PETRONAS has identified a large gas field as a potential CO2 storage reservoir in offshore Sarawak. Using the Carbon, Capture and Storage (CCS) concept, the plan is to inject CO2 into the reservoir for permanent storage with the purpose of mitigating the contribution of CO2 emissions to global warming. An important aspect of CCS is the Measuring, Monitoring, and Verification program, which is needed to ensure safe CO2 injection and storage. This includes understanding of the potential risks associated with leakage of stored CO2. This study focuses on the risk and impact of the CO2 once it escapes the overlying sediments and enters the marine environment.

To describe the behavior of leaked CO2 in the marine environment, it is important to understand the ambient flow conditions at the identified area that would govern the advection and dispersion processes in the seawater column. Hence, long-term 3D-hydrodynamic modelling is conducted to describe seasonal and inter-annual variations of hydrodynamic conditions at the area of interest. Supplementing the flow model is the coupled physical-chemical reactions that will occur if CO2 escapes into the seawater. As CO2 bubbles ascend in the water column, their volume changes because of gas dissolution and reduction in hydrostatic pressure. Additionally, as CO2 gas dissolves in seawater, concentration of the Dissolved Inorganic Carbon (DIC) increases, which in turn leads to reduction of pH in the seawater. Thus, the risk and effects of CO2 leakage to the marine environment will be reflected by reduction of the pH from its natural variation. Scenario of CO2 seepage from plugged and abandoned (P&A) well was simulated based on hypothetical leakage rates derived from previous studies.

Far-field modelling results of CO2 seepage from the P&A well suggest that no CO2 gas would reach the surface and escape into the atmosphere. The CO2 would dissolve rapidly above the seabed. Any reduction in pH values within the far-field is predicted to be within the natural variation of the seawater acidity with the varying climatic conditions. To fully capture the near-field dispersion effects, additional finer resolution modelling was performed for three representative climatic periods (or monsoons). Results suggest that the near-field plume where pH falls below 6.5 (threshold limit based on Malaysia Marine Water Quality Criteria and Standard) is usually confined within 100 m radius but may extend to 200 m from the leakage source. However, the near field model also confirms rapid dissolution of CO2 gas within the first 5 m water column above the seabed.

The study result can be used as an important input in designing X-Field's MMV operational plan in terms of optimizing sampling volume and frequencies for marine water monitoring purposes, which may result in significant operational cost reduction. Hence, similar study is recommended to be conducted with the same purpose in future CCS related projects due to its impact on the technical and economical value creation.

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