Abstract: Polymers comprising polyacrylic acid or polyacrylamide grafted onto a polyethylene oxide/polypropylene oxide copolymer and/or charged nanoparticles may be added to an aqueous downhole fluid having polymers therein to viscosify the aqueous downhole fluid using an amount of such grafted polymers that is less than the amount of polymers that would be needed to viscosify an identical aqueous downhole fluid absent such grafted polymers. The grafted polymers may confer a reversible viscosity property to the fluid when the grafted polymers are dissolved in water or brine in a certain pH range.