The Drill Stem Testing (DST)

Monday, September 21, 2009 by ShoXee

Drill Stem Testing

A drill stem test (DST) is a temporary completion whereby the desired section of the open hole is isolated, and relieved of the mud column pressure through the drill pipe (drillstem).The decision to run a drill stem test on a zone is often based on shows of oil in the cuttings which, in the opinion of the geologist or engineer, deserve detailed investigation. This may happen many times in the course of drilling a well, with as many as 20 or 30 tests being conducted on a single well. Although the cost of such detailed testing is quite high, it is much better to test for a productive zone rather than miss the zone entirely.

General Considerations

Proper planning and consideration of the factors involved are essential to successful testing. The following points are of particular importance.

1. Condition of the hole: The close tolerance between the hole and the tool assembly requires a full gauge, clean, well bore if the tool is to reach bottom in an undamaged, unplugged condition. Wall cake and cavings shoved ahead of the packer may plug the perforations and choke valve when the tool is opened. It is common practice to circulate for some time prior to testing, so that all cuttings are removed from the hole.

2. Pressure surges: The drill stem test conditions represent a severe case of pressure surge, because the lower end of the pipe is closed, necessitating displacement of the total drill pipe volume. Special consideration should be given to pipe running and pulling speeds to avoid undue pressure surges.

3. Operating conditions:

Length of section: The length and location of the test section governs the amount of tail pipe required and the choice of a conventional or straddle test. The testing of short sections are more conclusive, and are generally preferable. Also, the volume of drilling fluid below the packer should not fill the pipe to such an extent that its back pressure interferes with the test.

Packer seat: The packer seat location (while of no particular importance for tests run inside casing), is critical for a successful open hole test The test should be performed in atrue gauge hole, where the packer is opposite a dense, consolidated formation. Limestones, dolomites, anhydrites, or hard dense shales are all satisfactory.

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Choke sizes: The size of the bottom hole and surface orifices depends on the anticipated test conditions. The bottom choke is of prime importance and is used to govern the flow rate. The top choke is used primarily as a safety measure and should be considerably larger than the bottom choke, to minimize surface pressure in case a flowing test is obtained. The producing pressure drop around the borehole depends on the flow rate, which in turn is governed by the bottom choke size.

Use of cushions: This refers to the practice of placing a certain length or volume of liquid inside the drill pipe, rather than run it dry This is commonly done by two reasons: first, to reduce the collapse (external) pressure on the drillpipe in deep holes, and second to reduce the pressure drop on the formation across the packer when the tool is first opened.

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Length of test: This is difficult, if not impossible, to predict until after the test has commenced and some observations are available. In hard rock areas where the pipe is not in too great a danger of sticking, the flow test is often several hours long. If the fluid has not reached the surface within this period of time, it is usually desirable to leave the tool open, as long as appreciable entry into the pipe is taking place. The shut-in period after the flow test should be long enough to establish a stabilized static pressure.

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