A system for a wellbore can include a tubing string positioned downhole in a wellbore and a porous gravel pack. The porous gravel pack can be positioned in an annulus defined at least in part by a surface of the tubing string. The porous gravel pack can be made up of a plurality of grains. Each grain can include a core surrounded at least in part by a sheath. The core can comprise an expandable material that can expand in response to coming in contact with a fluid. The sheath can surround the core and can be deformable in response to the core expanding in response being exposed to a fluid.