30.1 Introduction

This chapter draws attention to the vital need for representative and justifiable error model inputs in the quest for safe and reliable surveying when using wellbore survey tools. The error model for a particular survey tool is intended to be representative of the average performance of a large number of surveys and for the well placement accuracy that can be expected using that tool for a planned survey. However, it is difficult to know whether or not these objectives are fulfilled unless the survey tool error model has been fully verified.

The use of non-verified error models may be adequate for assigning position uncertainties to surveys that can be tested relative to the chosen error model, with the help of the tests described in the chapter on survey quality control for example. Whether the error model used is valid for a large number of surveys is less important than whether it is valid for the given survey.

The use of non-verified error models is far more questionable if the error models are used to design optimal wellbore survey programs to ensure that the drilling target is intercepted in a safe and economical manner. The result might be the use of survey programs that are too comprehensive and overly expensive when the error models are too pessimistic, and give rise to an excessive number of failed surveys and/or bad wellbore placements when the error models are too optimistic. Methods are therefore needed to validate error models through the use of downhole data collected under real operational conditions. Such methods can be based either on direct measurements of the error terms involved, or on the comparison of multiple surveys in the same well.

The error model for a particular survey service is influenced not only by the choice of sensors and their configuration, but also by factors including the tool running configuration, choice of centralisation adopted, the platform from which the system is operated and the detailed operating/QC procedures applied when running the tool. It is therefore the responsibility of individual service companies to provide error model data based on the statistical analysis of real downhole data for each type of tool and service on offer. Simulation and theoretical analysis should be used wherever possible to support this process and so add credence to the statistical derivation.