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•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 Le < 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 Le≥40mm, the larger the Le, 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.