Study On The Mechanical Characteristics And Drilling Tendency Of A Static Push-the-Bit Rotary Steerable BHA

Rotary steerable drilling technology is currently an advanced technology in the drilling engineering field.At present,the problem of low build-up rate for the independently developed rotary steerable drilling system in our country restricts the commercial application of rotary steerable system.In addition,the instructions for well trajectory control is mainly determined by the on-site state in the process of using rotary steerable system to control wellbore trajectory.This is because of the inaccurate understanding of the drilling tendency of the bottom hole assembly(BHA)and lack of theoretical guidance for choosing the parameters to control well trajectory.The determination method has certain experience and limitations.In order to improve the build-up capacity of rotary steerable drilling system and provide theoretical basis for the determination of well trajectory control instructions when using rotary steerable tools,it is necessary to analyze and study the mechanical characteristics and drilling tendency of rotary steerable bottom hole assembly(RSBHA).The analysis of mechanical properties is the basis of predicting the drilling tendency of BHA and controlling the well trajectory.It is of great significance to guide the development and application of rotary steerable tools.In this paper,a prediction method of RSBHA build-up rate is established for the static push-the-bit rotary steering system,and the influence of different parameters on RSBHA build-up rate is analyzed.In order to improve the build-up rate,rotary steerable tool's structure and RSBHA is optimized.At the same time,the dynamic characteristics of RSBHA are analyzed,and the parameters are optimized from the perspective of improving the movement stability of the steering tool,which provides a theoretical basis for choosing the steering parameters and determining the well trajectory control instructions.Firstly,based on the balanced curvature method,a prediction model of drilling trend is established considering the influence of bit tilt angle and anisotropy.Then the basic idea of predicting RSBHA build-up rate is given by using the drilling tendency prediction model.The interaction between steering pads and wellbore is analyzed.Considering the tangential friction resistance between the steering pads and wellbore,the calculation model of actual steering force of the pads is established.The RSBHA continuous beam mechanical model is built by simplifying the steering pads and flexible short joint.Through combining RSBHA drilling tendency prediction model and the mechanical analysis model,a build-up rate prediction method for static push-the-bit RSBHA is formed.By comparing with field test data,the rationality and reliability of the prediction method are proved.On the basis of RSBHA build-up rate prediction method,the effects of rotary steerable tool's structural parameters,RSBHA parameters,drilling operation parameters,borehole geometric parameters and bit anisotropy index on the build-up rate are analyzed.It is found that the location of steering pads,the length and flexural rigidity of the flexible joint and the diameter of the lower stabilizer are the main structural factors affecting the build-up rate and the main sensitive control factors are the pads' steering force,weight on bit and bit anisotropy index.According to the influence of rotary steerable tool's structural and RSBHA parameters on the build-up rate,the structural and RSBHA are optimized to improve the build-up rate,which greatly improves the build-up capacity of RSBHA.Considering the characteristics of rotary drilling for rotary steerable drilling,a static pushthe-bit RSBHA dynamic finite element analytical model is established using finite element method.The initial stress of the drill string due to the borehole curvature is analyzed.Through the interaction model between the drill string and the wellbore,the constraint effect of the wellbore on the movement of the drill string is detailed described.The dynamic finite element model is solved by Newmark method.And then the dynamic finite element model is validated by BHA dynamic simulation experiments and field test data from BHA lateral motion state,bit torsional vibration and wellhead torque,respectively.The feasibility and accuracy of the model are proved,which provides a theoretical basis for the static push-the-bit RSBHA dynamic characteristics analysis.Using the RSBHA dynamic finite element analysis model,the effects of different drilling operation parameters(rotary speed,weight on bit,pads' steering force,tool face angle),borehole parameters(inclination angle,borehole curvature,borehole wall friction coefficient)and RSBHA structure parameters(mass eccentricity,distance between two stabilizers,single stabilizer structure)on the lateral motion and bit torsional vibration characteristics of RSBHA are analyzed.From the point view of improving RSBHA motion stability,the value range of each parameter is recommended.A static push-the-bit rotary steering well trajectory control method considering both RSBHA build-up characteristics and motion stability is established.When the requirement of well trajectory control is given,the rotary speed and weight on bit of drilling operation parameters are optimized by RSBHA dynamic analysis model to ensure tool's motion stability.And then the pads' resultant steering force is selected and determined according to the established build-up prediction method.The well trajectory control method provides a theoretical basis for the determination of the trajectory control instructions for the static pushthe-bit rotary steering.