Use a DTH button bit for hard geothermal rock when the rig can supply percussion and 0.7–2.4 MPa air pressure; run 1,000–3,000 lbf/in of diameter and 25–60 RPM. An insert tricone is the rotary alternative, using 3,000–8,000 lbf/in and 60–120 RPM with IADC 637 for hard formation.
Hard geothermal intervals combine high rock strength with a need for dependable hole geometry. Granite, basalt, and quartzite in the 15,000–30,000 psi range resist broad scraping action and demand concentrated contact. The bit must remove competent chips while retaining gauge over a sustained run. Selection also depends on the rig: a formation may suit DTH percussion, but the system is not viable if the available compressor and hammer cannot deliver the stated pressure range.
Why does DTH fit competent geothermal rock?
A DTH hammer places impact close to the bottom of the hole. Carbide buttons create point fractures, and slow rotation distributes those strikes around the face. The arrangement is well suited to hard, consolidated rock. It also gives the driller a direct relationship between hammer response, chip character, and penetration. Use the geothermal drilling archive for the application context and the hard formation archive for the fixed UCS and lithology class.
DTH is limited by equipment and hole conditions. Air pressure must remain 0.7–2.4 MPa. The brief provides no airflow-volume range, so a separate numerical demand cannot be claimed. Water-bearing conditions, depth, and return paths can alter cleaning response. A hammer that is not striking effectively cannot be corrected by applying rotary WOB as though it were a tricone.
How should thrust and rotation be introduced?
Use 1,000–3,000 lbf/in of bit diameter and 25–60 RPM. Thrust should maintain steady face contact without suppressing hammer movement. Rotation should index the buttons to new rock. An uneven wear pattern suggests that the bit is not sharing contact across the face. Before adding load, check alignment, button condition, and whether the hammer response has changed.
Hard geothermal rock may also be abrasive. Quartz content above 20% defines an abrasive formation in this dataset. If the chips show a quartz-rich band, expect faster gauge wear even though the UCS class has not changed. Reduce the interval between inspections and watch for rising drag on connections. A bit can continue drilling after losing gauge, but the resulting hole may be unsuitable for the next assembly.
| Hard geothermal option | WOB | RPM | Additional selection fact |
|---|---|---|---|
| Button/DTH | 1,000–3,000 lbf/in of diameter | 25–60 | 0.7–2.4 MPa air pressure |
| Insert tricone | 3,000–8,000 lbf/in of diameter | 60–120 | IADC 637 for hard formation |
| Hard formation | 15,000–30,000 psi | Granite, basalt, quartzite | |
Connection planning deserves attention before the hard interval begins. A DTH bit and hammer must match, while the rod string must transfer feed without introducing avoidable movement. For a tricone, the selected API Reg connection must correspond to the supported assembly. The permitted API Reg families are 2-3/8″, 3-1/2″, and 4-1/2″; the exact choice still depends on the equipment.
When should an insert tricone replace percussion?
An insert tricone is a defensible rotary choice when the hole program or available equipment does not support DTH, or when rolling contact produces a steadier run. Use IADC 637 for the hard formation class. Operate at 3,000–8,000 lbf/in of diameter and 60–120 RPM. The cones and inserts must remain matched to hard rock; a soft milled-tooth structure is not suitable for repeated granite contact.
The tricone introduces bearings, seals, and cone movement as additional condition points. High loading cannot compensate for a damaged bearing or chipped inserts. The fact table supplies no tricone flow number, so circulation should be managed from returns without borrowing the PDC 250–650 gpm value. That PDC figure applies only to the stated 8.5 in reference.
Review classification principles through the IADC. Geological background can be checked with the U.S. Geological Survey. The actual bit choice still rests on recovered lithology, rig capability, and dull condition.
What failure modes deserve an immediate trip?
For DTH, stop after a severe change in hammer response, repeated stalls, asymmetric vibration, or signs that returns have stopped carrying chips. Broken buttons, cracked insert pockets, body wash, and rapid gauge loss can worsen quickly. Flat buttons may produce a smooth but unproductive run, so the absence of shocks does not prove the bit is healthy.
For tricone, inspect chipped inserts, cone tracking, bearing condition, and outer-row wear. A hard or abrasive band can damage one cone more than the others. Continuing with a restricted cone can scrape the bottom rather than roll, increasing heat and producing an irregular hole. A recovered dull is evidence, not waste; it should be mapped against depth.
How does the run record improve the next interval?
Record bit type, diameter, thread or connection, WOB, RPM, air pressure for DTH, penetration, return character, and depth of lithology changes. Measure gauge at recovery. If a quartz-rich band caused most of the loss, separate that interval from the rest of the average. The next bit may need stronger gauge coverage rather than a wholesale change in drilling system.
DTH button bits are not automatically best for every geothermal hole. They require the correct hammer and air supply. Tricones are not immune to hard-rock wear or bearing problems. A strong plan states these restrictions before the run, operates inside the approved ranges, and chooses the next tool from measured evidence.