The present disclosure provides a casing seal for a wellhead, including a casing with an internal surface having an annular groove therein. A fluid seal is disposed in the annular groove and includes an internal bead for sealing against a cylindrical surface of an inner case or tube. A swellable material is disposed within the annular groove and surrounds the fluid seal. An energizing port is in communication with the annular groove for delivering fluid to the swellable material to cause the swellable material to expand and further load the fluid seal.