This chapter develop a simple methodology to estimate amount of long-term leakage, which can be utilized by those who do not have sufficient simulation resources to create a basis for CO2 capture and storage (CCS) accounting. The leakage coefficients for estimation are derived based on storage curve calculated by each characteristic. These are location and depth of injection. The methodology can provide the first order estimate of leakage of classified ocean storage of CO2 that would be a basis for developing accounting methodology. In particular, this is intended to contribute to accounting rules for CO2 capture storage proposed by the research project discussed in the chapter. Moreover, project planners and policy makers would utilize the methodology to examine the feasibility of some CO2 ocean storage projects. Difficulty of defining the leakage in ocean storage is due to the reservoir's open structure against atmosphere and continuous interaction between ocean and atmosphere. Under the current inventory system, "emission" is supposed to be counted. When this concept of emission is applied to the leakage in ocean storage straightforwardly, emission or leakage, which means that injected CO2 into ocean reservoir is released to the atmosphere, would not be observed because the partial pressure of CO2 in the atmosphere is higher than that of ocean constantly when viewed as a whole ocean and not local area, so that ocean consistently keeps absorbing CO2 from the atmosphere.
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