Do these two DST's run in the same wellbore show true depletion?
If not, what are the two possible clues that depletion is false?
Note: Review the charts once again before looking at the answers — you'll see the name of the testing company name has been added and DST 3 test results have been modified.
Neither of these DST's show valid true depletion.
Two Possible Clues (View Enlarged Diagrams)
a. The preflow pressure is higher than the pressure at the start of the main flow period (Point A). This is very obvious in DST 2, less obvious but still detectable in DST 3. This indicates tool plugging above the gauge.
b. The second clue is that in both DST's there is a squeeze pressure evident after the point at which the IHP was read (Point B). This was undoubtedly due to the tool sliding to bottom on "fill" in these bottomhole tests. (This is similar to the Example B of last month's Hugh Reid's Corner.
In summary, what happened was that:
The tool skidded to bottom resulting in a squeeze pressure and as the tool slid, fill was forced into the perfs, causing plugging above the gauges (probably in the choke or shut-in valve ports).
1. The squeeze pressure was "trapped below the plugging agent and was not dissipated during the very short preflow.
2. As a result the ISIP is too high.
3. When the tool was reopened for main flow, the tool plugged allowing release of the squeeze up the pipe. Notice in DST 2 this took 5-10 minutes and occurred erratically at start of main flow. In DST 3 it occurred immediately as the pressure dropped right away to the Baseline.
4. With release of the squeeze pressure during main flow, the FSI pressure is unaffected and valid.
So... the "depletion" is false.
A further clue (or at least grounds for suspicion) is the fact that the testing company which ran the DST is T.G. Eastland.
This company often had problems of this nature with charts of falsely high ISI pressures.
The tool configuration and ID must have been such that it was easy to plug. The bottom choke was only ½" so this may have been where the plugging occurred. It certainly was above the inside gauge (since the inside and outside gauges both showed this false pressure).
Readers who attended this author's introductory talk on Hydrodynamics of the Gilwood Sand at Mitsue may remember the "False ISI on the Eastland DST just updip of the field".
This was a rather critical DST in the Sixties prior to the discovery of the Mitsue field. Anyone working with a pressure/depth plot for reservoir continuity would have missed the barrier just downdip of this well and hence the vast Mitsue oilfield!