PLT is performed in cased-holes and sometimes in wells with open-hole bottom completions.
In both cases it's classified as cased-hole logging as the major part of the borehole is cased.
The PLT survey is conducted under flowing and shut-in conditions, preferably after reaching quasi-stationary conditions (when flow rate and pressure change negligibly during the survey).
The PLT is performed between two depth points: top of PLT survey and bottom of PLT survey, which is called PLT survey interval.
The primitive element of PLT survey is a single-run of PLT tool from top to bottom of survey interval (called "down-pass") or from bottom to top (called "up-pass") .
The typical logging procedure includes three different up-down passes at three different cable speeds, which results in totally 6 different passes.
For vertical well with up to 3,000 m depth and 100 m survey interval the typical duration of the survey would be between 4 hours and 8 hours and fits working day schedule at well location.
In deep wells and horizontals the PLT survey may last few days.
The PLT survey requires some pre-survey well-maintenance:
- ensure the wellhead valve and THP gauges are operational
- preliminary well testing to ensure the ell flow during the survey
- dummy run with a tool-equivalent to ensure the accessibility of the hole for the PLT toolstring
- Ppeferrably arrange flowrate measurements at wellhead
The PLT survey is normally conducted twice:
- during the stabilized flow and called "flowing passes"
- during the stabilized shut-in conditions and called "shut-in passes"
The usual choice of cables speeds is:
- 5 m/min
- 10 m/min
- 20 m/min
- 30 m/min
and logging engineer may wish to run the tool at all the above speeds or select three most appropriate speeds for a given flow conditions
In some cases the engineer may select less than 5 m/min pass and more than 30 m/min pass and in case of a real-time survey may decide selectively for different parts of survey interval.
Below is the table of popular suffixes for logging passes.
| Suffix | Flow status | Cable Speed | Up / Down |
|---|---|---|---|
| F1D1 | Flowing = Production / Injection | 5 m/min | Down |
| F1U1 | Flowing = Production / Injection | 5 m/min | Up |
| F2D1 | Flowing = Production / Injection | 10 m/min | Down |
| F2U1 | Flowing = Production / Injection | 10 m/min | Up |
| F3D1 | Flowing = Production / Injection | 20 m/min | Down |
| F3U1 | Flowing = Production / Injection | 20 m/min | Up |
| F4D1 | Flowing = Production / Injection | 30 m/min | Down |
| F4U1 | Flowing = Production / Injection | 30 m/min | Up |
| S1D1 | Shut-in | 5 m/min | Down |
| S1U1 | Shut-in | 5 m/min | Up |
| S2D1 | Shut-in | 10 m/min | Down |
| S2U1 | Shut-in | 10 m/min | Up |
| S3D1 | Shut-in | 20 m/min | Down |
| S3U1 | Shut-in | 20 m/min | Up |
| S4D1 | Shut-in | 30 m/min | Down |
| S4U1 | Shut-in | 30 м/мин | Up |
Follwowing the above conventions the log name TEMP_F1D2 means: "Temperature sensor, first flowing survey, down pass at second speed".