What is the production rate divided by the pressure difference in a well?

• A. Productivity Index (J)
• B. Skin
• C. Porosity
• D. Flow rate

Answer: (A)

The main concept here is the productivity index. It measures how much fluid a well can produce for a given pressure difference between the reservoir and the flowing bottomhole pressure. In formula form, q = J × (p_reservoir − p_wf). So the production rate divided by the pressure drop gives you the productivity index J. A high J means the well delivers more oil or gas for the same pressure drawdown, while a low J indicates limitations like lower formation permeability or skin damage that hinder flow. Porosity describes how much pore space the rock has and doesn’t by itself tell how easily fluids move for a given pressure change, and flow rate alone doesn’t account for the driving force.