Drill Stem Testing (DST) is a procedure for testing a well formation, with the drill string in the hole. It determines the fluid content of a reservoir and it’s ability to produce. These tests can be performed in both open and cased hole environment.
When drilling operations have been completed, it is important to determine whether to move on to the next phase – completion for production. Formation test ascertain if there are enough hydrocarbons to produce from the well, as it provides important information to design the well completion and production facilities. Drill Stem Testing is one of the various methods for formation testing.
In Drill Stem Test, the drill bit is removed from the drill string and a Drill stem testing tool assembly is attached. It is then lowered all the way to the formation and activated, measuring the flow of oil or gas. The amounts of hydrocarbons that flow into the drill string during the test and the recorded pressure are used to judge the production potential of the formation.
The Drill Stem Testing tool includes a perforated anchor at the bottom that allows the fluid to enter from the formation. A packer is used to isolate the formation from the rest of the bore hole, thereby forcing the fluid to enter the pipe. A series of valves open and close to control the flow of the hydrocarbons into the empty drill string. Additionally a Drill Stem Testing tool contains a pressure recording device that document pressure during the test.
Tools commonly used with DST are:
Reverse circulating sub: It is used to circulate fluid from the drill string after DST has been completed and drill string is ready to be removed. There are two types of these subs – Pump out sub and knock out (Bar drop) sub. In pump out sub, the port is opened by applying pressure to the inside of the drill string and shearing out the rapture disc. In knock out sub, a heavy bar is dropped to break the pin in the sub to open the port.
Rotating shut-in tool: A rotating shut-in tool is used to obtain flow periods and shut in periods. Available in two flows and two shut-in periods or three flows and three shut in periods, it is run immediately above the hydraulic multiple shut-in tool and operated by right hand rotation of the pipe. The tool is run in open position, but the hydraulic multiple shut-in tool prevents any fluid from entering the test string. Once the testing depth is reached, the multiple shut-in tool is opened and the first flow period begins. Rotating right hand, the tool will shut off the flow for the first shut-in period and rotating again will start second flow period.
Hydraulic multiple shut-in tool: It is a downhole valve that allows multiple opening and closing. This tool allows running into the hole with an empty test string. This eliminates the hydrostatic pressure on the formation at setting depth. Multiple shut-in tool is opened by slacking weight on it, and it is a time delayed tool (E.g: 10,000 lbs for 5 minutes). This delayed tool opening avoids accidental opening while running through tight spots in the hole. Picking up the tool string to the neutral weight will close the valve immediately.
Gauge (Recorder) carrier: It carries and protects downhole memory gauges. Gauge carriers can be run at multiple locations on the DST string. It contains two internal and two external recorder slots. The gauges are attached to the housing with metal seal connectors and are retained below the
O.D. of the carrier to protect the gauges from damage while running the string. The carrier can be configured to provide outside and inside position electronic recorders
Hydraulic jar: Hydraulic jar is used to free the stuck section of drill string in cased and open hole testing operations. The jar provides a temporary resistance that allows the drill pipe to be stretched. When the resistance is released, the jar body will fire rapidly upward to free the stuck string below. The impact force is dependent on the amount of over pull on the jar as it opens. It is easily reset for repeated use by putting setting down weight on the string.
Safety joint: The safety joint provides a means for releasing the tool string from above the packer in the event that packer cannot be released after jarring. This ensures recovery of DST tool and memory gauge data recorded during the well test. The safety joint is released with left hand rotation. The lower portion of the DST string can then be recovered with larger jars or fishing tools.
Packer: Apacker is used to isolate the formation from the rest of the bore hole, thereby forcing the fluid to enter the pipe. In Cased hole DST, a retrievable production packer is used and in open hole, an inflatable or compression set packer is used.
Perforated anchor: A perforated anchor is attached at the bottom of the tool string which allows the formation fluid or gas to enter the tool string pipes.
DST tools are available in sizes 3.125”, 4.250” and 5.00” and suitable for 10,000 psi WP and H2S service per NACE MR-01-75.