A Probabilistic Well Integrity Analysis Workflow for Leakage Risk Assessment: Case Studies for Shale Gas and Re-Use for CCS

Well leakage risk is a key concern for the safety and economics of geological CO2 storage. Maintaining the integrity of annulus cement in the wellbore during the lifetime of well is of paramount importance. Cement properties and the stress state in cement at the downhole conditions are however uncertain and difficult, if at all possible, to measure. To investigate effects of uncertain input parameters on well cement integrity, we performed a modelling study. The risk of cement sheath failure was assessed using the probabilistic numerical modelling of well integrity. Next, simulation results were fed into the statistical model based on Bayesian Belief Networks (BBN) for convenient data interrogation and risk analysis. The predictive ability of BBN models increases with the number of simulated wells, case studies and potential leakage scenarios. The modelling approach was applied to two case studies: (i) re-use of a gas well for CO2 injection in a depleted gas field in the offshore Netherlands, and (ii) a shale gas well in Poland. For the CO2 well, the predicted cement failure mode was debonding along the well cement interfaces leading to the creation of microannuli gaps, regarded as the most probable leakage paths. For the shale gas well, the likely failure mode was shear cracking. The probability of failure increases the most for cements with high stiffness and high shrinkage, while changes in the operational parameters considered here showed lower impact. The largest risk reduction in new wells could be achieved by cement formulations exhibiting lower stiffness (5 GPa) and smallest shrinkage levels practically achievable. The modelling methods presented here can be used for designing cement formulations for new wells, and for assessing the integrity and re-use potential of existing wells for CO2 injection.