Gas reservoirs often contain a proportion amount of naturally occurring CO2, ranging from a few percent up to 70% by volume. A carbonate gas field located in offshore Sarawak, eastern Malaysia has a very high concentration of CO2 and expected to be a world pioneer offshore CO2 storage in high CO2 field with the accumulated amount of 70 % mole of the gas in place. Carbon dioxide from the gas field is removed from the gas production stream in a central gas processing facility and then the CO2 is compressed, transported and stored underground in the carbonate reef reservoir of the same gas field. This study details the geochemical effects of CO2 injection into the field as part of the proposed field development. In this study, we ran a two-dimensional reactive transport models using TOUGHREACT. The model was run for 300 years at the temperature of 140°C and pressure of 352 bars. The CO2 was injected for 30 years below gas water contact (GWC) in the aquifer. The modelling results shows that the overall geochemical alterations of injected CO2 into solid phase are minimal and is confined to the region below the original gas water contact with the minimal additional impact on injectivity. This is due to the dry-out formation vicinity to the injection well that lead to the small amount of salt precipitation. These findings demonstrate the importance of geochemical reactions study when injecting CO2 into high CO2 gas field at high pressure and temperature.