Progress in monitoring strategies for risk reduction in geologic CO 2 storage

Abstract Commercial, demonstration and pilot geologic CO2 storage (GCS) projects have provided a wealth of practical experience in monitoring technologies. In this manuscript we review a few key projects, specific geophysical methods, well-based methods, geophysical workflows and some suggested best practices. These practices are often driven and governed by regulatory requirements (e.g. U.S. Environmental Protection Agency [US-EPA] Class VI underground injection regulation) and the need for effective reservoir management of pressures and volumes as well as the site-specific risks to containment, plume migration, and underground sources of drinking water. Reducing uncertainty in monitoring is likely to lead to reducing uncertainty in risk. One of our conclusions is the need for field experiment testing that is risk-driven. It is notable that GCS pilot projects have been designed for safe storage, and to date they have been successful. Technical challenges at all geologic CO2 storage sites include assuring injectivity and capacity predictions are reliable, the fate and subsurface topology of the CO2 plume is acceptable, regulatory permitting and compliance are accomplished in a timely manner, needed business assurances are supported by quantitative technical data, and the storage operation is acceptable to stakeholders. There is risk associated with all of these challenges necessitating a monitoring program utilizing both direct and indirect monitoring methods. Geophysical monitoring is viewed as a key component in approaching risk assessment because of the direct association of risk and monitoring in GCS. In this review we provide a framework for monitoring and then present some specific monitoring tools that may be useful for regulatory compliance, as well as technologies that are applicable to leakage detection. Since the Department of Energy National Risk Assessment Program (DOE NRAP) is focused on quantitative risk assessment, we focus on quantitative geophysical monitoring. Important concepts for a successful integration of monitoring and risk assessment include the following: Characterization of key parameters of risk and methods to monitor those parameters, the feedback between risk and monitoring, and the development of risk-based strategies to assess and update monitoring plans and tools. We briefly review current work in assessing uncertainty in important monitoring technologies, such as seismic and electrical geophysical surveys, and acknowledge that there is both temporal and spatial uncertainty in monitoring, in addition to the measurement uncertainty of specific tools. In our opinion monitoring approaches for subsurface imaging, such as 3D-reflection seismic, 3D-Vertical Seismic Profiling (VSP), cross well seismic, electromagnetic and electrical technologies, and well-log based measurements produce proxies for saturation and mass that, when properly calibrated and interpreted, are robust quantitative estimates of these key storage parameters. Understanding the uncertainty in these proxies is important in risk-driven monitoring. The cost of high-potential geophysical monitoring technologies impacts application of these methods to quantitative risk assessments. We therefore examine the concept of value of information (VOI) that is expected to play a role in selection of monitoring tools. We also conclude that monitoring of leakage scenarios has had minimal field validation and thus quantifying and reducing risk will require dedicated experiments that probe and test uncertainty in fault/fracture-zone and wellbore leakage.