Why is the production rate calculation for a well with an irregular drainage pattern valid only at early times?

• A. Because drainage patterns will change, or the average reservoir pressure will not decline uniformly within drainage areas because of different production rates and depletion rates.
• B. Because the reservoir pressure remains constant over time.
• C. Because drainage patterns will change, and the average pressure will not decline uniformly due to different production rates and depletion rates.
• D. Because the wellbore storage dominates at early times.

Answer: (C)

At the heart of this idea is how pressure within the drainage area evolves when that area isn’t symmetric. With an irregular drainage pattern around a well, different parts of the reservoir contribute fluid at different rates and deplete at different speeds. Early in production, the region that communicates with the well is relatively small and the pressure drop tends to look like a more uniform decline within that limited area, so simple rate calculations that use an average pressure work reasonably well.

As time passes, the drainage pattern changes because more distant zones start to contribute and each zone depletes differently. This causes the average pressure within the drainage area to fall nonuniformly, breaking the simple relationship the early-time calculation relies on. That’s why the production-rate calculation is regarded as valid only at early times.

Wellbore storage can influence behavior early on, but the key point here is that changing drainage patterns lead to nonuniform pressure decline, limiting the validity of the simple calculation to the early period.