Methane hydrate (MH) is found under the seabed in Japan's exclusive economic zone and is attracting attention as a domestic natural gas resource. One of the methods for extracting methane gas (MG) from sub-seabed MH reservoir is depressurisation. However, the production decelerates after a certain time partly due to the cooling of the reservoir caused by the heat absorption during MH dissociation. Therefore, several enhanced gas recovery (EGR) methods have been proposed, such as injecting CO2 into a sub-seabed sand layer to form CO2 hydrate (CDH) and, using the heat of CDH formation. This method is two functions: EGR and the reduction of greenhouse gas emission. In this study, a numerical method was developed to investigate the performance of the EGR from MH, using the heat of CDH formation. In our previous study, CDH formation model had been installed into a programme code for two-phase flow of CO2 and water in porous media. Based on this code, we newly implemented the state equation, enthalpy model, viscosity model, and heat conduction model of MG and the dissociation model of MH. Then, simulations were performed to form CDH in specific sand-mud alternate layers and to dissociate MH. As a result, it was suggested that increasing the CO2 injection amount potentially increases MH dissociation and the vertical distance between the mud layer capping the injected liquid CO2 and the bottom-simulating reflector of CDH is important to ensure a large CDH region to generate a large amount of heat. It was also shown that even after stopping CO2 injection, MH dissociation can continue for a certain period.