Research on the Method of Improving ROP and Designing Drill Bits to Mitigate Drillability of Bottomhole Rocks

In order to solve the problems of low rock breaking efficiency of drill bits and low rate of penetration (ROP) caused by great hardness, strong abrasiveness, high in-situ stress, and poor drillability of deep formation rocks, a new method of improving ROP was put forward to release the in-situ stress and reduce the drilling resistance of bottomhole rocks based on the analysis of existing speed increase technologies. Based on this idea, seven new types of drill bits were designed: bottomhole stress-induced unloading drill bits, concentrated energy attack-type unloading bottomhole stress drill bits, differential pressure drill bits, central differential pressure drill bits, drill bits under the joint action of induced unloading and abrasive jet, stairs type drill bits, and coupled self-excited axial impact and induced unloading rock-breaking drill bits. The ROP increase effect of two types of drill bits were verified through laboratory and well-site experiments. The central differential pressure drill bit increased its ROP by 30.01% in laboratory experiments, and the highest ROP increase in well-site experiments was 318.11%. The drill bits under the joint action of induced unloading and abrasive jet increased the ROP of limestone and red sandstone by 59.0% and 336.0% in laboratory experiments, respectively. The proposal of this method and the development of drill bits provide new ways for the ROP increase technology in deep formation with poor drillability.