Carbon storage is considered to be a necessary technology in bridging the transition to a low carbon energy mix.  Environmental risks related to the geological storage of CO2 such as well leakage, fault reactivation, seismicity and caprock failure, need to be minimized. In this work, we developed a probabilistic model to  assess the risk of cement sheath failure and the associated well leakage. The approach integrates a finite element model for well integrity analysis with a probabilistic model based on Bayesian Belief Networks (BBN).

In the Netherlands, the prime targets for future storage of CO2 are offshore depleted multi-compartment faulted gas reservoirs. The historical depletion provides a wealth of subsurface static geological data and dynamic pressure/temperature data. CO2 injection in the depleted reservoirs will lead to a significant drop in wellbore temperatures downhole. In case of well re-use, mechanical loads will be outside of the operating envelope for which wells were originally designed, raising the risk of cement damage. In addition, the properties of the aged cement and the formations are highly uncertain. This creates a challenge in deterministic modelling of cement stresses and failure expectations.

The probabilistic model provides (i) the probability of cement failure considering the distributions of the uncertain parameters and (ii) the range of related CO2 leakage in case of failure. The model has been applied to a case study to re-use an offshore well for cold CO2 injection, in a depleted gas field in the Netherlands. This case study will be used to illustrate the practical application of the probabilistic well integrity assessment tool.