Reducing the Uncertainties Associated with Unseen Borehole Inclination Changes between Directional Survey Stations on TVD Calculations and Post-Geological Interpretation
Authors: Hameed Mahmoud , Gouda Mohamed , Abouzaid Ahmed , Selim Mohamed , Alrushud Albaraa 
Published: February 2022

Abstract
It is well known in the Oil & Gas industry that Rotary Steerable Systems (RSS) are the most utilized and important drilling technology to optimize the placement of highly deviated and horizontal wells thus maximizing exposure to target reservoirs. RSS benefits over conventional motorized directional drilling are simply summarized as delivering a smooth borehole trajectory via continual tight control of borehole inclination and azimuth without any interruption to the drilling process. A vital measurement of the RSS system is the Near Bit Inclination (NBI due to its proximity to the drilling bit, its continuous real-time updates and tight tolerance range of (0.01 – 0.15 deg.) when correlated to conventional Measurements While Drilling (MWD) directional surveys. This gives NBI measurements a higher credibility as it provides better visibility on the detailed borehole trajectory and this will lead to enhanced decision making while drilling, as compared to the official borehole trajectory which is currently being mapped based on conventional MWD surveys "static" stations which are only taken over fixed depth increments (≈ 95ft MD, or a drill pipe stand length). The use of NBI provides a significant improvement over current True Vertical Depth (TVD) calculations using these stationary borehole inclination and azimuth measurementswhich will not be responsive to any wellbore trajectory changes between adjacent survey stations and the final resultant borehole trajectory will be a series of interpolations between thesesurvey stations. This in turn will have a strong influence on real-time geological interpretations like Formations tops, Formation true dip and associated True Stratigraphic Thickness (TST), which leads at the end to inaccurate well placement through target reservoirs as well as inaccurate reserve estimates. Considering post-well Geological work; subsurface maps need accurate True Vertical Depth Subsea (TVDSS) calculations for the planning of future lateral wells placement, especially those targeting thin reservoirs, in which reservoir navigation is itself a challenge.This paper sheds light on the importance of having real-time TVD calculations based on continuous updates of real-time NBI and borehole azimuth, rather than sparse updates from stationary MWD surveys. It also presents a few examples of effects of borehole inclination changes between stationary MWD surveys, either planned or accidental, on the TVD calculations and on final well placement
This paper is shedding the light on the importance of having Real-time TVD calculations based on continuous updates of Real-time NBI and borehole azimuth, rather than those updates from stationary MWD surveys. It is also presenting few examples about the effects of borehole inclination changes between stationary MWD surveys, either normal or accidental ones, on the TVD calculations and generally on the final well placement.