Before computer aided analysis of DSTs was available each operator had
their own rules of thumb of the "best" times to run the test. Now with
computer aided analysis all of this has changed, there is a quantifiable "best"
way to do it.
Elsewhere on the website is a page outlining in detail the times
Schlumberger and Amoco recommend.
The thought process goes like this.
The Initial Flow/Shut-in period is simply for measuring the original bottom
hole pressure. The only way to measure the pressure is to remove any drilling
fluid that has been lost to the zone. This fluid acts to raise the pressure.
However only the amount of fluid lost to the zone should be removed. Any
fluid removal over this volume acts to decrease the measured pressure.
If too much fluid is removed the pressure must be calculated instead of
The Final Flow/Shut-in portion is where the quantitative analysis is
performed. The accuracy of the calculation is directly proportional to the
amount of fluid withdrawn during the test. It is possible to introduce so much
fluid into the drill sting during the Initial Flow that the Final Flow period
introduces so little that calculations are impossible.
In summary Initial Flow periods longer than about 5 minutes prevent the
measurement of the true bottom hole pressure and they decrease the accuracy of
the Final Flow calculations.