Nonlinear Dynamic Modeling of Drillstring-Bit-Rock Coupling System Based on Bit/Rock Interaction Simulation

Summary In this paper, based on the dynamic model of the geometrically exact drillstring in horizontal well, a nonlinear dynamic model of drillstring-bit-rock coupling system is established. First, the contact model of drillstring and borehole wall in vertical well is extended to the contact model of drillstring and borehole wall in 3D well. Then a new numerical rock model is proposed, in which rock is discretized to spherical particles and each particle behaved in three states including undeformed state, elastic deformation, and crushing state. Equivalent stiffness is used to describe the mechanical properties of rock particles. The relationship between the equivalent stiffness of rock particles and mechanical specific energy (MSE) of rock is established. Finally, the coupling dynamic model of drillstring-bit-rock is established by coupling the dynamic model of drillstring, the contact model between drillstring and borehole wall, and the bit/rock interaction model. The coupling model is spatially discretized by quadrature element method; a mixed explicit-implicit numerical integration method is used for time integration; the nonlinear equations are solved by Newton’s method; and the solving efficiency is improved by using sparse matrix and parallel computing techniques. The numerical model is verified by laboratory experiments. By the above methods, the drilling process of an actual well is simulated. The results show that the mechanical model and numerical calculation method can capture the coupling vibration phenomenon of the drillstring-bit coupling system in real-time. The innovation of this paper is to propose a new drillstring-bit-rock coupling dynamic system model, including a new bit-rock model, so that the dynamic excitation of the drillstring system in the bit section is closer to the actual situation. It provides a new idea for the research of drill string-bit coupling system and lays a theoretical foundation for drilling optimization.