NIRE Annual Report		1998

Objectives

The objectives of this research are to develop polycrystalline diamond compact (PDC) drill bits which would be able to apply to hard rock drilling such as geothermal wells. The durability of PDC drill bits in hard and abrasive granite drilling has been improved at laboratory tests.

Results

Fig.1 shows one example of the durability tests for Sori granite (the uniaxial compressive strength is about 220 MPa). The bit used for the test is 142.88 mm-dia (5-5/8 in-dia) PDC drill one with ninety-one cutters of 8.2 mm-dia. As can be seen in the figure, the length drilled reaches about 26 m at the penetration rate of about 2 cm/min (1.2 m/h).

To apply PDC bits widely for hard rock drilling, not only the durability must be improved, but also the bit cost must be reduced. We fabricated two 66.0 mm-dia PDC drill bits using two types of PDC cutter available in the market. The bit cost of these ones is reduced to about two-third of conventional PDC ones with the same bit diameter. The test results for Sori granite using one of them is shown in Fig. 2 From the figure, it is obvious that the bit can drill granite for about 50 m at the penetration rate of about 3 cm/min (1.8 m/h). Even though the bit cost is reduced, these records in granite drilling are better as compared to those of the conventional diamond bits. In addition, we found that the cutter strength differs in two types of PDC cutter. It is very important to improve the durability to select PDC cutters with improved strength.

Selected Publications

1)	Improvement of PDC Bit's Performance at High Rotary Speed, Geothermal Resources Council TRANSACTIONS, 20, 503-508, 1996.

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