Abstract: This paper described seven expressions of the second category of water displacement curve $N_{p}=f((\frac{1-f_{w}}{f_{w}}))$,and pointed out that the formulae (9) ~ (14) are equivalent, which can be expressed as $N_{p}=a-b(\frac{1-f_{w}}{f_{w}})^{1/2}$, while water displacement curve expression (15) can be expressed as $N_{p}=a-b(\frac{1-f_{w}}{f_{w}})^{1/3}$.

Key words: Water drive, Displacement curve, Water cut, Rising velocity of water cut, Decline rate