Abstract: This paper presents the normalization processing of relative permeability curves from reservoirs, establishes the relationship between productivity index and water cut based on Darcy's law, and obtains the ultimate water cut from given well economic limit rate and reservoir limit producing pressure differential. The case study of low permeability reservoir demonstrates the validity and the practicability applied to determination of ultimate water cut in oilfield development.

Key words: water cut, productivity index, fluid productivity index, method