The invention provides aqueous based well drilling and servicing fluids containing a polymer viscosifier with enhanced thermal stability provided by a diacid or diacid salt preferably having about two to about eight carbon atoms, such as, for example, sodium oxalate. Cooling fluid is circulated by a submersed circulator, such as a motor, at a substantially constant velocity past a substance to be cooled. The articulated door also includes a flex point permitting selective deployment of the articulated door, such as a tambour door.