Well logging and drill-stem testing

After the bore hole has penetrated the possibly productive formations, these formations must be tested to determine if expensive completion procedures should be used. The first evaluation is usually made by well-logging methods, in which the logging tool is lowered past the formations while response signals are relayed to operators on the surface. 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 depend on difference in absorption of atomic particles. With well-logging tools, the exact depths of possibly productive formations are determined. Only a production test, however, can establish the potential productivity of a well.

The production test normally used is the so-called drill-stem test, in which the bore hole is sealed above and below the formation to be tested, with only the drill pipe open to the formation. The drill pipe is then emptied of the drilling mud so that the formation fluid may enter. When enough time has passed, the openings into the drill pipe are closed, and the pipe containing the formation fluid is brought to the surface for analysis. If there is gas present, the gas may flow from the top of the drill pipe during the test.