Subrat,

Good question. The short answer: PI is not a representative parameter for a well flowing under transient flow regime.

The reason is that rate is a function of time and thus for the same drawdown you can get different rates (i.e your rate vs. time is changing for the same drawdown).

When your wave is moving away from the wellbore, you can have short term production (flush) of the fluid near the wellbore (which is close so L is small in Darcy equation and thus Q is high for short period of time and it starts declining as the wave moves away and L increases). As you can see from the picture attached for transient flow, the pressure funnel radius increases with time and this is the reason you can see that Q in transient flow usually decreases with time due to the fact pwf is constant (THP/Choking is held constant in wells) and r is increasing (overall gradient drops in reservoir with time).

To quantify the production of well under transient flow, I would suggest you use dynamic simulation (most accurate as it also takes care of any areal and vertical variation in properties).

I hope this helps and all the best,

PS: pictures reference is Fekete.