A composite used to slowly release well treatment agents and/or tracers into a well contains a calcined substrate comprising a metal oxide coated onto at least a portion of a core and a well treatment agent adsorbed onto or absorbed into the interstitial spaces of the metal oxide coating of the calcined substrate. The core contains a material sufficient in strength to prevent closure of a fracture created or enlarged within a subterranean formation penetrated by the well at in-situ reservoir conditions.