A well system (100) includes a production string (130) adapted to extend from a wellhead (120) to a subterranean zone (110). The production string (130) includes one or more joints of tubing comprising a bore; a seal (150) actuable by a specified pressure in the bore to substantially prevent fluid communication in an annulus (140) between a first portion of a wellbore (115) and a second portion of the wellbore (115), and an intervention sub (200) comprising a body (210) and a plurality of ports (228, 230) adapted to allow fluid communication between the bore and the annulus (140). The annulus is disposed between an exterior surface of the production string (130) and the wellbore (115). The intervention sub (200) further includes a seat (226) adapted to receive a plug (224) from the terranean surface (105) and seal the bore to substantially prevent fluid communication through the ports (228, 230).