PNNL

Abstract

We evaluated and compared the effectiveness of four surface-based geophysical monitoring methods to downhole pressure and chemical methods to detect brine and CO2 leakage in underground sources of drinking water (USDW) overlying a CO2 storage reservoir. The six monitoring methods evaluated in this study are suggested in US EPA Class VI regulations for permitting CO2 storage projects. We chose to compare surface-based geophysical techniques with downhole pressure and chemical monitoring because they sample a large volume of the subsurface and have the potential to complement downhole methods. This assessment uses synthetic monitoring data generated from geophysical models of 400 simulated aquifer impact data sets at 13 time steps which capture leakage and hydrologic uncertainty inherent in geologic carbon storage (GCS) sites. 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 if the CO2 is trapped within the aquifer. Among the four geophysical monitoring methods, gravity appears to be the most effective detection method. Although downhole TDS and pressure monitoring methods outperform surface based geophysical methods, geophysical methods help reduce the false negative during the post injection site care because they detect impacted groundwater that downhole measurements do not. 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, delineate the Area of Review (AoR), and guide the monitoring design; (2) downhole TDS sampling to monitor groundwater quality and 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 CO2 and TDS plumes. It is important to combine downhole pressure and TDS measurements with geophysical methods that measure CO2 gas, such as gravity, and TDS, such as ERT, to improve leak detection and add confidence that “no detection” means no leak.