Design Considerations for a Hard-Rock PDC Drill Bit

Abstract

This paper discusses the potential for developing a polycrystalline diamond compact (PDC) drill bit for hard rock applications such as geothermal drilling. It is concluded that in order to use the inherently efficient drag cutting process in such severe applications, measures must be taken to reduce cutter penetrating forces in order to present thermally accelerated cutter wear and thereby improve bit life. A procedure is developed for determining the conditions under which water jets can be effectively used for this purpose by directing them at the rock surface ahead of individual cutters. It is concluded that hard rocks with compressive strengths as high as 30 kspi (200 MPa) may be drillable with a hybrid PDC/waterjet bit using pressures that conventional oil field pumping technology is capable of providing. Extremely hard rocks might be drilled with such a bit using nozzle pressures well below those required to effectively cut the rock with water jets alone.