A new method to improve rock breaking efficiency by using automatic unloading of bottom hole central stress

Abstract

The main reasons for slow drilling speed in deep formation are poor rock drill-ability, low cutting efficiency and short service life of rock breaking tools caused by high hardness and intensity of bottom hole rock and stress concentration of surrounding rock. In view of this, the concept of releasing rock stress in the bottom hole center to improve rock breaking efficiency and prolong the service life of rock breaking tools is put forward, the bit for automatic unloading of bottom hole central stress (AUOBHCS) is designed, and the testing experiment of rock breaking effect is carried out. The experimental results show that, firstly, the rock breaking efficiency can be improved by using automatic unloading of bottom hole central stress. When the rock breaking energy of the special bit is sufficient, the rock breaking efficiency in limestone and red sandstone can be increased by 1.59 and 4.36 times respectively, and the rock breaking effect still has a large room for improvement. Secondly, WOB and rotating speed are the key factors affecting the rock breaking effect of the bit used for automatically unloading bottom hole central stress, whose speed-up effect increases with the increase of rotating speed and WOB. Thirdly, the bit for automatic unloading of bottom hole central stress has the function of prolonging the service life of the cutter, and it should have some restraining effect on the “holing” of the bit. The above proposed idea of improving rock breaking efficiency and the laboratory test of rock breaking bit provide a new idea for the research and development of downhole rock breaking speed-up technology and tool, and open up a new direction for the drilling speed-up research in deep formation difficult to drill.