Weight-on-Bit Self-Adjusting Dual-Diameter Bit Leads a Step Change in Drilling Bit Technology

Abstract

Abstract With the deepening of the large-size well section, mitigating drilling vibrations and pushing the upper limit of performance while drilling in a single bit run remains a challenge for PDC bits, and the problem of slow rate of penetration (ROP) has seriously affected the drilling cycle and cost of the whole well. Recently, new Weight-on-Bit(WOB) Self-Adjusting Dual-Diameter Bit (WSDB) technology was introduced to help overcome these challenges. The structure of the bit is designed, the stress state of the stepped bottom hole is analyzed by using the general finite element software. The results of theories are corroborated with testing in the laboratory, which also demonstrates the ability of the technology to reduce drilling vibrations and push the performance envelope. The bit is mainly composed of four parts, the pilot section, the reaming section, the WOB regulating spring, and the transmission assembly. The technology provides the bit with the ability to continuously adjust its depth of cut control(DOCC) characteristics to suit the drilling environment and to mitigate of stick/slip vibrations. Because of the presence of the pilot section, the stress concentration area on the shoulder of the reaming bit is obviously reduced, and the stress unloading area in the junction of the reaming bit and the pilot bit is larger, therefore the rock breaking mechanical specific energy is reduced greatly in stress releasing area, thus the rock breaking efficiency is improved. Through the indoor comparison test, it is found that compared with the conventional PDC bit, the ROP of WSDB can be increased by up to 325% when drilling in limestone, which also indicates that Bit B has the characteristics of faster and more efficient drilling with low TOB. A method to make full use of bottom hole stress releasing effect and reduce rock compressive strength to improve the rock-breaking efficiency is put forward, and a matched WSDB is developed. The mechanical specific energy required in the rock-drilling process can be greatly reduced, at the same time DOC characteristics of the bit are dynamically adjusted, which could remit the stick-slip vibration caused by a rapid change of DOC, thus the rock-breaking efficiency is improved.