Position-uncertainty estimates are used to determine if there is adequate
probability of hitting a geological target, of avoiding collision with offset
wells, and of drilling a successful relief well in the event of a blowout.
These high-value decisions are based on tool-error-model predictions, the
validity of which is highly dependent on the application of rigorous
quality-control (QC) procedures to the survey data. Directional surveys that do
not conform to their model?s error predictions represent a risk in terms of
lost production, damage to infrastructure, and loss of life.
This paper documents weaknesses in conventional directional-survey QC
procedures. Theoretical considerations, statistical analyses of real survey
data, and real examples of failed surveys that have made it through
conventional QC procedures without detection are presented. The paper defines
principles for survey-program design and implementation to eliminate these
weaknesses. It proposes a new set of minimum requirements for survey
Where the cumulative QC data do not support the model, it is necessary to
adjust the model to fit the data. In the longer term, it might be possible to
rework the tools and procedures in the hope of meeting the original model's
specification, but the uncertainty estimate assigned to a survey must be valid
at all times.
The methods described in this paper are capable of ensuring valid positional
data and should be incorporated into standard practice. However, to implement
these techniques correctly and consistently, it is probably necessary to
establish a specialized supervisory function, responsible for the integrity of
wellbore positioning across a company's entire operation.
This paper is the product of a collaborative work within the SPE Wellbore
Positioning Technical Section (SPE-WPTS), and combines two previous papers on
the subject by Ekseth et al. (2006, 2007).
© 2010. Society of Petroleum Engineers
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- Original manuscript received:
3 July 2009
- Manuscript approved:
31 December 2009
- Published online:
21 June 2010
- Version of record:
16 December 2010