Introducing a fluid composition and a heat-generating fluid into a wellbore may improve timing and/or conditions of generating in situ heat downhole. The generated heat may be used to melt wax, and dissolve paraffins and asphaltenes or other deposits within the wellbore or in the reservoir itself. The fluid composition may include a base fluid and a metallic powder having a plurality of metallic powder particles. The base fluid may be or include a drilling fluid, a completion fluid, a stimulation fluid, a workover fluid, an activation fluid, and mixtures thereof. Each metallic powder particle may have a metallic particle core, and a coating disposed on the metallic particle core having a coating material. The metallic particle core may be released from the metallic powder particle. A heat-generating fluid may contact the released metallic particle core and thereby generate heat.