Theoretical Limits for Seismic Detection of Small Accumulations of Carbon Dioxide in the Subsurface
Seismic imaging has been used very successfully to map the location of injected CO2 at the Sleipner and Weyburn Projects (Chadwick et al., 2005; White, 2005). Theoretical calculations by Myer et al. (2003) suggested that the detection limits for cone-shaped accumulations would be in the range of 10,000 20,000 tonnes of CO2. However, based on actual experience, lower limits for detection of CO2 accumulations at the reservoir depth have been estimated to be in the range of several thousand tonnes of CO2. Recently, a VSP obtained at the Frio Brine Pilot demonstrated the ability to detect as little as 1600 tonnes of CO2.The ability to detect small quantities of CO2 that have unintentionally escaped from the storage formation is essential for providing early warning that the project is not performing as expected. The purpose of this study is to assess, from a theoretical and practical perspective, the minimum amount of CO2 that could be detected using surface seismic imaging as it migrated towards the surface. To this end, calculations have been made to assess the seismic response of accumulations of 1,000 tonnes of CO2 at depths of 1300 m, 1000 m, 800 m, and 500 m below the land surface. Sensitivity studies have been conducted to assess the influence of the accumulation thickness and saturation. Sensitivity studies will also be done to determine if even smaller accumulations could be detected at a depth of 500 m or less. In addition, the influence of multi-layer accumulations will also be assessed.