Analysis on the weight-on-bit transmission efficiency of the static push-the-bit rotary steerable system

Abstract

Now, the static push-the-bit rotary steerable system (SRSS) developed in China is being put into field tests and primary applications. During its normal drilling, higher frictional forces generated by the static pushing forces of the rotary ribs on the wellbore wall influence the weight-on-bit (WOB) transmission efficiency and the rate of penetration (ROP). In this paper, analytical models of the WOB transmission efficiency were established for the wellbore wall with and without ladders respectively based on the structure and working principle of the SRSS. And the variation rules of the WOB transmission efficiency with the nominal WOB, the frictional coefficient of wellbore wall, the pushing force and the front chamfer of the steerable rib were analyzed. And the following research results were obtained. First, the nominal WOB of this SRSS has to satisfy the minimum requirement and it is conducive to the increase of WOB transmission efficiency by increasing the nominal WOB appropriately. Second, whether there are ladders on the wellbore wall or not, the WOB transmission efficiency decreases as the total pushing forces of steerable ribs and the frictional coefficient of wellbore wall increase. And the WOB transmission efficiency in the case of wellbore wall with ladders is much lower than that without ladders. Third, if there are ladders on the wellbore wall, the front chamfer of steerable rib has a significant effect on the WOB transmission efficiency. The larger the front chamfer is, the lower the WOB transmission efficiency is (even self-locking). In conclusion, the research results play an important role in guiding the rational selection and control of the WOB at drilling site.