An Analytical Method for Determining the Motion Characteristics and Abnormal Conditions of Near-Bit Bottom Hole Assembly

The performance of relevant theoretical models combined with surface mud logging parameters has unfavorable accuracy in analyzing the motion characteristics and abnormal conditions of BHA.Based on the fast Fourier transform theory and the PDC bit teeth force bearing equation,one analytical method was established for modeling the motion characteristics of BHA and according to the one-dimensional unsteady flow theory and the acceleration theory,one analytical method for bottom hole abnormal conditions was set up.The theoretical analytical results aligned well with the field test data.As shown by the theoretical analysis and the measured results,in the measuring interval,there was a dominant frequency,whose rotating frequency was the same as that of PDM in the spectrograms of engineering parameters during sliding drilling.While during rotary drilling,there were two dominant frequencies present in the spectrograms of engineering parameters,whose rotating frequencies were equal to those of PDM and drill string respectively.In case of the drilling tools bouncing on the bottom,the time of the first casing pressure peak was twice that of the first annulus pressure valley,and the torque and vibration sensor readings changed dramatically.The research demonstrated that the dominant frequency characteristics of the bottom hole engineering parameter spectrograms could be used to distinguish the sliding drilling and the rotary drilling.Meanwhile,the rotation status of BHA and PDM could be obtained.The downhole abnormal conditions could be judged based on variations of casing pressure,annulus pressure,torque and acceleration sensor readings.This analysis method could provide a theoretical reference for safe and rapid drilling and the establishment of a downhole expertise system.