Analyzing the Risk of Well Plug Failure after Abandonment

All oil and gas wells will have to be plugged and abandoned at some time. The plugging and abandonment procedure must provide an effective isolation of the well fluids all along the well to reduce environmental risks of contamination and prevent from costly remedial jobs. Previous works have analyzed the plug behavior when submitted to local pressure or thermal changes but no work has looked to the effects of external pressure, thermal and stress changes resulting from a global equilibrium restoration in a hydrocarbon reservoir once production has stopped. This work estimates those changes after abandonment on a reservoir field case using a reservoir simulator in conjunction with a geomechanical simulator. Such simulations provide the pressure and thermal changes and the maximum effective stress changes in the reservoir cap rock where critical plugs are put in place for isolating the production intervals. These changes are used as loads in a well bore stress model that explicitly models an injector well and predict stress rearrangements in the plug after abandonment. Results obtained with the well bore stress model for a conventional class G cement plug show that the main risk of failure is tensile failure because of the low tensile strength of the cement. Actually, soft sealing materials or initially pre-stressed plug appears to be more adapted to the downhole conditions changes that may occurs after well plugging and abandonment

Cement-Polymer Composites for Oilwell Cementing

The selection of an optimal cementitious material is critical to maintain zonal isolation for the lifetime of oil & gas or gas storage wells not only for primary cementing but also after well abandonment. Polymer powder/cement composites present good mechanical and durability properties since polymer latex promotes improved adhesion and flexibility to the hardened cement paste. So, for well constructions in aggressive environment, polymer powder/cement composites would be good candidates. During the well construction, the cement slurry placement strongly depends on its rheological behaviour. In this paper, we studied rheological properties of polymer powder/cement composite suspensions: they present a transition from a gel to a liquid behaviour in oscillatory shear. The replacement of cement particles by polymer particles induces a decrease of the storage modulus in the gel state. The good quality of the polymer particle dispersion and their high affinity for cement particles were observed by Scanning Electron Microscopy (SEM). Solid state NMR (29Si and 27Al) and calorimetry allowed to highlight the influence of the polymer addition on both the hydration and hardening of the cement matrix. All these results helped us to understand the relations between the slurry composition, its rheological behaviour and the properties of the hardened materials which allowed us to design new cementing materials