Real-Time System to Calculate the Maximum Load of High-Frequency Torsional Oscillations Independent of Sensor Positioning

Abstract

Cutting forces at the bit or mass imbalances in downhole tools such as mud motors can cause severe vibrations in drillstrings and bottom-hole assemblies (BHA). Negative effects include reduced rate of penetration, low-quality measurements and downhole tool failures. A value that represents the real downhole vibration level is needed to perform a reasonable mitigation strategy. The most common values are statistical values derived from acceleration signals that are received from a sensor at a specific distance from the bit (DfB). The interpretation of an acceleration signal is limited if only one mode shape is dominantly excited. In this case, the measurement signal is very sensitive with respect to the DfB of the sensor placement. The derivation of a representative value for the severity of high-frequency torsional oscillation (HFTO) is shown that is independent of the sensor position. In the proposed approach, the dynamic torsional torque is used in addition to the tangential acceleration measurement. The frequency information of both measurement signals are determined and an analytical model is used to calculate the maximum value of vibration amplitudes that occurs along the BHA. The algorithm is implemented in the measurement-while drilling (MWD) tool for vibration and load measurements. The maximum load value in the BHA corresponding to HFTO can be sent to the surface in real time for interpretation by the driller. In a case study, different scenarios from the field are discussed. The maximum load values are compared to numerical simulations that show an excellent agreement. The maximum value calculated by the approach is factors higher than the values measured by the accelerometers. By using the algorithm-upgrade of the MWD tool, a representative measurement value for the severity of HFTO loads is derived. This is a clear advantage compared to tangential acceleration measurement only. The value enables the driller or an automated advisory system to initiate the optimal HFTO mitigation strategy that leads to reduced levels of vibration with the known benefits for the cost of a well.