Research on the working mechanism of the PDC drill bit in compound drilling

A combined ground-driven and down-hole-driven motor significantly improves the penetration rate of polycrystalline diamond compact (PDC) drill bits but greatly reduces bit life. By studying the mechanical behavior of the drill string in compound drilling, a kinematics model of a PDC drill bit under compound drilling was established, and the effects of transmission ratio, cutter layout, and drill string geometry on the cutting trajectory formed by the cutters were analyzed. Furthermore, a numerical drilling simulation of the PDC drill bit was developed and enhanced to research the kinematics and cutting regularities of the PDC drill bit in compound drilling. In addition to high rotation rate, nonparallel scraping of the PDC cutters and the unbalanced contact state between the cutters and the rock are the prime reasons for improvement of the penetration rate of the PDC bit in compound drilling. An error analysis comparing simulation and experimental results was performed and the two agree to within <20%. Based on the working mechanism of the PDC drill bit in compound drilling, technical ideas and implementation methods for customized designs of PDC drill bits are discussed.