In pressure buildup data, which two flow regimes cannot be distinguished separately?

• A. Steady state and transient
• B. Pseudo steady state and steady state
• C. Pseudo steady state and transient
• D. Diffusive and steady state

Answer: (B)

In pressure buildup data, the time history of wellbore pressure can look very similar for two situations: pseudo steady state and true steady state. In pseudo steady state, the pressure distribution around the well reaches a condition where the flow into the well behaves as if the driving gradient is constant, leading to a nearly constant rate of pressure change at the well. In true steady state, the flow and pressure field do not change with time. When you observe the buildup response, the wellbore pressure can rise (or fall) with a nearly constant slope in time for both cases, so the data cannot reliably tell apart a real steady-state condition from a pseudo steady-state condition. The practical upshot is that the late-time buildup pressure history is not enough to distinguish these two regimes without additional information or analyses.