Enhanced cement strength with selenium-modified nanocomposites synthesized via green chemistry

Maintaining zonal isolation and structural integrity in deep-well cementing operations under demanding downhole environments remains a challenge due to cement degradation and fluid migration. This study addresses this issue by incorporating a novel biogenically synthesized Se@Ag/AgO–ZnO nanocomposite into a Class G cement matrix blended with 10 wt% pozzolan to refine its microstructural, rheological, and mechanical properties. The nanocomposite was synthesized via a green co-precipitation method using Pinus nigra pollen extract as a reducing agent, followed by a mild thermal treatment at 200 °C. Slurries were prepared with nanocomposite concentrations ranging from 100 to 10,000 ppm at a water-to-cement ratio of 0.50 and evaluated at 80 °C. Dynamic light scattering confirmed that suspensions at or below 1000 ppm remained colloidally stable under 100 nm for 24 h. Experimental results revealed that adding 500 ppm of the nanocomposite optimized the system, yielding a sixfold decrease in permeability from 0.6 to 0.1 md and reducing API fluid loss from 20.3 to 2.6 mL after 30 min. Furthermore, the 7-day and 28-day compressive strengths increased to 4100 psi and 4800 psi, respectively, while the slurry thickening time remained stable at approximately 10.2 h. These enhancements are attributed to the synergistic nano-filler effect and accelerated nucleation of calcium silicate hydrate phases facilitated by the uniformly dispersed nanoparticles. Ultimately, this study demonstrates that low-dosage biogenic nanocomposite modification significantly enhances the durability and sealing efficiency of oilwell cement sheaths.