In order to secure long-term storage of CO 2 in the subsurface, one has to be able to model the mass transport of CO2. The physical properties of CO2 have a very strong influence on the mass transport. These physical properties, in turn are dependent on temperature (and pressure). Hence, the modelling of heat transport in the subsurface during CO2 injection should be part of mass transport. In the simple set-up examined in this thesis, the Joule-Thompson cooling (as a result of the pressure drop) plays an important role near the injection point. The amount of cooling at the injection point depends on the temperature of the injected CO2 (and of course on the pressure difference that brought about the cooling). The temperature at that region achieves a stable value with time. The impermeable layer should be deeper than the depth at which the CO2 becomes gaseous as it is easier to trap less mobile supercritical or liquid CO2.