Analysis of Torsional Stick-Slip Situations Observed with Downhole High-Frequency Magnetometer Data

Abstract

The use of downhole rotational speed measurements made at 300Hz gives new insight into the conditions under which stick-slip torsional oscillations occur. Observations made with high-frequency magnetometers while drilling two reservoir sections have shown, that for these wellbores, using top-drive speeds below 140rpm leads to severe downhole torsional oscillations, also in off-bottom conditions. Additionally, it was observed that downlinking the rotary steerable system (RSS) initiated heavy stick-slip and that reaming downward had a negative damping effect on downhole torsional stability. These observations have been compared and partly matched with estimations made with a transient hydro-mechanical model. Some ill-defined information had to be estimated, like the amount of flow diversion during the downlinking procedure, the coefficients of static and kinetic friction along the borehole, or the bit efficiency and aggressivity. Downhole measurements have shown that when the drill-string is subject to strong stick-slip conditions, the downhole rotational speed changes from stationary to more than 400rpm in just a fraction of a second. It is therefore important to utilize sampling rates that are compatible with such very fast events. In practice, that means several hundred Hertz. A challenge associated with analysis of downhole high frequency data is time synchronization and drift of clocks between the surface and downhole measurements. After time synchronization and correction, it appears that the downhole rotational movement is delayed by several tens of seconds compared to the actual top-drive speed. This leads to question whether rotating the drill-string off-bottom, typically done in order to break gelled-up mud prior to establishing circulation, has any significant the intended effects deviated wells as torque along the drill-string must be built up. Direct observations of downhole rotational speed at high frequency help in discovering conditions that were not suspected to lead to large torsional oscillations. This new information can be used to improve drilling operational procedures and models of the drilling process, therefore enabling increased drilling efficiency.