Mechanical models of drillstrings with drag reduction oscillators and optimal design methods of vibration parameters in horizontal drilling

Abstract

Engineering practices show that installation of oscillators on drillstring can effectively reduce the axial drag and inhibit stick-slip vibration in horizontal drilling. However, the mechanisms of axial drag reduction and stick-slip vibration inhibition of oscillators have not been fully revealed, and there is a lack of effective optimization design methods of vibration parameters. To solve the above problems, the mechanical models of drillstring with drag reduction oscillators are firstly built, which include axial vibration model in sliding drilling mode and stick-slip vibration model in rotary drilling mode. The nonlinear relationship between friction force and velocity is considered in the mechanical models. Next step, the optimal design models of vibration parameters are established, in which the drag reduction efficiency and the stick-slip reduction efficiency are taken as the objective functions respectively, and drillstring fatigue, hydraulic loss and so on are the constraint conditions. The optimal design models are solved with orthogonal experimental design. Finally, the above models are applied to horizontal drilling engineering in Chinese Jimsar oilfield.The results show that oscillators can significantly reduce the total friction forces on drillstring and keep a more stable weight on bit (WOB) in sliding drilling mode, and inhibit the stick-slip vibration of drillstring and fluctuation amplitude of WOB. The primary factors affecting the drag reduction efficiency in sliding drilling mode are vibration amplitude and number of oscillators, whereas the primary factors affecting stick-slip reduction efficiency in rotary drilling mode are rotary speed of drillstring and WOB. It is recommended to install 3 oscillators on bottom hole assembly of drillstring with vibration amplitude of 40 kN, rotary speed of 162 r/min, and the average WOB of 171 kN in case study, and then the drag reduction efficiency and the stick-slip reduction efficiency are respectively 37.47% and 74%.