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WFP is performed on stabilised flow  wellbore and reservoir flow and does not cover transient behavior which is normally covered in Well Testing domain.

The wellbore flow is called stabilised if the delta pressure across wellbore is not changing over time.

The formation flow is called stabilised if the well productivity index is not changing over time.

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It's important to remember the difference between constant rate formation flow and stabilised flow formation flow.

The stabilised flow  formation flow may go through a gradually changing flow rate due to formation pressure change, while the productivity index stays constant.

On the other hand, the constant rate flow  formation flow may not represent a stabilised flow  formation flow as the bottom-hole pressure and productivity index maybe still in transition after the last rate change.

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In practice, the productivity index is usually not known at all times as there is not no routine procedure to assess it.

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IPR – Inflow Performance Relation represnts  represents the relation between the bottom-hole pressure 

p_{wf} = p_{wf}(q)

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VLP – Vertical Lift Performance

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VLP – Vertical Lift Performance  also called Outflow Performance Relation or  Tubing Performance Relation represents the relation between the bottom-hole pressure 

p_{wf} = p_{wf}(q)

  which will be non-linear in generic case. 

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