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Background on wireline keyseating
Wireline keyseating is most frequently experienced during formation fluid sampling although it can happen with any logging string (stationary or moving). In this situation, if the cable cannot be freed, strip-over fishing is the only remedy; this is a cost and risk-intensive activity which may extend the logging job by several days and result in further inefficiencies such as wiper trips or pipe-conveyed logging to fully satisfy the formation evaluation objectives.The precursor of all cable sticking is slot cutting into mud-cake or formation. The slot is generated by sustained lateral pressure from the moving (tensioned) logging cable as it is run-in and pulled-out of hole. The heavier the tool-string the greater the applied cable thrusts; the higher the number of runs the more likely a slot will develop. Once a slot has been cut there are four possible mechanisms for cable entrapment (which may combine to result in a fishing job), as illustrated below:
Cable thrust visualisation
Below shows a fictitious S-shaped well, plotted to scale, with vertical depth indicated as TVDBRT. The upper greyed-out section is cased hole and below that is open hole, with dogleg severities shaded as per the legend on the top right. The red curve in the left hand track is the magnitude of the cable thrust with the tool logging up from TD; the thrust units are lbs/ft (or lbs/m if a metric well). The 3D arrows on the wellbore schematic indicate both magnitude and direction of the applied cable thrusts, illustrating low-side thrust in the lower drop-off section and high-side thrust in the upper-build section (intuitive, looking at the well profile). In this case, there is keyseating risk in the upper-build section, where the cable thrust has a peak value of ≈ 2 lbs/ft, located ≈ 5000 ft MD above TD. The higher the peak thrust and the further it is above TD the longer and more intense the cable cutting action will be; the mud-cake & rock strengths in the upper-build section are critical factors for keyseating risk (softer material may yield a slot and allow keyseating forces to act on the cable).
The cable thrust curve is computed via the cable force equations in the wireline tension modelling. Your logging provider should be able to evaluate the thrusts for different tool-strings and therefore make an assessment for your keyseating risk.
Keyseating risk factors
There are five (interlinked) factors which influence keyseating risk on a logging job:
- Wellbore design, trajectory & tortuosity.
- Logging environment and geological setting.
- Wireline conveyance system & tool-string design.
- Logging procedures, winch control & survey efficiency.
- Stuck tool and cable cycling policies.
These factors are discussed in SPE-193232-MS.
Candidate wells for keyseating risk
Any well with significant open hole directional work:
- J-shaped tangent wells (with build or drop outside the casing shoe).
- S-shaped wells.
- Tortuous side tracks.
- Tortuous injector wells (depleted)
With the following characteristics:
- Offset cable sticking experience, resulting in cancelled surveys, pipe conveyed logging or strip-over fishing
- Soft/pliable formations over the build or drop zones (Unconfined Compressive Strength, UCS < ≈ 5000 psi).
- Reactive mud and formations that may result in rock swelling (shale hydration).
- A long open hole interval (affording greater tensioned cable to cut a rock slot, e.g. > ≈ 3000 ftMD).
- Hole deepening and relogging (long soak time for the build section).
- Modest directional changes over permeable zones. For example, small kinks in the wellbore trajectory outside the casing shoe may be sufficient to generate cable thrusts that disrupt the mud-cake and induce differential sticking.
- Depleted formations, although high overbalance is not essential to generate high sticking forces
Example wellbore trajectories, where cable has been fished, are shown below (with open hole tortuosity increasing from left to right: the grey sections are cased, the green are open hole):
The first two wells are low tortuosity (low cable thrusts) and had pure differential sticking (large sands between cable head and CSG Shoe: most likely a mud-cake slot instead of a rock slot). As we move to the right there is a shift to slot cutting into rock and additional mechanisms to trap the cable, such as shale swelling resulting in slot compression.
Mitigations for keyseating
- Drill a straighter & shorter open hole section - if possible (tortuosity & cable thrusts → 0)
For wireline loggers:
- Record all signs of prior offset sticking (if available) and use that to benchmark the current well being logged.
- Reduce tool-string weights and minimize the number of logging runs (slot cutting is cumulative).
- Consider pipe conveyed logging if deemed too risky to log on wire.
- Consider setting temporary liner over high thrust zones (with slim thru-pipe logging tools).
- Consider deploying wireline standoffs on the cable to eradicate keyseating forces.
One Petro papers on keyseating prediction & mitigation
- SPE-174068-MS: Using Wireline Standoffs (WLSOs) To Mitigate Cable Sticking (2015)
- SPE-193232-MS: Wireline Cable Protection: Enabling Fluid Sampling In High-Risk Wellbores (2018)
For "thru-pipe" logging:
For the Gaia Cable Protection System (benchmarking & deployment of wireline standoffs): http://www.gcps.tech
[Guy Wheater, Ph.D. https://www.linkedin.com/in/guy-w-22942270/]