One of the critical issues that might happen in some geothermal wells is the failure of the cement sheaths and its debonding from the casing string. Cement shrinkage during setting is regarded as one of the main causes behind the formation of microannulus cracks. In this presentation, a new class of polymer-based expandable additives in the form of fibers is presented to compensate for cement shrinkage and address debonding issues of the geothermal wells. The proposed fiber additives are made from shape memory polymers. These fibers expand upon exposure to temperatures above a specific point, which is by design below the formation temperature at the cementing zone. Fibers expansion occurs before setting but after placement of the cement slurry to avoid microfractures. The expanded particles also helps sealing natural fractures to prevent cement loss to the formation. As a result of the controlled expansion of the cement paste, flow channels and fluid migration significantly reduce while maintaining the mechanical properties required for the mechanical integrity of the cement sheath. The bridging effect of the fibers helps control propagation and coalescence of fractures. Considering the inert property of the proposed additive, the water cement ratio and its chemical properties do not need to be revisited. The measured enhancement of cement toughness makes the cement system more resistance against cracking. The cement expansion, fluid loss, gel strength, compressive strength, ductility and tensile strength of the samples containing these fibers are examined by destructive and non-destructive methods as reported here. The proposed class of expandable additives can help the operators reach sustainable well integrity by improving the bonding between the cement and casing.