Lawrence Berkeley National Laboratory

The injection and storage of anthropogenic CO2 in deep geologic formations is a potentially feasible strategy to reduce CO2 emissions and atmospheric concentrations. While the purpose of geologic carbon storage is to trap CO2 underground, CO2 could migrate away from the storage site into the shallow subsurface and atmosphere if permeable pathways such as well bores or faults are present. Large-magnitude releases of CO2 have occurred naturally from geologic reservoirs in numerous volcanic, geothermal, and sedimentary basin settings. Carbon dioxide and natural gas have also been released from geologic CO2 reservoirs and natural gas storage facilities, respectively, due to influences such as well defects and injection/withdrawal processes. These systems serve as natural and industrial analogues for the potential release of CO2 from geologic storage reservoirs and provide important information about the key features, events, and processes (FEPs) that are associated with releases, as well as the health, safety, and environmental consequences of releases and mitigation efforts that can be applied. We describe a range of natural releases of CO2 and industrial releases of CO2 and natural gas in the context of these characteristics. Based on this analysis, several key conclusions can be drawn, and lessons can be learned for geologic carbon storage. First, CO2 can both accumulate beneath, and be released from, primary and secondary reservoirs with capping units located at a wide range of depths. Both primary and secondary reservoir entrapments for CO2 should therefore be well characterized at storage sites. Second, many natural releases of CO2 have been correlated with a specific event that triggered the release, such as magmatic fluid intrusion or seismic activity. The potential for processes that could cause geomechanical damage to sealing cap rocks and trigger the release of CO2 from a storage reservoir should be evaluated. Third, unsealed fault and fracture zones may act as fast and direct conduits for CO2 flow from depth to the surface. Risk assessment should therefore emphasize determining the potential for and nature of CO2 migration along these structures. Fourth, wells that are structurally unsound have the potential to rapidly release large quantities of CO2 to the atmosphere. Risk assessment should therefore be focused on the potential for both active and abandoned wells at storage sites to transport CO2 to the surface, particularly at sites with depleted oil or gas reservoirs where wells are abundant. Fifth, the style of CO2 release at the surface varies widely between and within different leakage sites. In rare circumstances, the release of CO2 can be a self-enhancing and/or eruptive process; this possibility should be assessed in the case of CO2 leakage from storage reservoirs. Sixth, the hazard to human health has been small in most cases of large surface releases of CO2. This could be due to implementation of public education and CO2 monitoring programs; these programs should therefore be employed to minimize potential health, safety, and environmental effects associated with CO2 leakage. Finally, while changes in groundwater chemistry were related to CO2 leakage due to acidification and interaction with host rocks along flow paths, waters remained potable in most cases. Groundwaters should be monitored for changes that may be associated with storage reservoir leakage.