Numerical simulation of breaking heterogeneous granite with PDC mixed-tooth bits
Unconventional oil and gas resources have complicated stratigraphic conditions,which bring great difficulties to their exploitation.In the process of exploiting unconventional oil and gas resources,the encountered dense strata have stricter requirement for the performance of drill bit.Conventional PDC bit mainly makes use of cutting teeth to cut and break rocks,producing weak aggressiveness,which not only decreases its lifespan and rock breaking efficiency greatly,but also increases drilling costs.To clarify the rock breaking mechanisms and effects of the PDC mixed-tooth bit and improve its rock breaking efficiency and lifespan,this paper establishes a heterogeneous granite model,compares the rock breaking effects of mixed cutter arrangement of axe-shaped,triangular,and circular PDC cutters using the finite element method,and selects the optimal cutter arrangement scheme.Then,the space between cutters is optimized using the response surface method,and the rock breaking effects and patterns of bits with different height differences between the front and rear rows of cutters are analyzed.Finally,the rock breaking processes of PDC bits with mixed cutters,axe-shaped cutters,and triangular cutters are comparatively analyzed based on field application.And the following research results are obtained.First,axe-shaped cutters possess stronger aggressiveness,facilitating the penetration into rocks,so they shall be arranged in the front row of the blade,while triangular cutters are more stable,so they are applicable in the rear row.Second,the optimal mixed cutter layout consists of axe-shaped cutters in the front row and triangular cutters alternating with axe-shaped cutters in the rear row.The optimal lateral distance from the rear-row triangular and axe-shaped cutters to the front-row axe-shaped cutters are 10.0 mm and 10.1 mm,respectively,and the optimal longitudinal distance is 7.0 mm.Third,after the rock breaking of the front-row cutters,the stress inside the rock can be released,which facilitates the rear-row cutters to break rocks,but the generated stress zone is limited,so it is recommended to keep the height difference between the front and the rear rows of cutters within 1 mm.Fourth,compared with the PDC bits with only axe-shaped cutters or triangular cutters,the PDC bit with the optimal mixed cutter arrangement increase the drilling footage by 21.4%and 44.1%,respectively under confining pressure.In conclusion,the drill bit with mixed cutter arrangement can achieve a high rock breaking efficiency and stability and realize a favorable drilling effect in deep and hard-to-be-drilled strata,so it is worth promoting and applying practically.
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