| The exploration and development of Oil-gas are on a path toward the deep and ultra-deep wells at present. With the increase of well depth, the drillability of rock gets worse. The rate of penetration (ROP) and the working life of general bit decrease, thus it can't meet the requirements of improving ROP and efficiency. Hybrid bit, which combines the structural features and the working principals of PDC bit and cone bit, will provide a new idea to improve ROP and rock breaking efficiency in hard formation. Currently, the research on hybrid bit in China and abroad is concentrated on its structural design, manufacturing, field application and evaluation of its applying result. However, little has been reported in the literture on the rock breaking mechanism of hybrid bit, and which happens to be the key of optimum structural design of hybrid bit and accelerating its popularization and application to improve ROP and efficiency. Therefore, the interaction mechanism between hybrid bit and rock is researched in this paper based on the theory of Hertz contact, rock mechanics and finite element method. Simultaneously, the methods of simulated analysis and bench test are employed in this paper. More detail works and achievements are as follows:(1) Paper summarizes the research on the structural design, simulated analysis, field application of hybrid and the rock crushing mechanism in China and abroad. Using the method combining the theoretical analysis with the simulation study and bench test to research the rock breaking mechanism and progress is put forward.(2) The rock breaking progress under the individual action of the PDC cutting structure and the roller-cone cutting structure is analyzed. On this basis, the rock breaking mechanism under the combined action of the two cutting structures above is researched. It can be obtained that the roller-cone cutting structure of hybrid bit crushes rock by fits and starts, the PDC cutting structure scrapes rock continuously, which supplement and promote each other.(3) Based on classical elasticity and Hertz contact theory, the contact mechanical behavior of the PDC cutter and the roller-cone cutter has been analyzed, the mechanical formula of hybrid bit breaking rock is derived, and the force comparison analysis between hybrid bit and PDC bit when breaking rock is carried out. The results show that hybrid bit has lower and more smooth torque and it can reduce torsional vibration effectively.(4) This paper has carried out the bench test of hybrid bit breaking rock, and the torque and weight on bit data is measured. Furthermore, the nonlinear dynamic model of hybrid bit breaking rock is established according to the practical boundary conditions of the bench test, and the simulating calculation is carried out. There is a relative error of below 8%between the average value of the torque and weight on bit data and the experimental result, which demonstrates that the finite element modeling method is feasible.(5) The finite element models of the fullsize hybrid bit and PDC breaking rock in deep hard formation are established, and the simulating calculation is carried out. The results show that the torsional vibration of hybrid bit is lower than that of PDC bit, reducing by about 50%, and which is consistent with the results of the theoretical analysis and drilling monitoring. The impact effect on rock of hybrid bit is comparatively greater than that of PDC bit, so hybrid bit has a high rock breaking efficiency in hard formation. The effect of hybrid bit on the rock adjacent to the wellbore is lower, which is beneficial to improve the borehole stability.(6) The effect of the relative height of composite structures on hybrid bit breaking rock is researched by both theoretical analysis and simulating calculation. The results show that the hybrid bit, in which the roller-cone cutting structures are ahead of PDC cutting structures within a rational range, can reduce the torsional vibration in hard formation, simultaneously, the impact effect on rock is more outstanding. Therefore, the service life and rock breaking efficiency of this kind of hybrid bit are longer and higher. |
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