Analysis And Computation Of Drag And Strength Of Drill Stem For Directional Well

Based on the theory of small displacement deformation of slender rod and the engineering characteristics of directional wells, the fundamental differential equation of static analysis of drill string is derived by considering the effects of torque and axial force on bending problems and the changes of acting position and direction of applied force and moment caused by deformation. Because of the use of the co-moving coordinate, the derivation process seems simple and clear. When dealing with the contact problem between borehole wall and drill string, the borehole wall is treated as an elastic body in order to fit for the actual drilling situation. Owing to the Double Non-linear of large displacement and contact problem, as well as the non-linear of the solved fundamental equation, it is necessary to use the complex function to convert the fundamental equation into variable coefficient nonlinear ordinary differential equation. After converting the iterative approximation equation into constant coefficient linear ordinary differential equation, the approximation value can be calculated by the iterative method on the complex plane. According to the actual calculation, this method has a fast convergence rate. On the basis of this method, the computation software which is used to get the drill string drag and strength in directional wells is developed by using database technology. The paper analyses the different force condition of the drill string under various borehole assembly, operating parameters and well conditions through a series of actual example calculation. The practical example calculation shows that the calculation model of the drill string strength and drag and torque is reasonable, the theoretical calculation results also have a good coincidence with the practical measurement results. The computation software can be used for calculating the drag, torque and hood load. Consequently, it can provide analysis means for the design and construction of directional wells, the prevention and treatment of accidents and the selection of drill rigs and drill strings.