Page 143 - Geothermal Energy Systems Exploration, Development, and Utilization

3.2 Drilling Equipment and Techniques 119

Figure 3.5 Rollercone- and PDC-drill bits. (From RockBit International, 2005.)

carbide (TCI). The rollers roll on bottom and crush the rock when the bit is
rotated. Rollercone bits are available for all kinds of formations to be drilled.
Generally, the teeth or insert size decreases for harder rock while the total number
increases.
• PDC bits: Thesebitshavefixedcutters distributed tothree andmoreblades.
The cutters are scraping on the bottom and are made of polycrystalline diamond
compacts (PDCs) to withstand quick abrasive wear. PDC bits are used in soft to
medium hard formations.
• Diamond bits, surface set: These are also fixed-cutter bits using scraping action
for drilling. Here, natural diamonds are fixed in the nonwear resistant matrix
body. When the diamonds are worn out the bit has to be pulled for change.
Surface set diamond bits are used in very hard rock. They normally give a fast
penetration rate in the beginning, which slows down when losing sharpness of
the exposed diamonds.
• Diamond bits, impregnated: Here, only small diamond particles are embedded
into a semiwear resistant matrix material which is exposed to slow abrasive wear.
This allows drilling as long as a certain height of the matrix–diamond mixture
still exists on the bit body. Impregnated diamond bits are used in ultrahard rock
and give the whole bit life a relative constant rate of penetration (ROP) when the
matrix wear characteristics fit to the rock to be drilled.
• Stabilizers and reamers: Stabilizers and reamers are tools which stabilize (cen-
tralize) the DCs in the open hole; they are equipped with smooth hard facing
made of tungsten carbide in the areas of contact to the borehole wall. Reamers