新型ＰＤＣビットによる地熱井の掘削 （第２報）

Abstract

The first field test by a new PDC core bit with 815/32 in. outside diameter in a geothermal well called HDR-2, drilled at Hot Dry Rock project site in Yamagata prefecture, resulted in granodiorite drilling of about 5m as described in the 1st report. From the results of the test, it became clear that the new PDC core bit has potential to drill geothermal wells and that there is some room for improvement concerning the arrangement of PDC cutters on the bit.Two improved type new PDC core bits and one conventional type new PDC core bit with the same design as the bit used for the first field test, were fabricated. And field tests of these bits were repeated in a geothermal well called HDR-3, which was drilled adjacent to the HDR-2 well, in order to evaluate the performance of the bits. Formation drilled is basement rock of granodiorite and its temperature is estimated to be around 250°C. Five drilling tests using the three bits mentioned above were conducted at the depths between 1, 627m and 1, 907m under almost the same drilling conditions as the first test.One improved type and one conventional type bits drilled hot, abrasive and hard granodiorite of about 10 meters respectively at the penetration rate of about 0.5m/hr. The drilled length and the penetration rate of the PDC core bits were greater than those of surface-set diamond core bits. It became obvious that the new PDC core bit can be applied to geothermal well drilling from the results of the field tests conducted in two geothermal wells. The performance of the improved type bits could not be understood correctly due to failure of cutters from a bit body etc. However, taking all results into consideration, the improved type bit was estimated to have better performance as compared to the conventional type bit.