| Cementing operations constitute a critical component in ensuring the integrity of wellbores, necessitating the provision of highly durable and mechanically robust wellbore cement for long-term zonal isolation throughout the well's operational lifespan. The integration of nano-carbon materials as additives has emerged as a promising avenue for significantly enhancing mechanical properties. This study conducts a comprehensive experimental assessment of the concurrent impact of nano carbon black (NCB) incorporation on cement sheath properties in both aqueous and hardened phases, simulating high-pressure and high-temperature (HPHT) conditions prevalent in wellbore environments. NCB particles are introduced as fillers at varying concentrations of 0.05%, 0.1%, and 0.2% by weight of cement (BWOC) into Class-G wellbore Portland neat cement, commonly employed in the oil and gas industry, to formulate cement slurry mixtures containing NCB. These mixtures are subsequently compared with NCB-free cement slurries. The analysis examines various critical parameters, including density, rheological characteristics, free fluid content, fluid loss, thickening time, sedimentation, compressive strength, tensile strength, porosity, and permeability, all assessed by the standardized testing guidelines of the American Petroleum Institute (API 10A and API 10B-2). Cement chemical additives are utilized as a reference formulation to maintain consistency in cement slurry composition across both pure cement slurries and those incorporating NCB. The findings reveal that the addition of NCB at the optimal concentration of 0.1% by weight of cement (BWOC) yields notable improvements in wellbore cement performance. This enhancement is attributed to several key factors: there is no significant impact on slurry density and rheological properties; the slurry exhibits enhanced stability, facilitating accelerated hydration. However, it is worth noting that higher NCB concentrations result in nanoparticle agglomeration, necessitating a rigorous dispersion process to maintain cement slurry stability. Compressive strength increased by 10.09% (API 10A), 13.66% (1-day curing, API 10B-2), and 12.35% (7-day curing, API 10B-2). Tensile strength notably increased by 77.46% (7-day curing, API 10B-2). These enhancements were accompanied by improvements in the cement matrix's microstructure, reducing the porosity and permeability of the cement sheath. Additionally, this formulation saved rig time by reducing wait-on-cement periods, thereby lowering field cementing costs and enhancing wellbore integrity over the well's lifetime. Furthermore, it reduced the need for interventions in response to realistic downhole conditions during production, positively impacting both cement sheath quality and environmental considerations. | Abstract |