Sub-seabed geological storage of CO2 in the form of gas hydrate is attractive because clathrate hydrate stably exists at low temperature and high pressure, even if a fault occurs by diastrophism like a big earthquake. For the effective design of the storage system it is necessary to model the formation of CO2-hydrate. Here, it is assumed that the formation of gas hydrate on the interface between gas and water consists of two stages: gas diffusion through the CO2-hydrate film and consequent CO2-hydrate formation on the interface, between film and water. Also proposed is the presence of a fresh reaction interface, which is part of the interface between the gas and aqueous phases and not covered with CO2-hydrate. Parameters necessary to model the hydrate formation in sand sediment are derived by comparing the results of the present numerical simulations and the measurements in the literature.