Linerless Casing Design. A Success Story of Synergy, Strong Planning and Flawless Execution Proving for Fhe First Time that a Significant Reduction in Well Construction Time is Possible in Nasr Field, Adnoc Offshore, Uae
Authors: Cantarelli Elena , Ahmed Mohamed Gamal , Benny Praveen Joseph , Al-Harbi Ahmed Abubaker , Ibrahim Ahmed Mohamed , Alhammadi Yousef Ahmed , Alzaabi Mohamed Abdulrahman 
Published: November 2019

With the objective to reduce the well construction time, ADNOC Offshore drilling team pioneered for the first time in Nasr field the linerless casing design which consist of merging the 8-1/2-in section into the 12-1/4-in phase. To maximize the savings, it is essential to drill the linerless section in one bit run, as multiple trips would defeat the purpose of implementing the new casing design.

The additional complexity of the 12-1/4-in section, now extended and landing directly above the M B reservoir, posed extra challenges at the beginning of the trial which required several unplanned trips due to bit damage or stuck pipe; the severe stick slip and high torque impacted the steerability and the rate of penetration (ROP). The development of a solid strategy to flawlessly drill the 12-1/4-in section was essential to effectively reduce the well construction time and exploit the linerless design potential.

The following solutions enabled the resulting fast-paced learning curve: polycrystalline diamond compacts (PDC) bit with conical diamond element (CDE) inserts to enhance the durability of the cutting structure while drilling hard stringers in K formation; motorized rotary steerable system (MRSS) to supply extra torque to the bit and compensate for the torque limitation of drillstring and top drive; anti-sticking bottom hole assembly (BHA) replacing 8-in jar and drill collars with 7-in jar and 5-7/8-in heavy weight drill pipe (HWDP) to minimize contact area and the risk of differential sticking; oil-based mud and later high-performance water-based mud, instead of the standard water-based mud to improve wellbore condition in shale intervals for safer and faster tripping; 24/7 real time wellbore surveillance service to monitor torque and drag, hole cleaning, shock and vibration, and drilling performance.

From the first trial, the new developed strategy enabled the drilling of the 12-1/4-in section in one run, achieving more than 20% IADC ROP improvement over previous linerless sections. Stick slip was considerably reduced resulting in consistent steerability and good hole quality; the bit showed almost green dull condition; the oil-based and high-performance water-based fluids maintained the wellbore in excellent condition maximizing the tripping efficiency. Furthermore, the linerless design enabled additional saving on subsequent operations for example the 7-in production packer could be pre-made with the lower completion, thus eliminating a dedicated run of two days. This proved that the linerless casing design is doable and can potentially reduce the well construction time over conventional casing design by at least seven days. Such result is not only an important milestone for Nasr field, but also for other fields that in future may adopt the same design, enabling additional saving to the operators.