Which reservoir state is defined by the absence of density changes over time?

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

Answer: (A)

Steady-state is the situation where properties do not change with time. If density stays the same as time progresses, there is no accumulation within the reservoir and the system has reached a time-invariant condition where inflow, outflow, and storage balance out. That time-invariance of density is the clearest indicator of steady-state.

Unsteady state would show density changing over time as conditions evolve, such as shifting pressure or temperature or changing production/injection rates. Pseudosteady-state describes a transient that can resemble steady behavior for certain variables, but it is not truly time-invariant. Dynamic equilibrium involves ongoing processes at equal rates, yielding no net change for the specific property, but the question’s criterion—absence of density change over time—best aligns with steady-state.