Step Change in Controlling the Gas-Cap in Highly Depleted and Fractured Formation
Authors: Al Sheidi Ali Salim , Al Balushi Hatim Abdul Raheem , Al Rawahi Zahran Ahmed , Al Harrasi Ahmed Saleem , Mansur Deutra , Al Farsi Mohammed Malallah , Al Rubaiey Hilal Sultan , Al Harrasi Nadir Ali , Choudhary Manish Kumar , Orta Cesar 
Published: October 2022

Abstract
This paper discusses the successful trials of a new approach to drill Formation-A Reservoir in Field-A. Field-A is one of the most challenging fields in the Sultanate of Oman due to the presence of a gas cap combined with total losses encountered when penetrating a highly fractured and depleted shallow reservoir. Several unsuccessful attempts had been made in the past to overcome the challenge of total losses using conventional methods (LCM, cement plug, etc). Since then, it was agreed to stop the attempts to cure these losses and control these wells instead with a unique well control situation through dynamic filling, which is a continuous pumping of water (70 - 120 m3/hr depending on the hole size) in the annulus. This method acts as primary barrier by having higher downward velocity of water than upward velocity of gas. However due to large amounts of water being pumped into the reservoir, it caused a commercial impact on oil production due to water flow back in production and large number of emissions (12 T/year/well) from this unproductive operation. The other downside of dynamic fill method is the risk of well process safety due to it relies heavily on human behavior and actions. There are many cases where the filling was interrupted by different factors.
The team carried out a study on the existing process of drilling 8 ½" and 6 ⅛" holes with a view of eliminating the need for dynamic fill practice. This led to the selection of Nitrogen Cap Drilling (NCD), which is a variation of the Managed Pressure Drilling technique (MPD) technique. NCD is a closed-loop drilling system with no returns to surface based on the presence of a nitrogen cushion establishing the communication between surface and a reduced liquid column in the annulus. The well is continuously monitored, and the annulus pressure is controlled through the wellhead pressure and a balanced fluid level without the need for filling while drilling with total losses.
In 2021, the team delivered 2 (two) trial wells with NCD technique successfully without interruption and saving approximately 8,300 m3 of water compared to conventional drilling. NCD allows the pneumatic pressure communication via a pressurized nitrogen column on top of the Heavy Annulus Mud (HAM) balancing the formation pressure. The positive wellhead pressure allows better control of the well behavior and monitor potential gas migration. A positive injectivity test confirmed the applicability of the NCD technique to contain the drilling fluid and drive the cuttings back into the formation. During the drilling process, well site water is injected down the drill string and HAM is injected intermittently in the annulus form the back side of the RCD based on the annulus surface pressure changes related to the gas migration. Tripping out of hole was done using NCD keeping the well balanced and monitored to avoid gas migration to surface.
This technology is a step change in our journey to address drilling high-risk Formation-A gas-cap wells in Oman with the NCD concept. It addresses the Well Process Safety concerns, generates a huge value by accelerating oil production from the drilled wells without affecting the nearby ones and finally improves the environment sustainability goals in PDO’s drilling operations.