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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.

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Summary Aiming at the difficulty of penetration in hard-plasticity formation represented by the Mahu oil field, a new type of chord-edge cutter is proposed. Through theoretical derivation and quantitative analysis of specific unit pressure (SUP) and breaking area, the penetration and rock-breaking area of the chord-edge cutter are studied, which clarifies that the rock-breaking efficiency of the chord-edge cutter is higher than that of the conventional cutter. The first part of the laboratory experiment investigated the drilling efficiency of two mini-bits on a special sample. The results show that the chord-edge cutter mini-bit has higher efficiency than a conventional mini-bit. The second part of the laboratory experiment investigated the influence of rate of penetration (ROP), revolutions per minute (RPM), and rock types on the drilling performance of chord-edge cutter mini-bit. The data show that the chord-edge cutter is more suitable for drilling hard-plastic rocks. It also reveals that the higher the RPM, the lower the stick/slip vibration, but the RPM above a certain value will lead to an increase in lateral vibration. ROP and weight on bit (WOB) are positively correlated. And the higher the drilling speed, the higher the stick/slip vibration and lateral vibration. To verify these conclusions, a field test is carried out in the hard-plastic formation of a well in the Mahu. In this test, compared with a conventional polycrystalline diamond compact (PDC) bit, a roller-cone bit, and a hybrid bit, the chord-edge cutter bit has the best drilling effect. Further, it is found that when using the chord-edge cutter bit, the high RPM and proper control of WOB can achieve a better drilling efficiency. This rule is mutually confirmed with the conclusion of the laboratory experiment. After the above research and its implementation, it can be concluded that the novel chord-edge cutter bit can achieve the research goal of higher efficiency, which provides a new idea to overcome challenges in the hard-plastic strata.