Leakage and Seepage in the Near-Surface Environment: An Integrated Approach to Monitoring and Detection

LEAKAGE AND SEEPAGE IN THE NEAR-SURFACE ENVIRONMENT: AN INTEGRATED APPROACH TO MONITORING AND DETECTION Curtis M. Oldenburg and Jennifer L. Lewicki Earth Sciences Division 90-1116, LBNL, Berkeley CA 94720 Abstract Monitoring and detection of leakage and seepage of carbon dioxide (CO 2 ) in the near-surface environment is needed to ensure the safety and effectiveness of geologic carbon sequestration. Large leakage fluxes, e.g., through leaking wells, will be easier to detect and monitor than slow and diffuse leakage and seepage. The challenge of detecting slow leakage and seepage is discerning a leakage or seepage signal from within the natural background variations in CO 2 concentration and flux that are controlled by a variety of coupled processes in soil. Although there are no direct examples of leaking geologic carbon sequestration sites on which to base a proposed verification approach, we have been guided by our prior simulation studies of CO 2 leakage and seepage, which showed that large CO 2 concentrations can develop in the shallow subsurface even for relatively small CO 2 leakage fluxes. A variety of monitoring technologies exists for measuring CO 2 concentration and flux, but there is a gap between instrument performance and the detection of a leakage or seepage signal from within large natural background variability. We propose an integrated approach to monitoring and verification. The first part of our proposed approach is to characterize and understand the natural ecosystem before CO 2 injection occurs so that future anomalies can be recognized. Measurements of natural CO 2 fluxes using accumulation chamber (AC) and eddy correlation (EC) approaches, soil CO 2 concentration profiles with depth, and carbon isotope compositions of CO 2 are needed to characterize the natural state of the system prior to CO 2 injection. From this information, modeling needs to be carried out to enhance understanding of carbon sources and sinks so that anomalies can be recognized and subject to closer scrutiny as potential leakage or seepage signals. Long-term monitoring using AC, EC, and soil-gas analyses along with ecosystem and flow and transport modeling should continue after CO 2 injection. The integrated use of multiple measurements and modeling offers a promising approach to discerning and quantifying a small CO 2 leakage or seepage signal from within the expected background variability. Introduction One of the outstanding challenges of geologic carbon sequestration is verification, that is, ensuring that carbon dioxide (CO 2 ) is not leaking from the intended sequestration formation and seeping out of the ground. The most straightforward way of verifying CO 2 sequestration would seem to be direct monitoring and detection of anomalous CO 2 in the near-surface environment. While catastrophic releases to the atmosphere, such as through well blowouts, will be obvious failures and therefore present no challenge for detection, slow or diffuse leakage and seepage of CO 2 will be much more difficult to detect, monitor, and quantify. The difficulty of observing and quantifying diffuse CO 2 leakage and seepage arises because there are large spatial and temporal changes in CO 2 concentration and flux in natural ecosystems, making the main challenge the detection of a CO 2 leakage or seepage signal from within the natural background variation. We have developed an approach for monitoring and verification that involves a variety of integrated measurements and modeling that could be used to discern a CO 2 leakage or seepage signal. In this brief paper, we summarize our proposed approach for geologic carbon sequestration verification. This approach is guided by results of numerical simulations of CO 2 leakage and seepage that we have carried out over the last few years, and by our experience in monitoring natural systems. Next we review controls on natural CO 2 in the shallow subsurface, and the technologies used for detecting and monitoring CO 2 in the near-surface environment. Finally, we present our ideas for an integrated approach to CO 2 verification. Corresponding author: Tel. (510) 486-7419, Fax. (510) 486-5686, Email: cmoldenburg@lbl.gov