Analysis Of Flexible Drill String Passing Capacity In Deflection Section Of Ultra-Short Radius Horizontal Well

Ultra-short-radius horizontal well drilling has become an important means for oil and gas field exploration and development,tapping potential and increasing efficiency due to its excellent flexibility,good drilling accuracy and low economic cost.Due to the complex drilling environment and unmatched drilling tools,flexible drilling tools are often stuck and fractured.At present,most of this technology uses field experience to guide the drilling technology,and there is little mechanical analysis of the overall slender flexible drilling tool.Ensuring the rationality and safety of the load transfer law of the articulated flexible drilling tool is the key to the success of ultra-short radius horizontal well drilling.Therefore,the research results are as follows:(1)According to the working principle of the flexible drilling tool for ultra-shortradius horizontal wells,considering the geometric problems of the articulated structure,and using theoretical analysis,a reasonable combination of flexible drilling tools is determined.At the same time,considering the problem of excessive wear resistance,a mechanical model that can reasonably describe the law of load transfer is determined.(2)Establish the transient dynamic model of the flexible drill string contact with the steering tube in the deflecting section.It is obtained that the axial force of the flexible drill string gradually increases with the increase of the well depth,and the torque gradually decreases with the increase of the well depth.When the radius of curvature is the same,the bottom hole axial force of the flexible drill string increases with the increase of the inclination angle,and the torque decreases with the increase in the inclination angle.When the well depth is the same,the larger the radius of curvature,the smaller the torque loss from the wellhead,and the greater the torque.(3)Establish the transient dynamic model of the flexible drill string contact with the shaft wall guide tube in the deflection section.It is obtained that the axial force of the guide cylinder is transmitted from the opening window,and there is basically no loss.The lost load is mainly the torque of the flexible drill string.The bottom-hole axial force of the flexible drill string increases with the increase of the radius of curvature,the bottom-hole axial force of the steering bore gradually decreases,and the total bottom-hole axial force of the flexible drilling tool gradually increases.(4)The safety evaluation of single-section flexible drill string under different working conditions is carried out.It is obtained that the maximum equivalent Mises stress of the horizontal section of the ultra-short radius horizontal well under the same state is larger than that in the build-up section.Under the ultimate load,the singlesection flexible drill string is safe.(5)Topology optimization for single-section flexible drill string.By increasing the contact area between the ball head and the ball seat,the maximum equivalent Mises stress of the flexible drill pipe can be reduced,and the service life of the flexible drill tool can be improved.Combining the optimized model with on-site investigation,the safety of the improved flexible drilling tool is significantly improved,and there is no drill tool accident at present.In summary,this paper systematically studies the load transfer law of flexible drilling tools and conducts safety evaluation and structural optimization of singlesection flexible drill pipes.It provides a reasonable mechanical basis for ultra-short radius horizontal well drilling and suggestions and directions for improving flexible drilling tools.