All DST charts are simply graphs of pressure versus time. Understanding the individual portions of the chart is fundamental to understanding chart interpretation.
There are normally three positions in the testing tool where recorders are run.
The Outside Recorder is positioned close to the perforated interval, with the pressure sensor on the outside of the drill string between the upper and lower packer. The Outside Recorder measures pressure changes at the zone of interest during the test period. In a mechanically successful test, this recorder will give you the most accurate indication of the reservoir pressure.
The Inside Recorder is positioned inside the tool assembly; thus the pressure sensor is inside the drill string. The Inside Recorder measures the pressure of fluid entering through the perforated interval into the inside of the tool. The Inside and Outside Recorders should mirror each other, unless the recorder malfunctions or there is plugging.
The Fluid or Flow Recorder is positioned above the shut-in/flow valve and has the pressure sensor inside the drill string. This recorder may or may not be present on older tests. The purpose of the Fluid/Flow Recorder is to measure the hydrostatic pressure of the fluid recovery.
A fourth, optional, recorder, may be run below the bottom packer in the case of straddle tests. (see Tool Diagrams) This optional recorder, called a Below Straddle Recorder, measures how successfully the bottom packer seat has held.
Each of these recorders will produce a different pressure response during the test. In a properly configured DST, all charts are inspected in the evaluation and interpretation process.
Under ideal test conditions, the recorders would reveal the following:
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lower k, no to minor plugging | |
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lower k, no to minor plugging | |
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pressure of fluid entry during flow periods | |
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bottom packer held; if it's the same shape of inside/outside recorder, then the lower packer failed |