PDC bits, which are widely used in oil and gas wells, have been generally thought to be unsuitable for geothermal wells, because the formations of geothermal fields mostly consist of hard, inhomogeneous and fractured zone, and the formation temperature is expected to be higher than oil and gas wells. In this report, the damage evaluation of the PDC bit based on the IADC dull grading system is described and compared with the structural analysis. In addition, the well logging data was analyzed to clarify the condition of the drilled formations. The PDC bit demonstrated a sufficient drilling performance in the field test, in which the drilled formations had an uniaxial compressive strength of 100 MPa or higher and were inhomogeneous and fractured under total lost circulation, and the highest formation temperature was 240 °C. Compared to a supposed case where the same formations were drilled with a conventional roller cone bit, the actual drilling rate (134 m/day) was doubled and the actual drilled interval (477 m) was more than 2.5 times, and the gauge loss was 1/16 in. or less. Furthermore, a laboratory drilling test conducted after the field demonstration revealed that the PDC bit has not reached its life and is thought to be still able to drill formations with a certain drilling rate. Several design factors of PDC bits for geothermal wells were discussed on the field demonstration and it was suggested that PDC bits have sufficient applicability to geothermal wells with appropriate bit design.
