Those of you familiar with well testing may be aware of a "drawdown/buildup" test. For those not familiar, this type of test involves flowing a well to create a pressure "drawdown", then shutting in the well to allow for a pressure "buildup". If done correctly, the pressure response during the drawdown and buildup can be analyzed to determine important things like permeability, skin factor, average reservoir pressure, and even boundary conditions.
A really simple rule of thumb for well testing is that a buildup test should be about twice as long as a drawdown. This means that if you flow the well for 1 hour, you should shut it in for buildup for about 2 hours. Often, we reverse-engineer this when designing a test. If you want a larger radius of investigation and you determine that you need a 12-hour buildup, then you should plan to flow the well for 6 hours prior to the buildup.
This rule of thumb is all about data quality and getting the most out of your test. Nothing bad happens if your buildup is too short or too long. You just might have data quality issues. If your buildup is too long relative to the drawdown, you may see some strange things happen on your derivative. These strange things can easily be misinterpreted as reservoir effects, when in reality it's just a mathematical limitation to the pressure transients.
There are a lot of other things that go into well test design as well. If you need help designing a well test, be sure to call your friendly, local well testing specialists (aka FyreRok).