SPE Lecture: 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 and manageable 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.  This will be illustrated by examples from the Australian public record, 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.

Speaker’s Biography:

Martin Storey is an independent practising Geoscientist with an academic background in Mathematics and Computer Science and over two decades of international experience in our industry, combining field operations and the acquisition, exploitation and management of well data at all stages of the life cycle.  He is based in Western Australia and works as a consultant and trainer, mainly in Petrophysics, data acquisition planning, and well data management, to a range of companies broad in size and spread.  He has a BSc in Mathematics and Computer Science from Stanford University, and an MSc in Electrical Engineering from the California Institute of Technology.  He just received the luxury watch that marks his 25-year membership in SPE.