INTERA, in association with Mitsubishi Materials Corporation (MMC) and Mitsubishi Corporation (MC), was recently awarded a contract with Japan Carbon Capture and Storage Company (JCCS), a consortium of over 30 companies, to support the geological sequestration of carbon dioxide (CO2) in Japan. Geological sequestration involves the long-term underground storage of greenhouse gases, such as CO2, typically in depleted oil and gas reservoirs. INTERA's scope on this project entails developing an appropriate methodology for guiding the assessment of risk of CO2 leakage via abandoned wells. Two hypothetical sites have been provided as the basis for numerical simulations to estimate the extent and properties of the CO2 plume in the reservoir as a function of time. Other detailed process modeling will involve a quantitative estimation of CO2 leakage via different potential leakage pathways (e.g., between cement seal and casing, or annular cement and formation) as a basis for carrying out a probabilistic treatment of leakage rates and amounts for different types of wells drilled into the CO2 reservoir. Ultimately, the results will be used to rank these different types of wells in terms of their potential for leakage.