Reducing TVD Uncertainties Using Dual Inclination and SAG Model

Abstract Uncertainties in True Vertical Depth (TVD), over the years in the practice of drilling, have been a concern for precise wellbore position and collision avoidance. Accurate well placement in thin oil bearing zones have necessitated service companies to design Bottom Hole Assemblies (BHA) capable of providing actual hole inclination with high precision. TVD accuracy is normally a function of Measured Depth error and Inclination error. These errors could arise from sensor calibration, misalignment error and BHA Sag. In highly deviated wells, inclination is the dominant contributor to TVD uncertainty. Reduced inclination uncertainty therefore results in reduced TVD uncertainty. By measuring the inclination of the well at same depth with different sensors positioned in different locations in the BHA in rotary or non-rotary mode, the improved accuracy in TVD is achievable. This technique, also known as Dual Inclination, is a survey method used to increase inclination accuracy, and thereby reduce TVD uncertainty. This paper discusses practical field cases for two long tangent and horizontal wells drilled at different locations of West Africa where Sag corrections and dual inclination were utilized to reduce the error in TVD prediction. The technique was adopted to avoid collision with existing and future wells and produce the oil rim in the zone of interest. Final results showed difference of 2ft and 3ft in TVD for both wells with 6ft and 4ft difference in vertical uncertainties respectively. The data utilized for this study were obtained from the well survey from the two sensors within the BHA. Also discussed are the Quality Control criteria to be met for right application of this technique. Recommendations for highly deviated wells and fields with greater well density are also discussed.