The vibration characteristics of drillstring with positive displacement motor in compound drilling Part 2: Transient dynamics and bit control force analysis

The transient dynamics of a drill string and the dynamic characteristics of bit control force during compound drilling have not been elucidated yet. A numerical simulation model of drill string with a positive displacement motor was established to analyze the transient dynamics of drill string. The calculation model of dynamic bit control force was proposed based on the results of the transient dynamics of drill string. The results show that, when compared with conventional drilling, the impact frequency and the mean and peak values of drill string-wellbore interaction were larger. The effect of compound drilling on the lateral vibration acceleration of drill string was greater than that on the longitudinal vibration acceleration. A heightened build-up effect was caused by the increase of weight on bit. The larger bend angle and higher position of the stabilizer were not conducive to the stable control of the well trajectory. The increased outer diameter of the stabilizer could reduce the build-up effect of the bottom hole assemble to a certain extent. The driller in the field control of a well trajectory should be based on the complex dynamics of the numerical model of the bottom hole assemble during compound drilling.