Embedding CO2 Leakage Modelling in MMV Planning for Marine Environmental Risks in an Offshore CCS Application
Authors: Tiwari Pankaj Kumar , Rashidi M Amir , Das Debasis Priyadarshan , Chidambaram Prasanna , Ebining Amir M Syafeeq , Tewari Raj Deo , Masoudi Rahim 
Published: October 2022

Abstract
Carbon Capture and Sequestration (CCS) has potential to provide an amicable solution in reducing CO2 emissions to the atmosphere for perpetuity with zero-degree failure. Geological storage sites are conventionally considered to be safe for CO2 sequestration and are thoroughly simulated for any potential leakage due to loss of structural integrity. The offshore CO2 storage site, if leaks, could potentially affect the marine environment eventually escaping into atmosphere. A prior modelling of potential leakage from identified threats and its impact on marine environment will ensure the safety and substantially aid site- specific and adaptive Monitoring, Measurement & Verification (MMV) planning.
Subsurface integrity study of the storage site integrated with coupled modelling contemplates longterm security of CO2 storage. For a leak to occur through the plugged &abandonment (P&A) wells, one or more barriers must fail. P&A wells integrity feasibility reveals the impact of CO2 interaction with non- CRA composite structure identifying possible leak paths. The probability of a leak from subsurface to the environment through each of the P&A well, depending on its attribution, was estimated. Barrier's failure probabilities in P&A wells are understood under various well leak estimation scenarios. Range of potential leakage rate, in the event of loss of well integrity, was evaluated. To demonstrate the impact of potential CO2 leakage in marine environment through the loss of subsurface or well integrity from identified locations, integrated CO2 dispersion simulation was carried out for multiple potential leakage scenarios and changes in marine water pH were analyzed.
Based on barriers reliability function, the well leakage modelling results suggest that minimum rate of CO2 leakage through the well path that can possibly reach to the seabed is 6-10 tCO2e/year and for extreme case it would be 500 tCO2e/year. The CO2 dispersion modelling were performed for these leakage rates at three different well locations. Three scenarios were considered and simulated for escaped CO2 gas in the water column and its impact on marine environment. The observed change in marine water pH values in near and far-field region were negligible or undetectable. Any reduction in the pH values was predicted to be within the natural variation of the seawater acidity at the storage site with the varying climatic conditions. The CO2 gas bubbles were predicted to be fully dissolved in the water column, no CO2 gas bubbles would reach the surface and escape into the atmosphere for the modelled leakage scenarios in water depths of 140m. In this paper, the integration of well leakage rate modelling with CO2 dispersionsimulation results are discussed to ameliorate the MMV planning for legacy wells to ensure that injected CO2 in the reservoir is intact and safely stored for hundreds of years post injection.