EFFECT OF SULFATE INVASION ON FLY ASH MODIFIED CLASS G CEMENT

This study investigates the effect of sulfate invasion on fly ashmodified Class G cement, with the aim of improving the durability and chemical resistance of oilwell cement systems exposed to sulfate rich environments. Class G cement is widely used in oil and gas well construction, however its long-term performance can be compromised by sulfate attack, which leads to the formation of expansive products such as ettringite and gypsum, resulting in structural instability, cracking, and loss of well integrity. In this work, Class G cement slurries were formulated with and without fly ash, alongside other additives such as bentonite, sodium chloride, and fluid-loss agents. Measurements of slurry density and thickening time were carried on both cement slurries.T he cement samples were immersed in sulfuric acid (H₂SO₄) solutions of varying concentrations to simulate sulfate exposure and weight change were taken over a period of 7, 14, 21, and 28 days. The fly ash modified samples exhibited progressive but controlled weight gain due to the formation of reaction products that initially filled pore spaces, indicating delayed sulfate penetration. Overall, the findings demonstrate that fly ash enhances the sulfate resistance of Class G cement by refining the microstructure, lowering permeability, and reducing the availability of reactive calcium hydroxide. The study recommends the incorporation of fly ash in oil-well cement formulations to improve long term well integrity in sulfate bearing formations.