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Drill Stem Test

The drill stem test is used primarily to determine the fluids present in a particular formation and the rate at which they can be produced. The test is run in a bare hole filled with drilling mud. Pressure exerted by the drilling mud in the well prevents fluids from flowing out of the reservoir rocks into the well. A hollow pipe called a drill stem is lowered down the well. The drill stem has two expendable, devices, called packers, around it. The drill stem is lowered into the well until one packer is just above the formation to be tested and the other below. The packers are then expanded to close the well above and below the formation. Sealing the well around the formation eliminates the pressure exerted by drilling mud on the formation. Water, gas, or oil can flow out of the formation and into the well. A trap door is opened on the drill stem and the formation fluids flow into and up the drill stem. If gas is present, it will flow up the drill stem and onto the surface where it is measured and flared (burned). Sometimes oil has enough pressure to flow to the surface during a drill stem test. Usually, however, the oil fills the drill stem to a certain level. This is measured and recorded. During the drill stem test, pressure of the fluid flowing into the drill stem is continuously being measured by an instrument in the drill stem. The trap door on the drill stem is opened and closed several times and fluid pressure build up and drop are recorded. These records are used by engineers to calculate formation permeability, reservoir fluid pressure, and extent of formation damage.