29.1 Introduction

Well path position uncertainty estimates, discussed in Chapter *, are used to determine if there is an adequate probability of hitting the geological target, of avoiding collision with offset wells, and of drilling a successful relief well in the event of a blowout. These are high value decisions and they depend heavily on the validity of the uncertainty estimates. However, error models are based on many assumptions. If the actual survey data are not acquired in conformance with these assumptions, the uncertainty estimate is invalid and it can no longer be assumed that the directional objectives for a well are being met. For this reason, it is dangerous to place any trust in an error model without an accompanying set of validating quality control (QC) measures. Therefore, a comprehensive set of QC measures, derived from the model, is required.

Various methods of QC are possible, including:
• Georeference tests whereby measurements of the Earth’s gravity field and its magnetic or spin field, derived from the tool’s sensors, are checked against independent measurements or estimates of these
quantities.
• Multi-station analysis of reference field measurements; an extension of georeference tests which becomes an option after measurements have been collected at multiple survey stations.
• Tests based on repeated or duplicate measurements at the same survey station and in-run/out-run comparisons.
• The use of independent data in the form of an overlapping survey provides additional reliability; the error models predict how well the orientation measurements and calculated positions provided by the respective
surveys should agree.

Although it cannot identify all potential errors, a second independent survey is the most powerful QC check. This chapter describes a comprehensive set of both the georeference QC tests and independent survey QC tests, including their limitations and operational recommendations. A few error sources, mainly linked to human inputs, cannot be fully tested by such data checks. They should in addition be subject to systematic manual cross checking to justify fully the use of a claimed error model. The highest level of reliability is achieved by the application of all of the QC techniques outlined here.

The proposed QC tests are intended to demonstrate that a directional survey does not contradict its error model uncertainty estimates. The limit values used in the tests must therefore be derived from the relevant model, and there is a need for an industry agreed confidence level. If applied correctly and consistently, the methods described here are capable of assuring valid attitude and positional data, and should be incorporated into standard operational practices.