A method of shale inhibition comprises adding a two-phase polymer system of a water soluble copolymer of vinyl pyrrolidone and dimethylaminopropyl methacrylamide and in-situ formed water-insoluble particles of said copolymer, or, alternatively, a water-soluble copolymer of methylvinyl ether and maleic anhydride, into a drilling well fluid under aqueous, high temperature, high salinity conditions.