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:
1. Permeability
The permeabilities calculated utilizing DST data are average effective permeablities. These are the best possible measurements since they are obtained under actual reservoir conditions.
2. Wellbore Damage
The determination of the magnitude of wellbore damage is one of the more valuable items to be calculated from drill stem test data. Many wells have been passed up and abandoned only because they were not evaluated fully with respect to wellbore damage.
3. Maximum Reservoir Pressure
If a stabilized initial or final shut-in is not obtained during the course of a drill stem test, it is possible to utilize the available data and perform a mathematical determination (extrapolation) of the stabilized reservoir pressure.
4. Depletion or Drawdown
If a particular reservoir is sufficiently small enough, any withdrawal of fluid will result in a pressure drop. This drop in pressure is called depletion or drawdown.
5. Radius of Investigation
Since physical removal of fluids from the reservoir occurs during a drill stem test, there is a definite effect on the surrounding formation. By using the data obtained from a DST, mathematical calculations may indicate the distance into the reservoir which was affected by the removal of reservoir fluids.
6. Barrier Indications (Anomaly Indications)
Many reservoirs are assumed to be homogenous in composition and the reservoir fluids contained are single phase in nature. This is not always the case; drill stem tests will often indicate changes or anomalies within the radius of investigation of a test. Determinations are possible utilizing test data to further determine the type of anomaly and its distance from the wellbore.
Permeability, wellbore damage & barrier effects evaluation using DST:
Reference:
- A dictionary for the petroleum industry, second edition, by the University of Texas at Austin.
- Testing Tools and Their Operation, Baker Oil Tools.
