When the pressure at different locations in the reservoir is declining linearly as a function of time, i.e., at a constant declining rate, the flowing condition is characterized as the pseudosteady-state flow. Mathematically, this definition states that the rate of change of pressure with respect
to time at every position is constant, or
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It should be pointed out that the pseudosteady-state flow is commonly referred to as
semisteady-state flow and
quasisteady-state flow.
Ahmed (2001)