Block polymers for the control of the filtrate

The present invention relates to the use of a block polymer as fluid loss control agent in a fluid injected under pressure into an oil rock, wherein: the fluid comprises solid particles and / or is contacted with solid particles in of the oil rock after its injection, the polymer comprising at least three blocks, including: - at least a first block which is adsorbed, preferably irreversibly, on at least part of the particles (p); and - at least one second block (B), of different composition from that of said first block, and average molecular weight greater than 10 000 g / mol and is soluble in the fluid (F). - at least a third type of block (A) or of type (B).