Different innovative experimental and numerical methods are presented to assess strategies and cementing materials aiming to improve well cementing, remediation of fractured cement sheaths and permanent well plugging. The work presented here shows that with minor adjustments, current cement formulations can go a long way in satisfying rigorous integrity criteria, both as well cement sheath and well plug. The methodology developed has two aims: the first is to test and model most realistic and relevant conditions, to probe and prioritize the issues in cementing and remediation; the second aim is to propose the best and most cost-effective solutions and test them for worst-case situations. Proposing improved cementing solutions for challenging wells implies having good numerical models, where the input parameters from dedicated laboratory tests contributes to calibration of the model and strengthen its predictive abilities. Experimental techniques involve testing the cement to formation interface tensile strength, cement sheath integrity under casing cyclic pressure, remediation fluid sealing testing on controlled fractures and numerical simulations of improved well plugging. Care is taken to include as relevant as possible stress and pore pressure conditions in these tests, both when preparing the cement sheath and when testing integrity and remediation scenarios. The different cement interface strength tests revealed the weakest link in the system and thus suggested a strategy of combining cement formulations with different stiffness inas a way to improve the sealing efficiency around the well. This strategy was explored numerically in the case of a cement well plug, suggesting an efficient and cost-effective method to abandon oil and gas or CO2 injection wells.