Backward Whirl Testing and Modeling with Realistic Borehole Contacts for Enhanced Drilling Tool Reliability

Abstract

Abstract Drillstring-borehole contact may result in backward whirl: a common and catastrophic phenomenon. To increase BHA (bottom hole assembly) reliability, reduce non-productive time (NPT), and ultimately improve service delivery, an in-house backward whirl testing rig is developed to investigate dynamic loads. Analytical and numerical models elucidate dynamic responses and are validated against backward whirl test results. A 6 ¾-in. BHA is tested in an 8.5 in. borehole using a continuous and discontinuous borehole profile. At a rotary speed of 60 rpm, and using a continuous borehole, lateral acceleration amplitudes range between 20 to 30 g with an RMS value of 3 gRMS. In comparison, a discontinuous profile results in acceleration amplitudes between 40 to 50 g with an RMS value of 7 gRMS. In regard to the discontinuous profile, the response spectrum shows a broader range of frequency content up to 200 Hz while distinct frequency peaks are evident in the spectrum when the continuous profile is used. Results indicate that backward whirl may occur from friction induced contact with different borehole profiles. Further, discontinuous profile yields drastically higher dynamic loads with a broader frequency spectrum than a continuous profile. Furthermore, validated models may serve as useful predictions for the backward whirl phenomenon and the system response. Backward whirl testing of full scale drilling tools with realistic, discontinuous borehole contact, supplemented with validated modeling, is a new approach to understand downhole dynamic events.