A short flow period at the beginning of a drillstem test. This period is followed immediately by a longer shut-in period to allow the pressure to closely approach initial reservoir pressure. The initial flow period is commonly 5 to 10 minutes, and the initial shut-in period is commonly 30 minutes to one hour. When plotted on a pressure buildup plot, extrapolation of the best straight line gives what is usually accepted as the best obtainable value of initial formation pressure.