| As a new form of one-cone bit, special-shaped one-cone bit is designed in order to overcome the defects of pre-wear of spherical one-cone bit in drilling hard formations with high abraseveness. At present, this kind of drill bit is still in research and development, and there are a lot of research blanks, including tooth-load which is one of top issues concerned by designers and users.Relying on the joint research task of " new technology of efficient drilling in deep and difficult formations" under national science and technology major projects of "key technologies of efficient drilling in complex formations for low permeability gas reservoir" this paper adopts finite element simulation method, bench testing, and field testing to study the basic rock-breaking theory of special-shaped one-cone bit. The research contents include bottom-hole pattern, rock-breaking mechanism, teeth-load and its distribution among teeth-rings, and teeth-failure mechanism, etc. The specific work and main conclusions are as follows:(1) Taking the patent of "a tapered one-cone bit" applied by Southwest Petroleum University as the prototype, two special-shaped one-cone bits with different cone-shapes are designed by using Pro/E;(2) The teeth-loads of the two bits are tested on experimental bench under different WOBs and rock types;(3) Bit-rock interactional models of the two bits are established and drilling processes are simulated under corresponding experimental conditions by finite element software LS-DYNA;(4) After data processing, comparision and analisis are made to the bottom-holes, WOBs and axial forces of the bits, shapes of time history curves of teeth-loads and its distributions among teeth-rings obtained by the two methods, and it verifies that the simulation in this paper based on FEM is reliable. The analysis indicates:1) The special-shaped one-cone bit breaks rock by the combined effects of impacting and shearing, which is different from the way of long distance scraping adopted by spherical one-cone bit, and it illustrates that the cone-shape will directly affect rock-breaking way;2) The distribution law of teeth-loads is mainly related to bit structure, and it has nothing to do with rock properties;3) Teeth-loads are unbalanced on different teeth-rings. The teeth-loads on bigger part of the cone are larger than that on the smaller one; meanwhile, teeth-loads on teeth-rings adjacent to transitional part of the cone are also larger. So the cone-shape and teeth-arrangement should be properly optimized.(5) One of the two bits is chosen for field test. It shows that the main failure mode of the bit is teeth-wear. On the basis of field test, the drilling process of the bit under real conditions is also simulated by FEM. Conclusion of finite element analysis is in consistent with the wear distribution law of the tested bit, and which proves the reliability of numerical simulation again. The research achievements in this paper help to gradually improve the fundamental rock-breaking theory of special-shaped one-cone bit, and have a certain significance in guiding bit design. At the same time, the numerical simulation method adopted in this paper is also applicable to tri-cone bit, PDC bit, hybrid bit, etc. The using of this method in bit design can effectively shorten the development cycle and reduce the cost. |
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