CO2 Leakage Marine Dispersion Modelling for an Offshore Depleted Gas Field for CO2 Storage

A depleted gas field situated in offshore Sarawak has been identified by PETRONAS as a potential CO2 storage development site. As part of the monitoring program, CO2 seepage risk and impact on the marine environment needs to be investigated and predicted. This study focuses on understanding the environmental risks associated with the potential seepage of CO2 gas at the depleted field within the 140 m water column through methods of numerical modelling. Leakage scenarios involving existing plugged and abandoned (P&A) wells as CO2 leakage pathways were modelled with leakage rates of 6 tonnes/year, representing a realistic rate and 500 tonnes/year which represents a more improbable and conservative scenario. The modelling period covers three representative climatic periods for the prevailing monsoons in the South China Sea (northwest, southwest and inter-monsoon). Simulation results showed that with the lower rate, changes to the seawater acidity within the far field region were negligible or undetectable. Under the high seepage rate, the pH plume footprint was predicted to extend beyond 200 m distance from the source point. However, the probability was estimated to be less than 1% while the vertical extent of the plume was limited up to 2 m above the seabed. For both scenarios, the CO2 gas were predicted to be fully dissolved within 5 m above the seabed. Therefore, it can be concluded that there is relatively low risk of impact at the storage field in terms of potential increase in seawater acidity if CO2 seepage occurs during the storage period.