Microstructure Behavior of Fly Ash-Based Geopolymer Cement Exposed to Acidic Environment for Oil Well Cementing

Acidizing is a widely used technique to stimulate an oil well in order to enhance its production. When portland cement is placed in contact with acid fluids, it will dissolve and, at some time, will be fully degraded because the acids have reacted with calcium silicate hydrate which resulted in the breaking up of the cement intergranular structure. This in turn will give rise to well integrity issue. Instead of introducing a new acid type, which may not be as effective toward reservoir formation, researchers have come up with a new cementing material, namely geopolymer cement. Literature has shown that geopolymer cement is unreactive toward conventional acids. In addition, its strength may be improved by introducing Nano-silica in the admixture. However, the studies were conducted at ambient condition. The objective of this work is to investigate geopolymerization mechanism and microstructure behavior of fly ash-based geopolymer cement with Nano-silica admixture. Two cement slurries, geopolymer and Class G OPC, were prepared in accordance with API RP-10B. The slurries were cured for 24 h at 130 ∘ C and 20.69 MPa at the HPHT curing chamber before being exposed to 15 wt% acid solution for 14 days. The reaction mechanism of the cement samples was investigated by using FTIR and XRD analyses. The results were further validated by carrying out SEM and EDS tests to evaluate the microstructure behavior and chemical compositions of the cured samples. Results show that fly ash-based geopolymer cement with 1 wt% of Nano-silica additive was the least affected cement samples after acid treatment as compared to a similar weight of Class G OPC.