Comparison of Analytical and Numerical Models on Torque and Hookload Calculation

Abstract

An analytical model for drillstring torque and drag is generated using a soft model. The soft model does not integrate all parameters affecting the drillstring behavior although some other researchers have taken the stiffness into account in the soft model. The torque and drag calculated by finite element method (FEM) is more accurate than the analytical model. The FEM can generate results that the analytical method cannot. This is because that the FEM takes both stiffness and some complicated boundary conditions into account. The program developed and presented in this paper can be used for torque and drag analysis in vertical, directional, horizontal, and other complicated wells under different drilling operational modes. Three examples on the calculation of torque and hookload are presented. The comparison between the analytical and numerical models was done, and the results were also compared with field data. The comparison shows that the result from FEM in one example matches the field data well but the analytical model doesn't get good results no matter how the friction coefficient is adjusted. The calculation of the contact force between the drillstring and the wellbore wall was conducted using the FEM. The calculated contact force estimates where the contact happens, as well as brings information about the location of possible drillstring sticking. The analytical model cannot do this, because the entire drillstring is in contact with the low side of the hole. The FEM program presented in this paper will enforce real-time analysis correctness of torque and drag together with the analytical model. The value of these torque and drag models could play an important role in real-time and planning of future drilling operations.