Assessment of the potential for CO2 leakage from geological storage sites is essential for the implementation of CO2 capture and storage in geological media. Possible pathways for CO2 leakage from a storage site include natural interruptions and breaches through the confining strata, faults and fractures, and degraded wells. Knowledge of the geology and stress regime is essential in assessing the potential for CO2 leakage through natural features and induced fractures. Assessment of the potential for leakage through degraded wells is much more difficult because of the large number of wells, the lack of knowledge about their condition, and the computational difficulties relating to the simulation of CO2 leakage through many wells across a multilayered succession of aquifers and aquitards. The large number of wells and the variability in their present and future conditions require a stochastic approach by which a large number of statistical realizations provides a probability distribution for CO2 leakage. The large disparity between the length scales associated with injected plumes and those associated with leakage pathways along wells leads to numerical intractability for statistical simulations. Semianalytical models, although constrained by assumptions needed to solve the mathematical system of equations, provide a framework for estimating the potential for and rates of CO 2 leakage through degraded wells. An example from the Alberta Basin in Canada provides an illustration of the types of information these models can generate. The models must be coupled to specific field observational and measurement programs to support full implementation of CO2 geological storage.
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