Abstract
We conducted a drilling test in a geothermal well located in Yamagata prefecture, using a new PDC core bit of 8-15/32in. outside diameter and 4in. inside diameter. The bit has fifty-seven new PDC cutters brazed on a bit body at both backrake and siderake angles of -10 degrees.
The bit is characterized by the new PDC cutter used for it. The feature s of the new cutter are as follows:
(1) The new PDC cutter has the same materials of a conventional one which has been developed by a maker in Japan for the drilling of hot and hard rock formations.
(2) A conventional PDC cutter, whose diamond layer is exposed, occurs chipping of a diamond layer readily in hard rock drilling. Therefore, the diamond layer of the new cutter was almost coated with a tungsten carbide layer to prevent the chipping.
The drilling test was carried out at the depth of about 1, 900m in the well. Rock drilled was granodiorite (about 1, 100kgf/cm2 uniaxial compressive strength) and rock temperature reached to around 250°C. Bentonite mud was used as a drilling fluid. The bit weight was changed between 0.6 and 12ft to obtain a constant penetration rate of about 2cm/min. The mud flow rate and the rotary speed were kept constant at 1m3/min and 27rpm, respectively.
Drilled length of the bit reached to about 5m. The degree of wear or chipping of the PDC cutters after drilling was estimated approximately 60%.
From the results of the drilling test, it became clear that the new PDC core bit can be applied to the drilling of heterogeneous, hot and hard rock formations.
