An operator successfully tested an offshore, exploration well in a High Pressure, High Temperature (HPHT) reservoir. Reservoir pressures were in excess of 10,000 psi and reservoir temperature of approximately 400°F. The tested intervals were behind 4.5-inch liner. Slimhole drillstem (DST) tools were considered but were not selected due to the restricted internal diameter and limited intervention capability. Instead, a cased hole DST method was selected in which conventional DST tools are set in 7-inch liner and tubing-conveyed perforating (TCP) guns are run in the 4.5-inch liner on the bottom of a long tailpipe.

The first DST has been successfully completed, with the pressure gauges located in gauge carriers in the 7-inch liner several thousand feet above the test interval. Pressure and temperature data from downhole gauges were analyzed and found to exhibit significant wellbore-phase redistribution effects due to gauge location. This has resulted in significant uncertainty in reservoir characterization. Solutions considering the HPHT and slimhole design were investigated.

We present a unique method of placing the downhole gauges on top of the perforating gun during an HPHT well test operations where formation temperature is more than 400°F. Five DSTs were performed using this technique with successful recording of downhole pressure and temperature data, despite the perforating gun shock and hostile environment encountered.

A comparison of pressure transient interpretations as well as a comparison of data gathered by the gauge above the guns (immediately above the perforations) with the data from gauges in the 7-inch liner, will be presented to show the potential for incorrect interpretation due to gauge placement. We will also discuss the unseen behavior of flowing gas temperature at perforation depth based on actual temperature data recorded.