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After perforating, a drillstem test (DST) is performed to test the producing characteristics of a formation. It involves lowering a special tool downhole attached to drill pipe or tubing without removing the drilling mud from the hole. Offshore the DST is run safely in the casing string and the well may be produced for several days. The test information ultimately allows an assessment of the rate at which a well will flow hydrocarbons and for how long.]
The DST tool is designed to determine whether a formation will produce fluids by dropping the pressure in the hole so that fluids will leave the reservoir. The tool is run into the hole with its valve closed so that mud is unable to enter. The inside remains empty and at atmospheric pressure. At the desired depth, usually near the bottom of the hole just above the potential reservoir, a metal cone will automatically slide into a rubber ring, called a packer, which is forced to expand and engage the wall of the hole. A valve is then opened in the tool. If the formation below the packer is porous and permeable then the pore fluids will enter the hole and flow up the pipe. If formation pressure is sufficient, they will flow out at the surface.
After a short flowing period the valve is closed and the pressure below the packer, measured by pressure recorders in the tool, is allowed to build up. It is customary to open and close the valve twice during the test period to indicate how the formation responds to production of fluid. Finally the packer is retracted letting the column of drilling mud back onto the formation preventing further flow, and the tool is pulled out of the hole. There are many variations in tool design. For example, if the well is deep and the pressure of the mud is very high or if the formation is at a low pressure the pipe can be partly filled with water or nitrogen to lower the pressure differential across the packer to better allow flow to the surface. If fluid will not flow to surface a valve can be provided that bypasses the packer. When a test is finished mud is pumped down the annulus forcing formation fluid up the pipe. This is called reversing out. It is also possible to have two packers so that an interval may be tested some way above the total depth of the hole. The packers are then called straddle packers.
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