Is While-Drilling True Vertical Depth True?
Authors: ElGizawy Mahmoud , Androsovich Natalia , Rachi Hakim , Abbas Nashat , Katheeri Yousef Al , Koakkadan Saleel , Noukhatha Jamal Al , Hargreaves Brad 
Published: November 2017

Abstract
The objective for drilling development wells is to penetrate the target reservoir, maximize recovery, and ensure maximum production. Determining accurate true vertical depth (TVD) in real time is essential to achieving these objectives in thin reservoirs. Ideally, encountered different formation tops while drilling should match the reservoir structure model within a reasonable accuracy; however, during drilling operations, formation tops frequently become shallower or deeper without any logical reasons. This article presents the causes of this uncertainty and provides a solution to the drilling operations problem in real time.
The majority of the drilling bottomhole assemblies (BHAs) consist of drive systems containing a positive displacement motor or rotary steerable system (RSS) and measurement-while-drilling (MWD) tool. The MWD tool includes the survey package that measures the inclination and direction of the wellbore, and a similar package is installed within the RSS. The technique, introduced in a massive field in the Middle East, uses the two survey packages within the same BHA to reduce the TVD uncertainty. Additionally, it increases the survey frequency without impacting rig time and mathematically reorients both survey packages to the center of the wellbore.
Results obtained by implementing this technique for the first time in the region are encouraging. The main factors affecting calculating TVD while drilling are measured depth, measured inclination, and survey frequency. The presented technique successfully minimized the errors associated with the measured inclination and errors associated with TVD calculation due to insufficient survey frequency. The presented advanced TVD survey management technique allows the well to be placed according to the plan and maximizes reservoir production. The accuracy of the calculated TVD using traditional MWD surveys is compared with that of this presented technique and shows a significant accuracy improvement. Consequently, when implementing this advanced technique, the formation tops are encountered as predicted by the reservoir structure model. Without implementing this technique, the encountered formation tops were found to be shallower in some wells and deeper in others due to the high uncertainty in the calculated TVD using the standard MWD surveys while drilling. This high uncertainty directly affects the correct well placement within the planned layers and negatively impacts the hydrocarbon production rate.
The benefits of this advanced TVD survey management technique in real time are significant. This technique provides high-accuracy wellbore positioning and TVD while drilling with well trajectory corrections being made available to accurately penetrate the target. Additionally, the advanced technique reduces the ambiguity in the different formation tops while drilling to enhance the reservoir production recovery, and eliminates the costs associated with drilling longer sections required to penetrate the target zone.