Strength Development Of Oilwell Cement Sheath Under The Influence Of High Ferrous Ion Concentration In Mixing Water
Obot Daniel Emem
Department of Petroleum Engineering, Faculty of Engineering, Niger Delta University, Nigeria.
Etebu Grace Ijeoma
Department of Petroleum Engineering, Faculty of Engineering, Niger Delta University, Nigeria.
Abstract
This study investigated the effect of high ferrous ion (Fe²⁺) concentration in mix-water on the compressive strength development of oilwell cement sheaths. Eight groundwater samples were collected and analyzed for physicochemical properties, then used to prepare cement slurries in accordance with API Specification 10A (2019). The slurries were cured under simulated downhole conditions to produce cement sheath cubes, which were subjected to compressive strength tests. X-ray diffraction (XRD) was employed to identify chemical compounds in failed cement sheath samples. Results show that compressive strength decreases as Fe²⁺ concentration in mix-water increases, particularly when levels exceed 0.9 mg/L. Compressive strength losses ranged from -288.37 psi to -587.70 psi under various curing conditions, with higher temperature and extended curing time exacerbating the reduction. Mechanistically, the declassification of calcium silicate hydrate (C-S-H) into Fe-S-H and the dissociation of calcium from Ca(OH)₂ into Fe(OH)₂ were identified as the primary causes of strength loss. Notably, applied pressure did not significantly influence the antagonistic behavior of ferrous ions. These findings highlight the critical impact of ferrous ion concentration on the mechanical integrity of Class G oilwell cement sheaths and underscore the need for water quality control in cement slurry preparation to ensure optimal downhole performance