Underbalance Coil Tubing Drilling in Depleted Tight Gas Reservoir with Closed Loop System - Contributed Towards Adnoc Sustainability Targets by Minimizing Hydrocarbon Flaring in High Sour Gas Well
Authors: Siddiqui Muhammad Ali , Al Hosani Abdullah Alhaj , Subaihi Maad Hasan Qayed , Al Janahi Mohammed Ibrahim , Zain Fawad , Baslaib Mohamed Ahmed , Al Zarouni Abdulla Haidar Abdulla , AlBlooshi Maryam Omar , Alhammadi Reem Mohamed , El-Meguid Mohamed Osama Abd , Ahmad Mubashir , Al Marzouqi Abdulla Mohammed , Al Mutawa Ahmed , Shahat Ayman El , Sumaida Ali Sulaiman Bin 
Published: October 2023

Abstract
With the depletion of reservoir pressure in low permeable formation, drilling (overbalance) always become challenging and thus the productivity due to high invasion. In order to opt the unconventional solution to maximize the recovery, it requires integrated approach such as underbalance coil tubing drilling with closed loop. The reason for opting the coiled tubing rather conventional underbalance drilling is due to high H2S in reservoir and closed loop system added to reduce emissions during projects.
Since the world demands more energy with less emission, Under Balanced Drilling is one of the solutions to prevent formation damage while drilling alongside of production from well. The UBCTD (Underbalance Coil tubing Drilling) planning requires detailed feasibility study, well configuration design along with completion design. Extensive safety reviews of all the surface network and equipment was conducted to minimize the risk of operations. Live Real time data accusation, remote well control system, continuous Nitrogen purging for handling of highly volatile condensate & H2S & SO2 dispersion modeling were conducted and implemented for successful operation.
ADNOC successfully conducted first trail of technology on one of the operating brown field. The well was freshly drilled, cased and cemented to the top of desired formation. The rig was released after running the completion and later coiled tubing brought to site to drill three lateral hole through specially designed BHAs (Bottom Hole Assembly) and maintaining continuous underbalance conditions.
Selection of appropriate material from sub-surface to surface equipment's is the key consideration for success of project. The well was drilled under-balanced to allow wellbore cleaning and returns (drilling / formation fluids) were routed to closed loop separation system. The produced gas & condensate was routed towards plant via the production line through compression system. The drill cuttings dumped to the mud pits and the drilling fluid was re-circulated. Flaring of high H2S & CO2 gases impacting the environment were totally removed.
Three laterals were drilled successfully with coil tubing underbalance drilling technology, the well was successfully tested with almost triple production from conventional lateral drilling. Further evaluation is in progress to develop the reservoir through technology. The trials proved a success in developing the depleted tight reservoirs with no Flaring thus saving environment and enhancing the productivity.
This paper will elaborate the integrated approach for planning UBCTD (Underbalance Coil tubing Drilling) operation without flaring considering the operational, Safety and subsurface aspects. Furthermore, it will highlight, how the pilot technology can contribute towards ADNOC strategy towards net zero emission as one of the key pillars of sustainability.