Loss circulation fluids contain aqueous based compositions having at least one alkali metal formate and at least one thermally activated self-crosslinkable gel forming material, wherein the composition is easily transported to a loss circulation zone and yet has ability to close off the loss circulation zone once thermally activated to reduce the loss of circulation fluids into flow passages of a subterranean formation. In addition, packer fluids contain aqueous based compositions which contain at least one alkali metal formate and at least one thermally activated self-crosslinkable gel forming material, wherein the packer fluid forms a thermal insulating fully hydrated gel once thermally activated.