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AAfter the borehole has penetrated a potential productive zone, the formations must be tested to determine if expensive completion procedures should be used. The first evaluation is usually made using well-logging methods. The logging tool is lowered into the well by a steel cable and is pulled past the formations while response signals are relayed to the surface for observation and recording. Often these tools make use of the difference in electrical conductivities of rock, water, and petroleum to detect possible oil or gas accumulations. Other logging tools use differences in radioactivity, neutron absorption, and acoustic wave absorption. Well-log analysts can use the recorded signals to determine potential producing formations and their exact depth. Only a production test, however, can establish the potential productivity of a formation.


The production test normally employed is the drill-stem test, in which a testing tool is attached to the bottom of the drill pipe and is lowered to a point opposite the formation to be tested. The tool is equipped with expandable seals for isolating the formation from the rest of the borehole, and the drill pipe is emptied of mud so that formation fluid can enter. When enough time has passed, the openings into the tool are closed and the drill pipe is brought to the surface so that its contents may be measured. The amounts of oil and gas that flow into the drill pipe during the test and the recorded pressures are used to judge the production potential of the formation. (If there is gas present in the formation, the gas may flow from the top of the drill pipe during the test.) Similar tools are available that can seal off and test a formation in a cased well bore or that can bring a small sample of produced reservoir fluid to the surface at reservoir pressure