High-Integrity Wellbore Surveying
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 validation.
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).