There are one of 4 methods that are used in oil wells
1. Constant PI (Productivity Index)traditionally used in high water cut wells
2. VOGEL which is used where more complex fluid mixtures are encountered
3. Composite IPR where the curve that is used is a hybrid of (1) & (2)
4. IPR plots using data supplied from actual well bore performance or reservoir characteristics studies
Water wells use constant PI (Productivity Index)as the fluid is less influenced by gas and not influenced by oil (gravity and decreasing volumes) etc.