DST's are run as an open hole temporary completion (short production test) within a drilling well. Essentially, a portion of the wellbore is isolated (see Packers) and the identified zone allowed to flow into the drill pipe. Recorders measure the pressure of the zone at the start and end of the Flow Period. After the flow(s), the time for the zone to "recover" its pressure is measured during the shut-in periods, as is the volume of fluid recovered.
The purposes of a DST are multiple. All of the data gathered is contained within the time versus pressure chart, within the recorders, and the fluid recovered inside the drill pipe at the completion of the test.
All drill stem test tools are comprised of:
1. rubber packers which expand within the well to isolate the desired interval to be tested;
2. a perforated flow sub to allow the formation fluid to enter the drill pipe;
3. a shut-in valve to open or close the flow path inside the drill pipe; and
4. pressure recorders to collect a graph, or chart, of pressure versus time.
Packers fall into two categories depending upon the mechanism used to expand and contract (the diameter of) the rubber element. Conventional packers expand by compression from the top and bottom of the element. The compression forces the rubber to expand outward until the wall of the drill hole is encountered. Inflate packers are constructed like balloons and are expanded by injecting drilling mud from the wellbore under pressure to expand, or inflate, the rubber element.
Fluid, flowing from the formation and entering the drill pipe, is trapped inside the drill pipe during the flow periods of the test. This fluid is collected, or recovered, as the drill pipe is raised out of the hole at the end of the test. The amount of fluid in the pipe is measured in terms of the length of the pipe filled, and is called the recovery.
The Shut-in Valve has many names depending upon the different testing company designs, but serves to allow fluid to flow into the drill pipe, or not. In this way a test can be constructed as a series of flow and shut-in periods. (see Chart Parts).
Recorders also fall into two categories, mechanical and electronic.
Mechanical recorders are comprised of a metallic "bellows" or Bourdon tube which expands or contracts as a result of a change in pressure. This bellows is attached to a stylus enclosed in a cylinder. The cylinder contains a brass "chart" coated with carbon black or white titanium oxide powder (oxide paint), and is driven along the length of the cylinder by a timing clock. As the pressure changes, the stylus rotates within the cylinder and scribes a mark in the coating of the chart. This rotation, coupled with the longitudinal travel of the cylinder with time, produces a continuous chart of pressure versus time. Mechanical recorders are usually accurate to within 1% of the total pressure range of the bellows (i.e., 100 kPa for a 10,000 kPa bellows). The primary mechanical gauges utilized in the DST industry are the Amerada Ak-1 and the Kuster K-3 recorders. See Orientation of Pressure/Time Axes to view a picture of charts from each recorder type. Each of these has an accuracy of approximately 0.25% of full scale deflection. (i.e., a 4000 psi gauge can read to ±10 psi or a 28000 kPa gauge can read to ±70 kPa).
Electronic recorders are somewhat more complex in design and manufacture but in general are comprised of a pressure sensitive transducer which converts changes in pressure to an electronic signal. (Quartz is one such material which possesses a property called the piezoelectric effect, whereby small changes in pressure exerted on the crystal cause the crystal to output small voltages.) These electronic signals are then converted to pressure readings, and are stored - along with time and temperature readings - as digital records in a memory chip. Electronic recorders are much more sensitive than mechanical recorders having accuracy's in the range of 0.1% of full scale. (i.e., ±10 kPa for a 10,000 kPa transducer).