reducing additive filtrate and well fluid

The invention concerns a water-based additive for well fluid, comprising an acrylamide and branched styrene sulphonate copolymer. It also concerns a water-based fluid comprising an acrylamide and branched styrene sulphonate copolymer. The additive is useful for controlling the filtering of a well fluid through the wall. The invention is advantageously applicable to drilling and/or completion of oil wells having a temperature higher than 120 DEG C.