An integral cement sheath that allows zonal isolation is crucial for safe and economical Oil and Gas production throughout the life cycle of hydrocarbon wells. For an integral cement sheath to achieve its goal, both short- and long-term cement properties must be tailored using cement additives to accommodate various cementing applications and well operating conditions. Although various types of additives exist, the current trend in reducing the carbon footprint motivates developing “greener” additives that are environmentally friendly and made from renewable and sustainable sources. Cellulosic particles, Cellulose nanocrystals (CNC) in particular, fulfil these requirements as it is characterized by superior properties. Several studies have showed the significant impact CNC has on the cement slurry; however, the major1ity of these studies were intended for the construction industry while the topic of hydrocarbon and geothermal well cementing was not thoroughly addressed. Increased degree of hydration, flexural strength, tensile and compressive strength, reduction in and alteration of various cement properties are observed upon adding CNCs to the cement. The significant impact CNC has on the cement slurry and the lack of data on the topic drives the interest in investigating the use of CNC as a high-performance cement additive. From this perspective, the main goal of this thesis is to study the CNC influence on well cement using API standard equipment and testing procedures. Based on the experimental findings, CNCs were found to influence cement properties by different magnitudes. For instance, a significant reduction in free water and shrinkage as well as a significant improvement in both long-term compressive strength and early compressive strength development were observed. In addition, increasing CNC concentration caused an increase in cement viscosity where a shear thinning profile is observed from rheological measurements. On the other hand, minor influences were observed in thickening time and static fluid loss. The observed reduction in fluid loss is attributed to the increase in viscosity caused by the addition of CNCs. Finally, compressive strength and rheological measurements were utilized to determine the threshold of CNC agglomeration. The optimum CNC concentration was also determined based on the most significant observed improvements in tested properties.