Drilling deep reservoir sections requires long drillstring and, consequently, long tripping and handling times. The long and heavy drillstring generates high side forces across the casing especially in S-shape and J-type wells that could affect its integrity. The operating envelope is affected and results in limited operational efficiency. A fit for purpose tapered drillstring was designed with the objectives of enhancing the operating envelope, minimizing the forces and stresses applied to both the drillstring and the casing, reducing the handling time to improve operational efficiency.

This newly engineered tapered string consist of 4" and 5-1/2" drill pipes, optimum number and size of heavy weight drill pipes and drill collars, and fit for purpose driving and steering mechanism to be utilized in both the 8-3/8" and 5-7/8" hole sections. The design is based on torque and drag simulations, drillstring dynamics and hydraulic analysis together with the driving mechanism and drill bit designs, paramount to reduce shock and vibration.

This novel approach was implemented successfully in various well types saving an average of 1.5 days per well. It also reduced both the downhole circulating pressure fluctuations and the Equivalent Circulation Density (ECD) by providing bigger annular clearance thus having a positive impact on differential sticking risks, side forces, and torque and drag.

This paper describes the engineering design and modeling undertook and the successful implementation of the tapered drillstring in deep and complex gas wells.

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