Initial shut-in period:

The comparatively short shut-in period following the initial flow period of a drillstem test.

This period is followed immediately by much longer flow and shut-in periods 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.