Research on Weight‑on‑bit Self‑adjusting Dual‑diameter PDC Bit Design and Effect Evaluation Utilizing Stress‑Releasing Effect of Rock at Bottomhole

摘要

Rock-breaking efficiency is not so satisfied using the conventional PDC bit, especially in the hard, soft–hard interbedded, andstrong abrasive formations in ultra-deep wells. A method is proposed to make full use of stress-releasing effect at bottomhole,thereby improving the rock-breaking efficiency, and a supporting weight-on-bit (WOB) self-adjusting dual-diameter PDCbit (WSDB) has been invented. The WSDB can dynamically adjust the depth-of-cut control characteristics, which couldimprove the rate of penetration (ROP) while mitigating the stick–slip vibration caused by the rapid change of depth of cut.The stress field of the rock at the bottomhole is calculated with a numerical method, and the working mechanism of the bitis analyzed. The application effect of the WSDB is evaluated through laboratory tests preliminarily. The results show thatthe stress concentration area on the shoulder of the reaming bit is decreased obviously and a larger stress unloading areaemerges in the intersection between reaming and pilot bit. Thus, mechanical specific energy (MSE) can be greatly reducedwhen the reaming bit drills into this section. Indoor tests show that the maximum ROP of WSDB can be reached up to 70%higher compared to the conventional PDC bit, and higher ROP could be obtained with a relatively lower bit torque. Withdrilling parameters greater than the critical revolution and WOB, the reduction in MSE can be reached up to 36% duringthe rock-breaking process. Making further efforts into the optimization of bit structure to reduce the vibration intensity isexpected to achieve more improved bit performance.