Distortions in the reservoir response due to wellbore storage. The characteristic trends are an early unit slope trend with pressure change and the derivative overlain on the log-log plot, followed by a "hump" in the pressure derivative that gradually disappears as reservoir trends become recognizable. Complex behavior in the wellbore, such as wellbore phase distribution, can result in a more complex transient trend. A crucial part of the transient analysis is to distinguish the effects of wellbore storage from the interpretable reservoir response.