the invention provides brine based well drilling and servicing fluids which exhibit relatively thermally stable rheological and filtration characteristics . the fluids comprise a brine , an amorphous silica viscosifier , a biopolymer viscosifier , a fluid loss control additive , and a bridging agent . by utilizing both a silica viscosifier and a biopolymer viscosifier to provide the desired rheological characteristics , the fluids have enhanced thermal stability thus maintaining their suspension and hole cleaning properties over an extended temperature range .