Depth is the most fundamental logging parameter, tying together the vast array of subsurface measurements made. Along-hole depth forms the basis of essentially all aspects of our downhole industry and is the common reference for all subsurface measurements.
Previous studies have shown that as along-hole depth increases there is an increasing spread of differences between wireline and driller’s depths. Logging while drilling (LWD) depths are based on driller’s depths. These LWD depths are often used to initially define along-hole events, and are taken as a reference to take early decisions and make public announcements. A source of depth error and uncertainty is the common practice of operators to take LWD depth as a final reference, to avoid modifying net-pay public statements made early based on LWD data. Subsequently recorded wireline depths are often different, with the difference being perceived as a nuisance, because all the depths announcements otherwise need to be revised. However, the real consequences of depth-determination shortcomings are often only seen long after drilling and logging the well.
This paper reviews the key methodologies used in wireline depth determination and outlines a technique for wireline depth elastic-stretch correction and for stick-and-pull (also called stick-and-slip) correction. The waypoint methodology is proposed as a method of closing the gap between the various elastic-stretch corrections used. The correction is based on surface and cablehead tension measurements. These operational techniques can be used by all wireline service providers in helping to improve consistency and accuracy in the correction of wireline depth for elastic stretch, particularly as applied in first primary logging (first run in hole). The techniques outlined take in account complications caused by long-reach wells with complex trajectories and tension regimes.
By reviewing the various methodologies used to determine wireline depth, it is hoped that at least the wireline side of the discussion can be understood. The paper is aimed at opening up the debate on how the industry should deliver depth—the essential single parameter that links all subsurface data together.
The paper does not address a number of related subjects, including the economic need for absolute and relative depth accuracy, driller’s depth measurement and correction, thermal correction of wireline depth, (initial) permanent deformation (also called plastic stretch or, incorrectly, inelastic stretch) of wireline cables, and wireline cable stretch-coefficient determination. Neither does the article include a discussion on uncertainty, a key component to measurement integrity. These issues are each subjects for further investigation and debate.