What is the purpose of a drill stem test in oil and gas exploration?

In my opinion the most crucial purpose of DST is the determination of permeability. The real time permeability data obtained gives an actual idea of how the fluid will flow from the formation into the wellbore. This helps in determining the economic viability of the well.

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. 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 packer or packers are set to isolate the zone from the drilling fluid column. Drill-stem tests are performed to confirm or prove the presence and the producibility of oil and gas that is detected by the other services.

Drill stem tests involve connecting a measurement device to the bottom of the drill stem, also in place of the drillbit, and lowering the system into the well, all the way to the formation. The instrument is activated at the bottom of the well, measuring the flow of oil or gas for a specified amount of time

The main objective of a DST is to evaluate the commercial viability of a zone's economic potential by identifying productive capacity, pressure, permeability or extent of an oil or gas reservoir. The test is an important measurement of pressure behaviour at the drill stem and is a valuable way of obtaining information on the formation fluid and establishing whether a well has found a commercial hydrocarbon reservoir.

Important factors in determining the success of the DST include: well depth; duration of the test; time required for hydrocarbons to reach the surface; fluids recovered in the drill pipe; initial and final flow pressures, indicating the increase in flowing capacity of the well; and the shut-in bottom hole pressure, which signifies the maximum reservoir potential. It is important to decide whether a well is commercially productive. Typically, one or more formation test is performed to determine if the well is productive or not.

DST is act as a temporary completion after drilling a well. It will helpful to safely handle the flow for clean out operation and carried out different well test such as build up, drawdown and fluid sampling. The well test will the give the idea about the productivity, reservoir description etc. Fluid sampling will gives the idea about the type of fluid and fluid characterisation.

DST string can be combine with TCP perforating gun assembly in one single run for both exploration wells and development wells at multi purpose. DST string is included packer, ball valve, circulating valve, downhole gauge, shock absorber, hydraulic jar all place above TCP Firing Head and perforating guns, DST string can be use for overbalance and underbalance Perforating by the lighter fluid weight is set above the ball valve is closed when RIH, DST string play the role in well control process after perforation well, and circulating well fluid until safe to POOH.