Active geothermal systems in Indonesia (Sarulla, Muara Laboh, Rantau Dedap) and the Philippines (Makiling-Banahaw, Tiwi, Bacman) share a common stratigraphic architecture. From surface downward:

The critical drilling challenge is not peak hardness — it is the rapid alternation of hardness values within a single bit run. A transition from 90 MPa welded tuff to 180 MPa dense basalt over 3–5 metres forces every cutter element to absorb a shock load on entry and a relaxation load on exit, repeated thousands of times per hour. This cyclic loading regime is more destructive than either hardness value alone.

Well-documented field data from producing Indonesian fields places bottom-hole circulating temperatures between 180 °C and 240 °C at 2 000–2 500 m TVD. Drilling fluid is typically aerated water or natural water-based mud, which provides limited cooling efficiency compared with conventional oil-field muds. Two chemical hazards are specific to geothermal wells:

•  Chloride brines (Cl⁻ concentration: 2 000–18 000 ppm) attack carbide substrates through selective dissolution of the cobalt binder — a process that takes hours at surface but accelerates dramatically above 150 °C.

•  CO₂ and H₂S partial pressures in the circulating fluid promote stress-corrosion cracking at bearing seal interfaces and threaded connections.

Any drill-bit specification for geothermal service must address both the mechanical and chemical environment simultaneously. A bit optimised for UCS alone will fail through chemical degradation; a corrosion-resistant bit with inadequate impact toughness will fracture mechanically.

Conventional PDC cutters (PCD diamond table bonded to a tungsten carbide substrate) represent the default choice for moderate-hardness formations worldwide. In geothermal service, they encounter two independent failure mechanisms that can operate simultaneously:

The consequence: standard PDC bits in Indonesian or Philippine geothermal wells commonly achieve only 40–80 m per run in the andesite–basalt transition zone, against 200–400 m per run in comparable hard-rock oil-field wells. The temperature and chemistry of geothermal service, not the rock hardness alone, drives the performance gap.

TSP cutters address the thermal degradation failure mode directly. The manufacturing process removes substantially all residual cobalt from the diamond lattice through an acid-leaching step, producing a cobalt-free PCD layer typically 400–600 µm deep. Without cobalt as a graphitisation catalyst, the diamond table retains its hardness to approximately 1 200 °C — well above any contact temperature achievable in drilling.

The trade-off is mechanical: cobalt also contributes ductility to the substrate–diamond interface. TSP cutters are more brittle than standard PDC in pure impact loading. This is why TSP elements are almost always used in a hybrid configuration:

•  TSP cutters occupy the critical abrasive-wear positions (gauge row, outer cone) where thermal loading is highest.

•  Standard high-Co PDC cutters (12–14 wt% Co) fill the inner cone and nose positions where impact loading dominates and temperatures are lower.

•  Some designs use TSP as fixed-cutter backup elements set slightly behind the primary PDC row, engaging only when primary cutters wear below a threshold depth.

Reported field performance of TSP-hybrid bits in Indonesian geothermal wells (Sarulla field, 2019–2023 operational data): average run length in andesite–basalt transitions increased from 65 m (standard PDC) to 140–180 m (TSP hybrid), with a corresponding reduction in trips of approximately 45%.

TCI (tungsten carbide insert) tricone bits operate through a fundamentally different cutting mechanism: rolling cone geometry allows inserts to crush and chip rock rather than shear it. This impact-based mechanism is inherently less sensitive to formation hardness variation — the insert geometry does not change when the formation changes stiffness. This makes TCI tricone bits the conservative default for exploratory geothermal wells where the stratigraphy is incompletely characterised.

Key performance characteristics in volcanic geothermal formations:

The critical limitation of TCI tricone bits in geothermal service is bearing seal integrity. Sealed-bearing designs use elastomeric O-rings and metal-face seals rated for 150–180 °C maximum. Above this threshold, seal materials soften, allowing drilling fluid and formation brine to enter the bearing race. Once chloride-bearing fluid contacts the bearing steel at 200 °C, fatigue life collapses from thousands of hours to tens of hours.

For wells where BHT exceeds 180 °C, open-bearing TCI designs with metal insert seals rated for high-temperature service are available but must be specified explicitly. Standard catalogue TCI bits are not suitable for deep geothermal service without this modification.

3.  Selection Matrix: Matching Tool to Formation and Depth

The following matrix consolidates the analysis above into a practical field-selection guide. Inputs required: current TVD, prevailing UCS from offset well logs or cuttings analysis, measured BHT or gradient extrapolation, and chloride content of the circulating fluid.

4.  Operating Parameter Recommendations

Tool selection determines the ceiling of achievable performance; operating parameters determine whether that ceiling is reached. The following guidance is specific to volcanic geothermal wells — general hard-rock drilling parameters are insufficient because they do not account for the thermal and chemical environment.

•  For TSP-hybrid PDC in andesite (UCS 100–160 MPa): WOB 60–90 kN for a 215.9 mm bit. Higher WOB does not increase ROP proportionally but increases cutter contact temperature and accelerates thermal degradation.

•  For TCI tricone in basalt: WOB 80–120 kN. TCI inserts require higher WOB to initiate rock fracture compared with PDC shear cutting, but exceeding this range crushes inserts rather than rock.

•  In fractured or cavernous intervals: reduce WOB by 25–35% from the base parameter and accept lower ROP. Bit drops into cavities at full WOB cause catastrophic impact damage within seconds.

•  PDC and TSP-hybrid: 60–100 RPM in hard volcanic (UCS > 140 MPa). Higher RPM increases cutter surface velocity and therefore contact temperature. In a 200 °C BHT environment, limiting RPM is the most effective single lever for extending PDC thermal life.

•  TCI tricone: 80–140 RPM. Tricone cutting action requires cone rotation, which scales with string RPM. Below 60 RPM, cone rotation may stall intermittently, concentrating wear on a subset of inserts.

•  In transition zones (rapid UCS change): reduce RPM to the lower end of the applicable range before the transition and increase only after 10–15 m of stable drilling in the new formation.

•  Minimum annular velocity 0.45 m/s for effective cuttings transport in aerated water-based mud. Inadequate cleaning in basalt generates coarse cuttings that regrind under the bit, increasing abrasive wear 2–3×.

•  Bit hydraulic horsepower (BHHP): 2.5–4.0 kW per cm² of bit face area for PDC in hard volcanic. This range balances cutting cooling against the risk of hydraulic erosion of altered formation zones.

•  For TSP-hybrid bits: maintain minimum flow rate even during slow drilling in hard sections. TSP cutters cool less efficiently than standard PDC (no cobalt-metal thermal bridge) and are more dependent on fluid cooling at the cutting face.

•  Torque oscillation amplitude > 25% of mean torque at constant WOB/RPM: early indicator of cutter chipping or insert fracture. Pull out if sustained for > 15 minutes.

•  D-exponent deviation > 0.3 units with no lithology change: indicates bit wear or cutter damage before surface inspection confirms it.

• Temperature-corrected ROP (adjusted for mud density and bit hydraulics) declining at > 15% per 50 m: thermal wear is outpacing the mechanical cutting rate; consider early trip.

5.  Post-Run Bit Inspection: Reading the Evidence

In geothermal wells, pull-out-of-hole (POOH) decisions are expensive — rig time at 200 °C wells runs USD 25 000–50 000 per day. Accurate post-run inspection reduces the risk of repeating the same bit selection or parameter error on the next run.

6.  The basic conclusion is drawn: Select tools based on different rock strata

Geothermal drilling in Indonesian and Philippine volcanic formations demands a tool selection logic that is fundamentally different from conventional hard-rock practice. The difference is not the rock hardness — it is the combination of thermal exposure, chemical attack, and rapid lithological alternation operating simultaneously.

Standard PDC cutters fail through thermal degradation above 180 °C BHT and through cyclic impact fatigue in alternating volcanic sequences. TSP-hybrid designs address both failure modes by eliminating cobalt from the diamond layer and concentrating impact-resistant grades at the mechanically loaded positions. TCI tricone bits remain the appropriate choice for exploratory wells with undefined stratigraphy and for fractured-zone drilling, provided that bearing seals are rated for the actual thermal and chemical environment — not generic catalogue specifications.

No single tool type is optimal across all depth intervals in a geothermal well. The highest-performing programmes treat the selection matrix as a live document, updated after each bit run with inspection data, and adjust both tool specification and operating parameters accordingly.

© 2026 SUNGOOD TECH |  www.zzsungood.com  | Technical data compiled from published geothermal field operations literature and engineering reference standards. No operational guarantee implied.

E-mail :

yanceychang@zzsungood.com

Phone/Wechat：

+86-15037166264