Basics of a DST...
When the hydraulic tool is opened, the test zone is opened to atmospheric pressure. This happens almost instantaneously, and the pressure drop is recorded. This is called the pre-flow period, and its purpose is to relieve the hydrostatic pressure from the annular space within the tested interval.
The length of the pre-flow period can be determined by the surface blow, according to the following:
*Approximately 5 minutes in duration if the permeability is estimated to be greater than 15 md.
*Approximately 10 minutes in duration if the permeability is estimated to be less that 15 md.
If the pre-flow period is too short the hydrostatic pressure will not be dissipated and the following shut-in period may be under the influence of “hydrostatic super-charge” effect.
At the end of the pre-flow period the tool is closed and the pressure below the packer is allowed to build. This is called the initial shut-in pressure.
Initial Shut-in Period
The purpose of the initial shut-in period is to record the reservoir pressure before any production has occurred. It is important to have an initial shut-in period long enough to extrapolate a maximum reservoir pressure. Many times the initial shut-in period is too short to determine a reliable extrapolated reservoir pressure which can make it more difficult to determine if the reservoir is of limited area extent.
When the initial shut-in is completed the tool is once again opened. The purpose of this second flowing period is to allow reservoir fluid and gas to enter the drill string. Analysis of the final flow data will help to determine the flowing capabilities of the tested reservoir. Depending on conditions, when the tool is opened the pressure will drop from reservoir pressure to the pre-flow pressure and will record the weight of the formation fluid entering the drill string. If gas is present the flowing pressure will reflect the upstream pressure of the gas flow.
Final Flow Period
The duration of the final flow period should be approximately 60 to 180 minutes, depending on conditions and estimated permeability. The air blow at surface will indicate whether formation fluid or gas is entering the drill string. If gas flows to surface a stabilized measured rate is desirable for proper reservoir evaluation.
When the final flow period is concluded the tool is again closed for a second shut-in period which stops the flow of fluid and gas into the drill string. The pressure below the packer is then allowed to build. The duration of the second shut-in period should be approximately 1 ˝ to 2 times as long as the second flowing period, depending again on conditions and estimated permeability. In low permeability zones longer shut-in times are necessary for proper reservoir evaluation.
Second Shut-in Period
The purpose of this second shut-in period is to once again measure the reservoir pressure after a certain amount of production has occurred. Proper evaluation of the second shut-in data will help determine if the tested reservoir is of limited area extent. Skin Damage, Permeability, Radius of Investigation and other reservoir parameters can also be determined.
Several types of reservoir analysis are now available including log/log diagnostics.
And model verification or history matching.
At the conclusion of the second shut-in period, the packer is released which allows drilling fluid to flow from the hole into the test zone and the hydrostatic pressure is then recorded. Because the pressure is equalized, the packer can be easily removed from the packer seat.
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BJ's Drill Stem Testing