In this post we will be looking at the meaning of Drill Stem Test in Oil and Gas, components of Drill Stem Test tool and Applications of drill stem testing.
INTRODUCTION
Drill stem testing involves deploying a series of tools known as a test bottom hole assembly. A basic drill stem test consists of a packer or packers, which act as an expanding plug to be used to isolate sections of the well for the testing process, valves that may be opened or closed from the surface during the test, and recorders used to document pressure during the test.
In addition to packers a downhole valve is used to open and close the formation to measure reservoir characteristics such as pressure and temperature which are charted on downhole recorders within the bottom hole assembly. A DST can also obtain actual fluid samples from the formation. From detailed analysis of the pressure recordings and of the fluids, the operator can infer a great deal of information about the reservoir.
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In the 1950s, Schlumberger introduced a method for testing formations using wireline. The Schlumberger formation-testing tool, placed in operation in 1953, fired a shaped charge through a rubber pad that had been expanded in the hole until it was securely fixed in the hole at the depth required. Formation fluids flowed through the perforation and connecting tubing into a container housed inside the tool.
When filled, the container was closed, sealing the fluid sample at the formation pressure. The tool was then brought to the surface, where the sample could be examined. In 1956, Schlumberger acquired Johnston Testers and continues to perform drill stem tests and wireline formation tests in both open and cased holes.
What is Drill Stem Test in Oil and Gas?
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. It can also be seen as a method of formation evaluation. The test allows fluids in the formation being evaluated to flow into the drill stem and to the surface. Pressure-recording devices in the drill stem test tool measure and record pressure in the well while it is both flowing and not flowing (shut in).
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Components Of Drill Stem Test Tool
The drill-stem tool has four basic or compulsory components that should exist in every tool assembly made by any service company. These four components are: the pressure recorders, the test valves, the hydraulic by-pass or equalizing valves and the packers.
- The Pressure recorders (the Brain of the DST tool assembly): The pressure recorders used in the DST tool are usually sensitive for low pressure gauges as much as ½ psig and for high pressure gauges as much as two thirds of the maximum pressure of the well.
- The Test valve: one of its main functions is to provide a passage for the fluids to flow into the drill pipe when the packer is set at the annulus. Second, it prevents the drilling mud to enter the empty drill pipe while running in the borehole.
- The hydraulic by-pass valve: The hydraulic by-pass valve has a role in relieving the pressure in the borehole. It provides an additional space for the drilling mud to pass around the packer while running in or pulling out of the borehole. It allows the drilling mud under hydrostatics pressure to flow downward through the packer mandrel at the end period of the test into the holes below the packer.
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- The Packers: Their main functions are to seal the annulus space and prevent the down-hole movement of the drilling stem. This sealing mechanism prevents the vertical flow of the fluids in that section of the well. Packers very often improve the production rate of the well by directing the formation fluids to one way passage rather than going into the annulus space.
The basic component of the DST tool there are other auxiliary components that most of the modern DST assemblies are composed of, which include the following:
- The Shut-in pressure valves
- The Disk valves
- The Reverse Circulating valves
- The Jar
- The Safety joints
- The Anchor shoe (Bullnose jetting nozzles)
- The Bottom Hole Choke
- Perforated pipe
- The Surface Control Head (Christmas Tree)
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Applications of drill stem testing
- Cased Hole stimulation: cased hole drill stem testing uses a retrievable production packer that is set above the zone of interest. The two types of cased hole testing are pressure operated and mechanically operated.
- Open Hole: open hole drill stem testing can be the most economical way to determine productive capacity, pressure, permeability or the extent of an oil or gas reservoir.
- Alternate Procedures: Depending on testing objectives and scope of work, drill stem testing may also be performed in combination with various other exploration and completion process such as fluid loss control and well control, closed chamber tests, well stimulation, and a combination of DST and TCP.