Drill_Stem_Test Operation (DST)

Well tests conducted with thedrillstringstill in the hole. Often referred to as DST, these tests are usually conducted with a downhole shut-in tool that allows the well to be opened and closed at the bottom of the hole with a surface-actuated valve.

One or morepressuregauges are customarily mounted into the DST tool and are read and interpreted after the test is completed. The tool includes a surface-actuatedpackerthat can isolate theformationfrom theannulusbetween the drillstring and thecasing, thereby forcing any produced fluids to enter only the drillstring.

By closing in the well at the bottom,afterflowis minimized and analysis is simplified, especially for formations with low flow rates. The drillstring is sometimes filled with an inert gas, usually nitrogen, for these tests. With low-permeabilityformations, or where theproductionis mostly water and theformation pressureis too low to lift water to the surface, surface production may never be observed.

In these cases, the volume of fluids produced into the drillstring is calculated and an analysis can be made without obtaining surface production. Occasionally, operators may wish to avoid surface production entirely for safety or environmental reasons, and produce only that amount that can be contained in the drillstring.

This is accomplished by closing the surface valve when the bottomhole valve is opened. These tests are called closed-chamber tests.Drillstemtests are typically performed onexplorationwells, and are often the key to determining whether a well has found a commercialhydrocarbonreservoir.

The formation often is not cased prior to these tests, and the contents of the reservoir are frequently unknown at this point, so obtaining fluid samples is usually a major consideration. Also, pressure is at its highest point, and the reservoir fluids may containhydrogen sulfide, so these tests can carry considerable risk forrigpersonnel. The most common testsequenceconsists of a shortflow period, perhaps five or ten minutes, followed by a buildup period of about an hour that is used to determine initialreservoir pressure.

This is followed by a flow period of 4 to 24 hours to establish stable flow to the surface, if possible, and followed by the final shut-in orbuildup testthat is used to determinepermeability thicknessand flow potential

Credit to: Schlumberger Oilfield Services