US Patent 7647970 Self-sealing well cement composition

A cement composition is provided including a cement and a low reactivity particle coated with a swellable polymeric material. The low reactivity particle is coated with a swellable polymeric material at a concentration of about 10 weight percent to about 50 weight percent, based on the weight of the cement. The swellable polymeric material swells to a larger volume when exposed to one or more hydrocarbons, depending on the selected polymeric material. The cement composition is used in a method as a self-sealing cement for cementing an oil and gas well, thereby minimizing or mitigating the unwanted migration of water or hydrocarbons.