Novel Engineering Approach Utilizing Underbalanced Coil Tubing Drilling Solutions for Tight Sour Gas Carbonate.
Authors: Abd EL Meguid Mohamed Osama , EL Shahat Ayman , Bin Sumaida Ali Sulaiman , Al Mutawa Ahmed Abdulla , Saleh Abdalla Saleh , Almazrouei Saeed Mohamed , Yousfi Fawad Zain , Baslaib Mohamed Ahmed , Al Ali Abdulrahman Hasan , Al Hosani Mariam Ahmed , Al Bairaq Ahmed Mohamed , Mohamed Ihab Nabil , Ojha Aditya , Alawadhi Fatima Omar , Solaiman Tarek , Almteiri Nama , Bernadi Bondan , Rangel Pedro , Basha Maged , Zhylkaidarova Sholpan , Wahbi Ali , Jumagaliyev Yerlan , Rennox John 
Published: October 2022

Abstract
Due to the declining reservoir pressures in some of its onshore gas carbonate fields, ADNOC decided upon an initial 3 well UBCTD, (Underbalanced Coil Tubing Drilling), campaign in its onshore Asab and Bab fields, with 2 wells to be drilled in Asab and 1 in Bab. Both target fields have high H2S concentrations up to 6% and ADNOC undertook the necessary candidate selection process, Basis of Design, and equipment selection to enable them to drill these wells using UBCTD techniques. Due to the high H2S content, it was required that a closed loop system design was implemented, which was the 1st successful one implemented in the Middle East.
The project's given objectives were analyzed, and the planning was conducted considering the different aspects to achieve ADNOC's objectives and expectations.
Several challenges were faced during the designing phase which had to be resolved prior the operations start-up. These challenges included extended drilling reach, closed loop returns handling system, handling high H2S levels at surface amongst others.
One of the main design objectives, the drilling reach, was improved by optimizing the trajectories Dog Leg Severity, (DLS), and Bottom Hole Assembly, (BHA), configuration. Instead of a conventional mud motor, a turbine was used to give power to the bit and allowed having a lower Weight on Bit, (WOB), to drill the formation, thereby increasing the depth of the section. The trajectory was planned in a way to maximize the reservoir contact within the production layers and reduce footage in the non-productive zones between the producing formations, therefore maximizing the well productivity. Increasing the well production was key to the project economics and to prove the value brought by the UBCTD to ADNOC's hydrocarbons production.
Several business disciplines collaborated closely under the IWC, (Integrated Well Construction), stewardship to provide practical solutions and design a system specifically tailored to achieve the objectives and overcome the various challenges associated with this project.
The final solution was a closed loop system capable of:removing solids/drilled cuttings from the system.measuring flow rates of different fluid phases (gas, condensate & water).treating and removing H2S.exporting gas and condensate to ADNOC's production facility.whilst drilling the well in Underbalanced conditions.
After the operations start-up on the 1st well, the returns handling system was modified to improve the efficiency and enhance the safety of the personnel and equipment.
This paper will discuss the design and planning involved in the successful drilling of these three wells and the operational challenges and mitigations encountered while drilling.