Trends in Well Depth Errors

Well depth was measured until about two decades ago using a relatively standard procedure based on wireline logs, and it was then essentially fit-for-purpose. The measurement was subject to errors but once identified, these were generally understandable by all involved. The measurement and definition of well depth have since become less set due to a combination of complications, including: the frequent use of Logging While Drilling (LWD), of highly deviated well trajectories, of floating rigs, the evolution of tool string configurations, the curtailment of logging programs, the systematic use of computers to store and exploit the depth-based data, etc. Concurrently, the training of the personnel involved in the acquisition, exploitation and management of well data including the well depth has also been reduced, while new or improved data acquisition technologies, integration methods and exploitation techniques require or assume well depths of greater accuracy and precision. The industry is now acquiring higher value / higher cost well data indexed against a depth that has a greater uncertainty than before, and generally without the information required to mitigate and quantify this uncertainty. Remarkably, operators have not expressed widely a concern about this deterioration of well depth.

It is submitted that blunders in well depth, i.e. errors due to human mistakes rather than to uncertainties intrinsic to the measurement methods are now much more common than before. This will be illustrated by simple real examples and by compilations of observations made on large sets of wells, to demonstrate that the examples are symptomatic rather than anecdotal. When these errors become apparent, they are frequently airbrushed and the problem itself is repressed on account of it being “too hard” or perhaps “too late”. Errors tolerated at the start of a workflow or a project, for instance when converting time to depth or when propagating well data into an inversion cube, are then carried throughout all the subsequent work, amplifying uncertainties at every step.

This is not “the best we can do” with respect to well depth: there are in fact simple ways to mitigate this consequential uncertainty, the prize being a lesser uncertainty in most activities that rely on or use depth-indexed well data. The solutions are essentially procedural and they often have no incremental cost. In practice, the results of work using a better well depth tend to fall into place more readily and more consistently, yielding tangible reductions in uncertainty.