DST Overview

Being involve in exploration well drill stem test (DST) planning and operation provide a value and provide more appreciation on the reservoir engineering works.


Why do we need DST for exploration well? This would need to be agreed before the operation. But basic needs for exploration well is of course to get the reservoir properties of a newly discovered field. Hydrocarbon shows from the logs will dictate whether to go for a DST or not. Logs can only shows the presence of oil or gas in the reservoir, but whether it can be produced or not, we would need to flow it to surface to get confirmation. Then, we could collect the PVT fluid for the analysis especially when we would need quiet a volume of it.


How do we execute a DST then? We would need a proper planning before conducting DST, both on operation and test design (flowing sequence). Operationally, we would need to consider the personnel and equipment are all ready. If we talk about the offshore operation, we would need to ensure equipment are in very good condition and tested before being mobilized to the rig. On the test design, we would need to clarify our objectives in conducting the test. Plus, we need to agree in our team on the test sequence, how long we're going to flow the well plus the buildup period.

After the well reach the target depth, DST string will be lowered down into the well. Depending on type of testing, perforation would be commenced in cased hole testing or else open hole testing would be suitable for strong formation (normally basement reservoir). While run in hole, enough cushion will be fill up inside the string so that when the valve opened for flowing, it will prevent big surge of pressure (due to pressure difference at above and below the valve). Next, perforation will be done by the gun specialist at the correlated depth (by referring to the log).

Downhole tester valve will be open to allow for the first flow of the hydrocarbon to the surface. The well need a cleanup period in order to remove previously filled cushion fluid and remaining mud. After continuously stable hydrocarbon flow was established, next is to start the flow period for rate measurement and fluid sampling. The well will be shut in for the reservoir properties determination where the downhole gauge will ensure all the changes in pressure are recorded. As per previously said, the flow sequence and timing will be based on the specific objectives of the DST. An extended well testing is somewhat a different kind of DST which is flowed for a long period in order to assess the production sustainability.

What do we do with the result? Basically the surface data (rates, gas gravity, GOR, water cut) will be combined with the recorded downhole data (pressure and temperature) for the analysis which we called "pressure transient analysis". As the name suggest, pressure transient analysis is the evaluation of behaviour of the pressure propagation characteristic from the well to the formation. The analysis result will be suggest the well and reservoirs parameters (reservoir pressure, permeability, skin, wellbore storage, existence of nearby boundary, etc).