperform geophysical methods in detecting deep plumes, geophysical methods may help reduce the false negative during the post injection site care because they detect impacted groundwater where downhole sensors may be absent. This analysis informs future monitoring plans by defining the size of leaks that can be detected for a range of techniques and the need for using complementary methods that measure CO2 gas and changes in groundwater chemistry spatially in addition to discrete point source measurements. A comprehensive monitoring plan consists of (1) site specific reservoir, wellbore leakage and geochemical models to predict likely aquifer impact; (2) downhole TDS sampling to provide an early indication of brine and CO2 leakage; (3) downhole pressure monitoring to track migration of the CO2 plume; and (4) surface-based geophysical methods to track the shallow leakage plumes. It is important to combine downhole measurements with geophysical methods that are sensitive to CO2 gas and TDS to improve leak detection and add confidence that no detection means no leak.