This paper presents a new Flexible Bottomhole Assembly (BHA), which is classified as a coupled driven drilling BHA of the power anti-deviation, with the technical superiority of anti-deviation and high ROP. The coupled driven drilling has simultaneously two driving powers: one is from kelly/rotary-table or top-drive; another is from positive downhole motor (PDM). Comparing with the traditional BHA, the new BHA for coupled driven drilling uses a short flexible sub and the appropriate PDC bits with high hydraulic efficiency and high cleaning and cooling capacity, which has rationally distributed the stabilizers' spacing, and has scientifically designed the structure of PDM centralizer's size.

The new BHA used for coupled driven drilling is based on an integrated mechanical model. Through the analysis on the kelly/rotary-table and rotor centrifugal force and their transmission efficiency, the fulcrum role to PDM centralizer and its leverage ratio, the formation force and the geometric anti-deviation force, and the coupling analysis on the above factors, this paper has set up an anti-deviation force's calculation model of the coupled driven drilling, a relationship model between the allowable well-deviation and the critical WOB, a PDC bit gauge length design model, and the structural designed model of PDM centralizer. According to the above integrated model from the view of the whole system, a set of software moduli, a coupled driven drilling BHA mechanical analysis and drilling parameters optimization, is developed to determine the optimum BHA through different formation parameters, drilling parameters, the design requirements to well-deviation and ROP. The new BHA and the matching technology have been in successful application in the part of oil blocks of the Bohai Sea in China, the ROP and well-deviation meet the design requirements, and borehole quality has been improved significantly.

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