What reservoir state exists when the pressure change with time varies with location?

• A. Unsteady or Transient State
• B. Steady-State
• C. Pseudosteady-state
• D. Dynamic Equilibrium

Answer: (A)

When pressure changes over time and that change depends on where you are in the reservoir, the system is in a transient (unsteady) state. In a steady-state situation, the pressure at any fixed point does not change with time, so dP/dt would be zero everywhere. But here, the pressure signal is evolving and the timing of that evolution varies with distance from sources or boundaries, which means the pressure field is still developing rather than settled. This is exactly what happens during production or injection, as the pressure disturbance propagates through the reservoir and different locations respond at different rates. Pseudosteady-state is a more specific, often later-time condition where the pressure pattern resembles steady flow, while still changing with time, so it isn’t the general description requested by the prompt. Dynamic equilibrium is a broader term and doesn’t precisely capture the evolving, location-dependent pressure behavior described here.