The ongoing challenge of mitigating climate change has emphasized the importance of technologies like Carbon Capture and Storage (CCS). Among various options, the sequestration of CO2 in depleted gas fields presents a promising solution, particularly due to the availability of well-characterized geological data and the inherent capacity of these fields to store large volumes of CO2. However, injecting CO2 into these reservoirs, introduces significant thermal and geomechanical challenges. This thesis investigates the near wellbore effects during CO2 injection, focusing on the Joule- Thomson cooling effect, which can lead to substantial temperature drops and associated risks such as hydrate formation, wellbore thermal stress, and potential over-pressurization.