Theoretical study of water saturation in oil-water fluid flow area is meaningful to the high, stabilized production
and the improvement of ultimate recovery. Combining relational expression between relative permeability and water
saturation with the fractional flow equation produced an expression of water cut changed by water saturation; Introducing
Vieta's theorem to the differentiation of water cut gave a theoretical expression of water saturation in oil-water fluid flow
area; Introducing the concept of frontal water saturation to these expressions give the continuous expression of water saturation.
Applying this formula to certain sandstone reservoir in China achieves good results: In oil-water seepage process,
water saturation is increasing by frontal distance, and the frontal distance increases over time. This new water saturation
equation lays a base for the study of residual oil mobility and enhanced oil recovery.