Elastic parameters of a new cement system can be specified to meet requirements for long-term mechanical durability. These cement systems also can contain expanding additives to prevent loss of zonal isolation caused by changing downhole conditions. Cements with a high tensile strength to Young's modulus ratio and with a low Young's modulus value compared with that of the rock are the best cements for mechanical durability. Wells cemented with expanding cements in unconsolidated sandstones risk debonding between the cement and the casing string as the cement expands radially outward in the direction of least resistance. Expanding conventional cement systems work best in hard rock formations.