Abstract: A packer for use inside a casing of a subterranean well includes a resilient element, a housing and a rupture disc. The resilient element is adapted to seal off an annulus of the well when compressed, and the housing is adapted to compress the resilient element in response to a pressure exerted by fluid of the annulus of a piston head of the housing. The housing includes a port for establishing fluid communication with the annulus. The rupture disc is adapted to prevent the fluid in the annulus from entering the port and contacting the piston head until the pressure exerted by the fluid exceeds a predefined threshold and ruptures the rupture disc.