Abstract: In accordance with wellbore instability induced by bound water on clay surface, the types and boundary line of bound water on clay surface is quantitatively determined by ion exchange.The results show that the bound water on clay surface includes strong bound water, loose bound water(osmotic bound water)and free water. The relative vapor pressure p/p_s=0.9 or 0.98 is the special moisture, the fomer is the boundary line between
the strong bound water and the loose bound water(osmotic bound water)on clay surface, the latter is the boundary line between the loose bound water and free water on clay surface.

Key words: clay, bound water, boundary line, ion exchange, special moisture