Drill Stem Testing

After drilling a wildcat well, potential zones of interest in the subsurface may be tested
for hydrocarbon presence by running a DST (drillstem test). Drillstem Tests are run more
frequently in open hole than cased hole. A drill stem test will provide the first opportunity
to collect a major sample of reservoir and to evaluate reservoir flow potential.
After drilling and casing well, a DST assembly is attached to the drillstring and run in the
hole. A downhole shut in valve is installed in the DST Assembly so that the well can be
shut in downhole. The well is perforated with completion fluid I the hole of appropriate
density to achieve underbalance.
Underbalance is achieved when the hydrostatic pressure due to the column of completion
fluid is less than the reservoir pressure. A packer is set in the hole prior to perforating to
prevent reservoir fluids from contacting the casing. The well is flowed at pre-determined
rates and the flowing bottomhole pressure is recorded in a downhole memory guage.
More than one intervals can be tested in this manner.
After the flow test, a pressure build up test is conducted. The well is shut in and the
pressure is allowed to build back to the initial reservoir pressure. From the data collected,
reservoir permeability and skin factor can be obtained. Skin factor is a measure of the
damage done to the formation by drilling mud liquids and solids, which may plug the
pore throats of the reservoir rock, resulting in restriction to fluid flow in the reservoir.