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DRILL STEM TEST INTERPRETATION
Earlougher, Advances in well test analysis, SPE Monograph Volume 5, 1977
DRILL STEM TEST
DST provides a temporary completion which allows pressure testing of part of a formation
DST provides: Sample of reservoir fluidIndication of well production rateStatic and flowing reservoir pressures (electronic gauges with surface readout)Short pressure transient test (pressure buildup)
Interpretation of DST data provides:Estimate of formation permeabilityIndication of formation damageIndication of initial reservoir pressure
DRILL STEM TEST TOOL
The DST tool isolates a formation interval from the mud column, allows formation fluid to flow into the drillpipe and continuously records pressure
DST FLOW PERIODS
The initial flow period is usually short (5‐10 min) – release hydrostatic mud pressure and any formation supercharging due to filterate invasion
The initial shut‐in period is usually of the order of 1 hour – allow the pressure to approach a stabilised formation pressure
The second flow period is usually approximately 60 min
The second shut‐in period is usually 60‐120 min
DST TEST PRESSURE BUILDUP ANALYSIS
The second shutin pressure buildup is analysed
DST TEST PRESSURE BUILDUP ANALYSIS
Pressure buildup is analysed using a Horner semi‐log analysis
DST TEST PRESSURE BUILDUP ANALYSIS
Pressure buildup is analysed using a Horner semi‐log analysis
DST TEST PRESSURE BUILDUP ANALYSIS
Production time is taken as the length of the preceding flow (second) period but if the first flow period is long then the production time is the sum of the duration of the two flow periods
DST TEST PRESSURE BUILDUP ANALYSIS
In liquid producing wells the production rate usually decreases with time as a result of the increasing bottom hole pressure as the liquid head builds up with production. The change in rate is usually neglected and the average rate over the production period is used
Average flow rate – measured rate for a stabilised flow to surface or feet of oil recovery in drillpipe
DST TEST PRESSURE BUILDUP ANALYSIS
The sand thickness is read from logs over the test interval
Since DST involves a sub‐surface shutin wellbore storage effects are usually negligible
Since the test time is short boundaries are not normally encountered and the reservoir is treated as infinite acting
DST TEST PRESSURE BUILDUP ANALYSIS
1. Plot pressure vrs time for the entire test period and check the validity of the test
See DST notes for more examples of problematic tests
DST TEST PRESSURE BUILDUP ANALYSIS
2. Produce a file of pressure‐time data for the final shutin pressure buildup period in preparation for a Horner semi‐log plot analysis
3. Determine the production time (duration of final flow period)
4. Determine average flow rate – measured rate for a stabilised flow to surface or feet of oil recovery in drillpipe
5. Determine actual sand thickness for producing interval from logs
6. Perform Horner semi‐log analysis and determine formation permeability, skin and initial reservoir pressure
As the DST is usually unaffected by wellbore storage effects and boundaries are not encountered the Horner analysis follows along the lines of an ideal pressure builduptest