This chapter develops a simple methodology based on quality of cap rocks to estimate amount of CO2 leakages. To quantify quality of cap rocks, Cap-rock Quality Factors (CQF) is developed. CQF is calculated reflecting features of site, height of cap rock, depth of the site and possibility of three types of leakage, leakage via matrix of cap-rock, leakage via fracture of cap-rock, and leakage via wells. Two different interpretations of CQF, Conduit Model and Membrane Model are used to estimate amount of leakage by each categories. Conduit Model can be generally applicable to certain structural seal category and Membrane Model applicable to uncertain seal category. The methodology can be utilized by those who do not have sufficient simulation resources and provide the first order estimate of leakage of classified geologic 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 and storage (CCS) proposed by research project. Moreover, project planners and policy makers would utilize the methodology to examine the feasibility of some CO2 geologic storage projects.
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