Drilling Through Fractured Gas Bearing Formation to Access Previously Inaccessible Parts of an Offshore Reservoir in Abu Dhabi Using Pressurized Mud Cap Drilling - Case Study
Authors: Jadallah Omar Nazih , Saleh Mujahed , Benberber Mohamed Rebbou , Arvind Upadhyay , Diltaiyev Zhanibek , Suryavanshi Vijay Himmatrao , Al Kasasbeh Ameen , Moslim Samaawi , Abdelrahman Ahmed Abdelraheem , Abdel Sattar Mohamed , Al Hammadi Yousefl , Al Katheeri Yousif Saleh 
Published: December 2021

Drilling through fractured gas bearing formations to access the oil reserves underneath has been one of the most challenging tasks for the drilling Team due to the embedded risks such as; total circulation losses, Gas migration, well control issues, hole instability, cutting beds accumulation and stuck pipe.

This paper explains an approach in drilling fractured gas bearing formations that was performed for the first time in offshore Abu Dhabi field-A, Pressurized Mud Cap Drilling (PMCD).

Drilling through fractured Gas bearing formation causes the loss of the mud column and the consequent intrusion of hydrocarbon gas to the wellbore, thus initiating well control response, which adds to the flat time and might cause cutting slippage, stuck pipe and eventually loss of well objective.

PMCD is best suited to deal with such situation, as it allows drilling to continue under the mentioned circumstances by filling the well with sacrificial fluid while the well is closed, fractures take seawater, cuttings and the formations pressure lefts the underbalanced annular fluid to reduce losses volume.

Two wells were drilled successfully using the PMCD technique in Field A where the anticipated fracture gas bearing formations system was encountered shortly below the 9-5/8″ casing shoe. The performance increased substantially in the second well as lessons learnt were implemented to avoid any time loss. Drilling the 8-1/2″ Hole section started in well #2 conventionally with required 200 psi overbalance mud weight, the drilling fluid system is directly changed to sacrificial fluid (Sea water) once the fracture system is hit and total losses observed. A light Annular mud (Seawater) is pumped in the well's annulus. After having stable PMCD parameters, drilling continued at an ROP of 100-150 FPH. TQ & Drag real-time monitoring & intermittent pumping of 3 × 50 bbls weighted HVP to clean bit & BHA from cuttings were essential to avoid getting the pipe mechanically stuck.

The 6,710 ft section was drilled successfully, Striped BHA Out of hole, Ran 7,160 ft of 7″ Liner, perform cement Job & achieved isolation. Comparing with offset wells drilling conventionally in field-A through the gas bearing fractured zone, PMCD saved +/− 44 days of the well time, cost and achieved the target. and greatly improved the operational safety by providing closed-loop drilling.

The PMCD application on the two wells is the first of its type in offshore Abu Dhabi, it allowed accessing parts of the reservoir that have been inaccessible due to the fracture system. Additionally, it increased safety of operation & saved rig days that would have been spent in treating losses and well control operation.

Pressurized Mud Cap Drilling application in field-A provides a solution for a wider implementation in developing fractured gas cap resources in future.