Benefits of Conducting a Pressure Drawdown Test

By Chad Cluver | Tue, 1 Sep 2009

There are many reasons to test a well. At DRC the most common type of test we see is the Pressure Build-Up.  It is easy to conduct as it simply requires shutting the well in from stable flow.  However, it requires the well to be shut-in which results in lost revenue to the operator. Low permeability wells require a long time to test which exacerbates the loss of revenue concern.  To avoid the loss of revenue concerns, many operators have turned to the Pressure Drawdown Test to obtain their critical reservoir properties.

A Pressure Drawdown Test is simply measuring the change in BHP during a period of production on a constant choke setting (not constant rate).  A well may decline in rate on a fixed choke, which is natural and can be accounted for in the analysis.  Changing the choke to maintain a constant rate introduces extra transients which can interfere with the test interpretation.  Usually the Pressure Drawdown Test begins with the well in a shut-in state, however it may also be conducted by greatly increasing (doubling) the current rate.  The obvious benefit here is that the well is being tested and produced at the same time.  This allows uninterrupted cash flow while simultaneously providing the engineer with information on the reservoir such as permeability, skin, and reservoir size.  New wells are ideal candidates for drawdown testing as they are already in a shut-in state with uniform reservoir pressure.  This early data obtained when bringing the well online for the first time can prove invaluable later in the life of the well.  A well which has been shut-in for a long period of time for a workover or recompletion presents a good opportunity to conduct a drawdown test.  In the event of planned maintenance where the shut-in is scheduled, the drawdown test may even be coupled with a build-up test.  This can help eliminate any uncertainties in the interpretation of the data from the tests.

A drawdown is less subject to phase re-segregation, cross flow problems, and thermal effects in the tubing compared to a build-up.  If there is multi-phase flow in the reservoir or if the rock is highly compressive then a drawdown test can yield more reliable results than a build-up.  Boundary/reservoir-limit testing is also typically done with the drawdown test.  Also, reservoir dimensioning can take a long time, on the order of weeks or even months to see all limits, making a build-up test impractical; therefore these tests are typically done by performing an extended drawdown.