In an embodiment, a sealant composition comprising a swelling agent is displaced into a wellbore penetrating a subterranean formation to maintain isolation of the formation. The sealant composition may be displaced into a permeable zone through which fluid can undesirably migrate. In another embodiment, the sealant composition comprising the swelling agent is displaced into the wellbore to reduce a loss of a fluid to the formation during circulation of the fluid in the wellbore. The fluid may be, for example, a drilling fluid, a secondary sealant composition, a fracturing fluid, or combinations thereof. The sealant composition may be displaced into a flow pathway through which fluid can undesirably migrate into the formation. In both embodiments, the swelling agent contacts water such that it swells and thereby reduces fluid flow through the permeable zone/flow pathway. The swelling agent may comprise a crosslinked co-polymer.