A composition and method is given for a well control fluid that is injected into a well during completion or remediation and prevents fluid flow from a formation into the well during operations (such as shut-in, or placement or adjustment of tools/hardware) when no fluid flow is desired. The composition includes a viscoelastic surfactant fluid system (VES) and a decomposable first solid material, preferably in fiber form, that forms a pack or plug in the well at the location where fluid would otherwise leak off, and then decomposes without intervention to release a product that is a breaker for the surfactant or the micelles of the VES. The composition optionally also contains a pH control agent and a second solid (that may also be a fluid loss agent) that affect the decomposition of the first solid.