Wireline cable sticking is most frequently experienced during formation fluid sampling. 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. In this paper, the authors outline a proven engineering approach that permits the systematic identification of high-risk wells for cable sticking, where the deployment of wireline standoffs (WLSOs) may facilitate safe and efficient sampling on wireline.
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, referred to hereafter as "cable thrusts". A sophisticated 3D cable force model evaluates the cable contact zones and thrusts along the open hole section under analysis. For differential sticking, a cable contact and sticking limit in mud-cake is computed, having adjusted the pore pressures to offset data. For keyseating, the well is benchmarked against wells with similar trajectories, gained from a global study on cable sticking. Sticking-linkage is also assessed, whereby overpulls from tool sticking may induce cable sticking, via amplified cable thrusts against mud-cake or formation. To reduce cable sticking risks to an acceptably low level an array of WLSOs may be deployed, combined with optimised logging procedures and winch techniques.
Operators are now employing this approach on a regular basis as an important risk management tool, from the well planning stage through to completion. WLSOs have been successfully deployed for 40 sampling runs on 25 wells, in fields with a history of severe cable sticking or fishing. Prior signs of sticking have largely been eliminated, resulting in significantly reduced well costs, whilst avoiding pipe-conveyed logging or LWD, and without consideration to the value of lost data or samples. Well Engineering may now use this approach to design and optimize ambitious well trajectories, potentially saving a hole section, in the full knowledge that future wireline fluid sampling can be performed within the bounds of acceptable risk. Real-time risk assessments have proven invaluable, to track the risk trend as the well is drilled, leading to a final decision on the requirement for WLSOs, with additional optimisation of the sampling procedures if needed.
Operational best practices and lessons learned are summarized, as well as an outline for future R&D to understand and reduce wireline conveyance risks even further.