The rock breaking mechanism of a combined high-voltage electric impulse-PDC bit drilling technology.

• A new drilling method combining HVEI drilling with mechanical drilling is proposed. • A combined HVEI-PDC bit is designed in this paper. • The HVEI-PDC rock fragmentation model is established in this paper. • Electrode spacing significantly affect the efficiency of the combined bit. Low rock breaking efficiency of drilling bit is one of the key factors restricting the large-scale development of oil gas in deep formations and geothermal energy resources in hot dry rock (HDR). The high-voltage electric impulse (HVEI) drilling technology has unique advantages in deep formation and HDR drilling. However, existing electrode bits form low-quality borehole walls and are not capable of performing directional drilling. To broaden the scope of engineering application of HVEI drilling, a new method combining HVEI drilling with mechanical drilling is proposed in this paper. Based on the structure of the existing electrode bits, a combined HVEI- Polycrystalline Diamond Compact (PDC) bit is designed, and the rock breaking mechanism of the bit is studied. A dynamic damage model (DDM) of rock containing an electrode bit is established on the basis of HVEI drilling's basic process. Furtherly, the HVEI-PDC rock fragmentation model is established by coupling the DDM with the heterogeneous granite model which is based on Voronoi subdivision. Through the analysis of granite fragmentation in HVEI-PDC drilling, relevant conclusions are obtained: Compared with conventional PDC bit, there is no significant difference in the torque variation of the combined bit during drilling. When the electrode spacing L e < 40mm, the energy consumption of the combined bit is higher than that of the conventional PDC bit at low rotating speed, and the efficiency is reduced. When L e ≥40mm, the larger the L e , the more obvious the speed-up effect of the combined bit compared with that of the conventional PDC bit, and the lower the drilling cost. The conclusions of this paper are expected to provide a reference for promoting HVEI drilling to achieve wider industrial applications. [ABSTRACT FROM AUTHOR]