Drill-Stem Testing Methods

Drill-stem testing provides a method of temporarily completing a well to determine the productive characteristics of a specific zone. As originally conceived, a drill-stem test provided primarily an indication of formation content. The pressure chart was available, but served mainly to evaluate tool operation. Currently, analysis of pressure data in a properly planned and executed DST can provide, at reasonable cost, good data to help evaluate the productivity of the zone, the completion practices, the extent of formation damage and perhaps the need for stimulation. A drill-stem test provides an estimate of formation properties and wellbore damage. These data may be used to determine the well's flow potential with a regular completion that uses stimulation techniques to remove damage and increase effective wellbore size.

Reservoir characteristics that may be estimated from DST analysis include:

·      Average effective permeability. This may be better than core permeability since much greater volume is averaged. Also, effective permeability rather than absolute permeability is obtained.

·      Reservoir pressure: Measured, if shut-in time is sufficient, or calculated, if not.

·      Wellbore damage: Damage ratio method permits the estimation of what the well should make without damage.

·      Barriers, permeability changes, and fluid contacts: These reservoir anomalies affect the slope of the pressure buildup plot. They usually require substantiating data to differentiate one from the other.

·      Radius of investigation: An estimate of how far away from the wellbore the DST can "see".

·      Depletion: Can be detected if the reservoir is small and the test is properly run.

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