Recent deepwater discoveries in CO2-laden presalt regions have posed new drilling and cementing challenges associated with salt and CO2. Salt can react and gel cement slurry prematurely, resulting in pumping and displacement issues. Further, creeping salts can compromise the mechanical integrity of the cement sheath. Carbonic acid under downhole conditions can react with the cement sheath and adversely affect its integrity. Design of a cementing solution for such environments requires a holistic approach that addresses the chemical and mechanical effects of well operations on the cement sheath during the life of the well.
A combination of chemistry, engineering modeling, thermodynamics, chemical reaction, and experimental characterization was used in this study. The work evaluates the influence of dissolution of different levels and types of salt on the cement slurry's physical properties and modeling the cement sheath response for salt creep loads. It also involved conducting long-term high-pressure/high-temperature (HP/HT) cement carbonation experiments and developing a mathematical model to predict the long-term possibility of corrosion under downhole conditions.
Slurry was designed such that the blend is suitable for salt and presalt zones. By virtue of its chemical nature, the slurry resists dissolution of salt and hence does not cause premature gelling. Mechanical characterization and further engineering analysis predicted that the cured cement had good resistance to failure caused by mechanical loads resulting from salt creep. The slurry was tested to confirm that it mitigates the effect of CO2 corrosion on cement under static and dynamic test conditions. The developed model predicted accurately the extent of corrosion expected under downhole conditions.
The present work provides a solution to cementing salt and presalt zones, with a sound understanding of the associated risks and their mitigation. The cement designs and engineering models developed as part of this study can help reduce the risk of damage to the cement sheath and casing collapse across salt zones. They also help mitigate the corrosion of cement when exposed to CO2 present in presalt zones.