Loss of wellbore fluids (such as drilling fluids, completion fluids and workover fluids) into the flow passages of a subterranean formation may be reduced or eliminated by introducing into the wellbore in communication with the formation a composition containing a composite of a deformable core having a hardened coating which contains a viscosifying agent and, optionally, a crosslinking agent. The viscosity of the composition increases in-situ as the viscosifying agent, and optional crosslinking agent, disassociate from the deformable core and react. A fluid-impermeable barrier is thereby formed. The composites may also be used in spacers, well cements, workover and completion fluids as their need arises.