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Laboratory scale evaluation of compliant drillstring with PDC bits in geothermal applications

Barnett, Laymon
Polycrystalline diamond compact (PDC) bits have shown tremendous performance compared to roller cone bits and have gained attraction in deep geothermal drilling applications. While PDC bits can reach a higher rate of penetration compared to other bit types, they are more prone to drilling vibrations, especially torsional vibrations. Drillstring vibrations are one challenge that hinders efficient drilling and causes downhole tool failures. The objective of this work is to investigate the effects of drillstring vibrations on drilling performance by utilizing a laboratory testing facility. The test facility is equipped with an advanced drillstring simulator using suspension and torsional springs systems mimicking the natural vibrations modes of a field drillstring. Drilling tests were performed with various drillstring configurations which include rigid, flywheel, torsional compliance, axial compliance, and combined axial-torsional compliance. Two 3¾ inches PDC bits, with different designs, were used to evaluate the different drillstring vibration modes' effect on the bits' rate of penetration and output torque. The drilling tests were conducted at three constant rotational speeds of 80, 120, and 160 RPM, and the axial load varied by approximately 500 lb increments from 1500-5500 lb. The testing results from torsional compliance testing showed decreased performance relative to the rigid drillstring results by a margin of 35%. Additionally, the torsional compliance configuration facilitates more stable torque, and thus superior drilling over the rigid configuration when both configurations are in the inefficient phase. The axial compliance drillstring maintains a 5-20% margin of improved performance over the combined axial-torsional compliance in the 4-blade testing. The results of the 5-blade bit in axial compliance showed superior performance compared to the combined compliance by 17%, 38%, and 65% for the 80, 120, and 160 RPM tests, respectively. Overall, the experimental results showed that drillstring compliance configurations enhance drilling performance at the low weight on bit (WOB), during inefficient drilling, and decrease the drilling performance in the efficient drilling phase, i.e. high WOB.