A Drill Bit and a Drilling Motor With Embedded High-Frequency (1600 Hz) Drilling Dynamics Sensors Provide New Insights Into Challenging Downhole Drilling Conditions

Abstract

Summary Drilling motors are typically used in every well drilled globally with conventional steerable bottomhole assemblies (BHAs) and powered rotary–steerable BHAs. Downhole drilling dysfunctions are common when mud motors are pushed to the limit for maximum drilling performance. High–frequency (1600 Hz) continuous–recording compact drilling dynamics sensors were embedded into the bit, bit box, and top subassembly (sub) of the motor to better understand the dynamic role played by drilling conditions in different shales throughout North America. In the drilling industry, most downhole measurements for drilling dynamics use relatively low–frequency sensors (up to 100 Hz). Typically, the measurements are burst and not continuous. These low–frequency burst acceleration devices cannot reliably measure high–frequency torsional oscillations (HFTOs), which are known to be problematic while drilling in certain shale basins. Newly developed high–frequency (1600 Hz) compact drilling dynamics sensors can now be embedded into the drill bit, mud–motor bit box, and top sub to record three–axis accelerations continuously at high–speed sampling rates. The embedded sensors do not add any extra length to the steerable motor and, therefore, capture the true dynamic response of the system. Embedding the high–frequency sensors at both ends of the mud motor provides two unique data sets of dynamic measurements. With conventional steerable motors and motor–assist rotary–steerable systems (RSSs), HFTO dominant frequencies between 100 and 400 Hz were commonly observed. In some cases, HFTO dominant frequencies between 400 and 700 Hz and their harmonics were captured, which have not been previously reported. In most cases, the HFTO-induced acceleration amplitudes are between 20 and 200g (where g is the gravitational acceleration = 9.806 m/s2) peak (or between 40 and 400g peak to peak). On some occasions, ±200g self–perpetuating HFTO-induced accelerations were recorded in memory where their calculated angular acceleration is more than 25 000 rad/s2. The transitions between low–frequency stick/slip and HFTOs were captured in high–speed recording. Negative string–rotation speeds were commonly observed at the top sub of the motor while in the rotary mode. It was noted that the bit would slow down to stop but never turn backward, resulting in the backward rotation of the motor top sub. During very–high–amplitude multiple–axis shocks at the bit, there was a significant increase in temperature because of the loss of energy from bit dysfunctions. The newly reported drilling dynamics phenomena, such as multiple dominant HFTO frequency shifts, microsticks, and microslips, will be detailed in this paper. Monitoring and understanding high–frequency drilling dynamics dysfunctions allow us to make systematic changes to bits, BHAs, and drilling parameters to reduce the dysfunction magnitude, improve the overall drilling efficiency, and minimize component wear.