A sealing composition for sealing an annulus of a wellbore includes from 20 weight percent to 97 weight percent epoxy resin based on the total weight of the composition. The epoxy resin comprising at least one of 2,3-epoxypropyl o-tolyl ether, alkyl glycidyl ethers having from 12 to 14 carbon atoms, or a compound having formula (OC2H3)—CH2—O—R1—O—CH2—(C2H3O), where R1 is a linear or branched hydrocarbyl having from 4 to 24 carbon atoms. The sealing composition also includes from 1 weight percent to 20 weight percent curing agent based on the total weight of the composition. Methods for sealing a wellbore annulus or casing-casing annulus includes introducing a spacer fluid to the wellbore, introducing the sealing composition to the wellbore, displacing the sealing composition into the annulus, and curing the sealing composition.