When drilling has reached total depth and the formation to be evaluated has been determined by samples, logs, and cores, a Drill Stem Test may be ordered.
The testing equipment is attached to the end of the drill string. A hydraulic valve system keeps the string dry as it is being lowered into the wellbore. The testing equipment also incorporates a sealing device or packer that effects against the wellbore.
When testing equipment has reached the prospective formation, the formation is isolated from the rest of the wellbore by the packers. A valve opens and the formation is allowed to produce into the dry drill pipe.
At this time, a graphic pressure versus time chart of flow performance is recorded. This chart is produced by mechanical recorders and/or electronic pressure/temperature gauges. The data derived from the Drill Stem Test (DST) give a computation of the formation’s permeability, damage ratio, productivity index, transmissibility and radius of investigation.
After the operator determines that a formation test is to be conducted, the test tool is assembled and lowered into the wellbore. A graphic chart is obtained from two recorders.
Drill stem test (DST) is the conventional method of formation testing and reservoir evaluation which obtains reservoir data under dynamic (rather than static) conditions. A DST is essentially a temporary completion, a method of evaluating reservoir formations without costly and time-consuming completion procedures.
The basic drill stem test tool consists of a packer or packers, valves or ports that may be opened and closed from the surface, and two or more pressure-recording devices. The tool is lowered on the drill string to the zone to be tested. The packer or packers are set to isolate the zone from the drilling fluid column.
The valves or ports are then opened to allow for formation flow while the recorders chart static pressures. A sampling chamber traps clean formation fluids at the end of the test.
Analysis of the pressure charts is an important part of formation testing.
Data attainable from a Drill Stem Test
Normal data recovery from a Drill Stem Test includes items such as fluid recovery and description, blow descriptions test times, mud and hole data and the pressure/time data as recovered from the chart record. These items are reported from the field and recorded on a field data sheet or envelope.
In addition to field data (direct information), additional reservoir characteristics may be calculated utilizing the test data recovered in the field (indirect information). Some of these reservoir characteristics are: