EFFECT OF SULPHATE INVASION ON SILICA FUME-MODIFIED CLASS G CEMENT

In aggressive subsurface environments such as oil and gas wells, Class G cement is vulnerable to degradation by sulphate invasion, especially under sulfuric acid exposure, that causes expansive reaction, mass loss and reduced structural integrity. This study examines the effect of various concentration of sulfuric acid on the durability of silica fume-modified Class G cement with the aim of assessing the resistance of the Class G cement to sulphate attack and identify the best formulation which will be used to improve the integrity of wells in sulphate containing formation such as that in Niger Delta of Nigeria. Class G cement samples (320 g per batch) were modified with silica fume at 18% BWOC, bentonite as a density reducing agent (60g), anti-foaming agent (0.02% BWOC), fluid loss additive Biacel-FL (0.26 gal/sk) and dispersant Marabond (0.02% BWOC). The slurry was prepared at a water to cement ratio of 0.44, and mixed at a rate of 4000 rpm for 35 seconds and casted into moulds and cured for 14 days at ambient conditions. Cured specimens (initial dry weight approximately 126 g) were immersed in sulfuric acid solutions ranging from 0.2 to 1.0 mol/dm3 (molar mass 98.08 g/mol, 19.616 g for 0.2 mol/dm3 in 1 dm3) in the presence of up to 28 days to simulate sulphate invasion properties assessed included density (reduced to 10.9 ppg from 11.4 ppg base), thickening time (4:15 hours at 90degF, decreasing with Sharp mass losses were observed by 28 days, increasing with the acid strength (1.0-15.0%), which was attributed to leaching of calcium and dissolution of the matrix. Comparative analysis showed that silica fume reduced losses by 40-60% with respect to unmodified cement due to denser microstructure and low porosity of cement. These results highlight the efficiency of silica fume in preventing sulphate-induced degradation at mild concentrations (