Depth is widely portrayed as the most important formation evaluation measurement. Reviews of current practices for all conveyance methods, and spot checks of individual logs, confirm that — in practice — depth is not given the attention and effort required. Inadequate depth control procedures are in place, and the guidelines and procedures in place are not always strictly followed by the logging companies and seldom enforced by the operators.
The technical and cost implications of poor depth control can be significant. Misinterpretation of fluid contacts can lead to errors in field models, misplaced wells and sub-optimal field development. Despite such possible consequences, no noticeable effort to improve depth control for wireline (WL) nor Logging While Drilling (LWD) data acquisition can be identified.
To improve depth control to what it should be, the petrophysical discipline should enforce first and foremost a proper calculation of True Along Hole (TAH) depth, for wireline, and especially for LWD logs. Second, the basic principles of metrology should be strictly followed. Hencelogging companies should provide precision and accuracy estimates for the TAH depth of downhole horizons. Third, logging companies should provide all relevant depth QC and audit trail records with every log. Fourth, operators should enforce such practices.
Providing a TAH depth for wireline will not be very complicated. Current practice already provides stretch corrections for the logger's depth. Since all the relevant inputs required are recorded by the logging company, refining and improving the currently applied correction procedures will be relatively straightforward.
LWD logs are normally referenced to the "driller's" depth, provided by the drilling company. But driller's depths are significantly different from the required TAH depth because no stretch corrections are applied. The required corrections involve data input from several parties and thus integration is the key to success. Ideally the logging service providers should provide TAH as part of their service. But if this were not possible, an alternative platform for such integration could be (advanced) mudlogging services, where borehole related correction parameters are already measured and the necessary set up to apply the corrections is readily available.
For True Vertical Depth (only), near future new bore hole gravity technology may provide an alternative approach with significantly better precision and accuracy than wireline or drill pipe combined with deviation surveys. But even then, TAH depths from the conventional conveyance systems — i.e., cable, pipe, coil and wire, for every single log—remain necessary.
