Geologic storage of carbon dioxide as a climate change mitigation strategy: performance requirements and the implications of surface seepage

The probability that storage of carbon dioxide (CO2) in deep geologic formations will become an important climate change mitigation strategy depends on a number of factors, namely (1) public acceptance, (2) the cost of geologic storage compared to other climate change mitigation options, and (3) the availability, capacity, and location of suitable sites. Whether or not a site is suitable will be determined by establishing that it can meet a set of performance requirements for safe and effective geologic storage. To date, no such performance requirements have been developed. Establishing effective requirements must start with an evaluation of how much CO2 might be stored and for how long the CO2 must remain underground to meet goals for controlling atmospheric CO2 concentrations. Answers to these questions provide a context for setting performance requirements for geologic storage projects. According to the results presented here, geologic storage could be an effective method to ease the transition away from a fossil-fuel based economy over the next several centuries, even if large amounts of CO2 are stored and some small fraction seeps from storage reservoirs back into the atmosphere. An annual seepage rate of 0.01% or 10(-4)/year would ensure the effectiveness of geologic carbon storage for any of the projected sequestration scenarios explored herein, even those with the largest amounts of storage (1,000 s of gigatonnes of carbon-GtC), and still provide some safety margin. Storing smaller amounts of carbon (10 s to 100 s of GtC) may allow for a slightly higher seepage rate on the order of 0.1% or 10(-3)/year. Based on both the large capacity of geologic storage formation and the likelihood of achieving leakage rates much lower than the rates estimated here, geologic storage appears to be a promising mitigation strategy.