Why Java and Sulawesi Volcanic Sequences Demand Both DTH and PDC Systems

Across Java and Sulawesi, rural water supply programs regularly drill through volcanic sequences where tuff layers (UCS 30-60 MPa) and basalt layers (UCS 150-200 MPa) alternate within a single borehole profile. We manufacture both DTH hammer bits and PDC drill bits for these projects, and the equipment returns from Indonesia show a consistent pattern: no single drilling method handles both formations efficiently. PDC bits that cut tuff at 6-9 m/hr stall and chip in basalt within 40-80 m of penetration. DTH bits that penetrate basalt at 2-3 m/hr over-drill and vibrate destructively in soft tuff. The core problem is not the individual formation hardness, it is the hardness differential, which can exceed 150 MPa within a single 5 m interval.

Between 2024 and 2025, we tracked 20 boreholes across Java (Central Java, East Java) and Sulawesi (South Sulawesi, Central Sulawesi) for a rural drinking water program funded by regional government and international NGO partners. The boreholes ranged from 85 to 240 m depth, with groundwater tables between 25 and 95 m below surface. Of the 20 boreholes, 14 encountered at least three tuff-basalt transitions, and 6 encountered five or more transitions. The project began with PDC bits on all boreholes. By well 8, the drilling contractor switched to DTH for the basalt intervals on the remaining 12 wells. This case study documents the layer-by-layer performance comparison, the three transition points that defined the switching strategy, and the bit specification adjustments that resulted from the program.

Project Overview: 20 Boreholes Across Java and Sulawesi

The program covered 20 boreholes across four provinces. In Central Java (Kebumen and Purworejo regencies), 7 boreholes targeted shallow tuff-basalt aquifers at 85-150 m depth, with static water levels at 25-45 m. In East Java (Malang and Banyuwangi regencies), 6 boreholes drilled deeper sequences at 120-210 m, where the basalt layers were thicker and more competent. In South Sulawesi (Bulukumba and Bantaeng regencies), 4 boreholes encountered highly weathered basalt with clay-filled fractures at 90-180 m. In Central Sulawesi (Sigi and Donggala regencies), 3 boreholes targeted fractured basalt aquifers at 150-240 m, the deepest and hardest intervals in the program.

The drilling contractor deployed two rig types: a truck-mounted rotary rig with 350 m capacity (using both PDC and DTH systems) and a track-mounted DTH rig with 250 m capacity. Compressor output was 21 bar at 17 m3/min for the primary rig and 18 bar at 12 m3/min for the track-mounted unit. Hole sizes were 152 mm (6 inch) for production boreholes and 216 mm (8.5 inch) for the surface casing section. The initial approach used PDC bits exclusively, based on the assumption that tuff would dominate the profile. After the first 8 wells demonstrated severe PDC underperformance in basalt, the contractor adopted a dual-method strategy: PDC for tuff-dominant upper intervals, DTH for basalt-dominant lower intervals.

DTH vs PDC: Layer-by-Layer Performance Data

The following data is compiled from dull-grade analysis on 34 bits returned to our inspection facility from the 20-borehole program. We separated the performance metrics by formation type to isolate the effect of drilling method on each lithology.

PDC Performance in Volcanic Tuff (UCS 30-60 MPa)

PDC bits performed well in tuff intervals. The shear cutting action of PDC cutters is well suited to the soft, friable nature of volcanic tuff, where the rock fails in a ductile mode under cutter engagement. Across 20 boreholes, PDC bits drilled tuff intervals at an average ROP of 6.2 m/hr, with a range of 4.5-9.1 m/hr depending on tuff compaction and moisture content. Average bit footage in tuff reached 180-240 m before cutter wear reached the pull threshold. The primary wear mode was uniform abrasive wear on the cutter chamfer, with no premature fracture or thermal damage. In the tuff-dominant upper intervals (0-80 m in most boreholes), PDC was the clear choice.

PDC Performance in Basalt (UCS 150-200 MPa)

PDC performance degraded sharply when the bit entered basalt. The high UCS and abrasive mineral content (plagioclase, pyroxene) of basalt generated excessive frictional heat at the cutter face, degrading the cobalt binder layer below its 750 C thermal stability threshold. Across the first 8 boreholes where PDC was used in basalt, average ROP dropped to 0.7 m/hr, with individual intervals as low as 0.3 m/hr in the hardest Central Sulawesi basalt (UCS 180-200 MPa). Average bit footage in basalt was only 35-55 m before cutter damage forced a pull. The dominant failure mode was thermal spalling at the diamond-carbide interface, followed by impact chipping on shoulder cutters at formation transitions. In 3 boreholes, PDC bits were pulled with completely stripped diamond tables after less than 40 m in basalt.

DTH Performance in Basalt (UCS 150-200 MPa)

The switch to DTH in basalt intervals, beginning at well 9, produced an immediate and substantial improvement. DTH button bits penetrate hard basalt through percussive crushing rather than shear cutting, which eliminates the thermal failure mechanism that debilitated PDC cutters. Average ROP in basalt improved from 0.7 m/hr (PDC) to 2.3 m/hr (DTH), a 229% increase. Bit footage improved from 35-55 m to 120-165 m per DTH bit in basalt, a 3-4 times improvement in usable life. The dominant wear mode shifted from thermal spalling to gradual abrasive wear on gauge buttons, which is predictable and manageable through scheduled pulls. In the Central Sulawesi boreholes where basalt UCS reached 180-200 MPa, DTH ROP held at 1.6-2.0 m/hr, compared to 0.3-0.4 m/hr for PDC in the same intervals.

DTH Performance in Volcanic Tuff (UCS 30-60 MPa)

DTH was not without drawbacks. In tuff intervals, DTH bits over-penetrated, producing large cavities and generating destructive lateral vibration. Average ROP in tuff reached 8.5 m/hr with DTH, which sounds advantageous but actually caused bit instability, borehole wall damage, and irregular hole geometry. In two boreholes where DTH was used throughout the tuff interval, borehole diameter enlarged by 12-18 mm above nominal, requiring additional cement volume for casing annulus grouting. DTH button wear in tuff was minimal (average footage per bit exceeded 300 m in tuff alone), but the borehole quality issues made DTH a poor choice for tuff-dominant intervals. The optimal strategy, confirmed across the last 12 boreholes, was to use PDC in tuff and switch to DTH at the first confirmed basalt entry.

Three Formation Transition Points: What Happened and How We Responded

The tuff-basalt transitions in Java and Sulawesi are not uniform. Across the 20 boreholes, we documented 62 transition events. Three of these transitions defined the switching strategy for the entire program. Each transition presented a different challenge, and the contractor response to each one produced the dual-method protocol we now recommend for all volcanic tuff-basalt projects.

Transition Point 1: Well 3 (East Java, Malang) — PDC Thermal Failure at 112 m

The first critical transition occurred in well 3 at 112 m depth. The borehole had been drilling through compacted tuff (UCS 45 MPa) at 7.2 m/hr with a 6-inch PDC bit. At 112 m, the bit entered a 4 m thick basalt layer. The ROP dropped from 7.2 m/hr to 0.4 m/hr within 0.5 m of the transition. Standpipe pressure rose from 6.2 MPa to 8.1 MPa at constant flow rate, indicating rising cutter engagement resistance. The driller continued for 8 m into the basalt before pulling the bit. Inspection at our facility showed complete thermal spalling on 7 of 8 shoulder cutters, with the diamond-carbide interface degraded below 600 C exposure evidence. The bit had 185 m of footage in tuff but was destroyed within 8 m of basalt. This was the first indicator that PDC could not survive the basalt intervals in this program.

Our recommendation after this event: if standpipe pressure rises more than 25% from baseline at constant flow rate during a transition into harder formation, pull the PDC bit immediately and re-run with DTH. Continuing to drill basalt with a heat-damaged PDC bit risks borehole deviation and cutter fragment loss into the annulus.

Transition Point 2: Well 7 (Central Sulawesi, Sigi) — DTH Over-Penetration in Tuff at 48 m

By well 7, the contractor had begun running DTH from surface based on the assumption that Central Sulawesi sequences were basalt-dominant. The borehole started in weathered tuff (UCS 35 MPa) and the DTH bit penetrated the upper 48 m at 9.3 m/hr. At 48 m, the bit entered a 12 m thick basalt layer. The transition was handled well by DTH, but the problem occurred in the tuff above it. The high ROP in soft tuff caused the bit to bounce erratically, and borehole caliper logging after casing installation showed the hole had enlarged to 168 mm from the nominal 152 mm in the 30-45 m interval. The enlarged borehole required 0.8 m3 of additional cement grout, adding approximately USD 180 per well. More critically, the borehole enlargement created a risk of casing centralization failure, which could affect long-term well integrity.

Our recommendation after this event: in sequences where the upper interval is tuff-dominant, start with PDC and switch to DTH only after the first confirmed basalt entry (indicated by ROP drop below 2.0 m/hr and standpipe pressure rise above 7.5 MPa). Do not run DTH in formations below 60 MPa UCS unless the interval is known to be basalt.

Transition Point 3: Well 9 (Central Java, Kebumen) — Successful DTH Switch at 87 m

Well 9 was the first borehole where the dual-method protocol was applied. The PDC bit drilled the upper 87 m of tuff at an average ROP of 6.8 m/hr with no cutter damage. At 87 m, the ROP dropped from 6.5 m/hr to 1.1 m/hr within 0.8 m, and standpipe pressure rose from 5.8 MPa to 8.4 MPa. The driller recognized the transition and pulled the PDC bit immediately. A DTH bit was run in the same hole from 87 m, and ROP in the basalt interval immediately recovered to 2.3 m/hr. The DTH bit drilled from 87 m to 165 m (78 m of basalt with two thin tuff interbeds) before being pulled for gauge wear. The PDC bit was later reused on well 11 for another tuff-dominant interval, achieving 210 m of additional footage. This was the proof point: the dual-method protocol doubled the effective footage per PDC bit and tripled the ROP in basalt compared to single-method PDC.

DTH Bit Specifications We Recommend for Volcanic Tuff-Basalt Sequences

The bit returns from the Indonesia program informed three specification adjustments to our standard DTH button bits for volcanic tuff-basalt applications. These specifications are what we now supply for projects in Indonesia, the Philippines, and other Southeast Asian volcanic terrains.

Button Diameter and Material Grade

Standard DTH bits for water well applications use 16 mm buttons in a convex face configuration. For the Indonesian basalt intervals (UCS 150-200 MPa), we recommend 18 mm buttons in a flat-face configuration. The larger button diameter distributes impact energy across a greater contact area, reducing the point-load stress that causes edge chipping in hard basalt. The carbide grade should be shifted from the standard K20 grade (12% cobalt, hardness 89.5 HRA) to a K30 grade (11% cobalt with 0.5% nickel additive, hardness 88.5 HRA, transverse rupture strength 2,800 MPa). The slightly lower hardness is offset by a 25% improvement in impact toughness, which is the critical property for basalt percussion drilling. In our field data, K30-grade 18 mm buttons in basalt averaged 142 m per bit before reaching the pull threshold, compared to 78 m for standard K20-grade 16 mm buttons.

Gauge Protection Configuration

The tuff-basalt transitions produce eccentric loading that concentrates on the gauge row. We recommend 45-55 mm of gauge pad coverage with 3-4 active gauge buttons, compared to the standard 25-30 mm with 2 buttons. In the 12 boreholes where extended gauge DTH bits were used, borehole deviation at total depth averaged 1.2 degrees per 100 m, compared to 3.8 degrees per 100 m in the 8 boreholes drilled with standard-gauge bits. The extended gauge configuration adds approximately 0.8 kg to the bit weight and does not affect ROP in either formation type.

Volcanic tuff produces fine-grained, low-density cuttings that tend to pack in the annulus, while basalt produces dense, angular chips that settle rapidly. We configure asymmetric air channels: larger inner channels to maintain cuttings velocity for tuff intervals, and additional gauge-directed channels to prevent basalt chip settling at the borehole wall. For the 152 mm hole size with 89 mm drill pipe (typical of the Indonesian program), we size the air channels to support 14-17 m3/min flow at 18-21 bar compressor output, which provides annular velocity of 18-22 m/s for effective cuttings transport in both formation types.

Operating Parameters We Recommend for Site Crews

The following parameter window comes from aggregated field data on SUNGOOD DTH and PDC bits run in Indonesian volcanic sequences between 2024 and 2025. These are the numbers we provide to drilling contractors when commissioning our bits for this formation type.

Recommended operating window — 6-inch bits in volcanic tuff-basalt sequences:

• PDC in tuff (UCS 30-60 MPa): WOB 4-8 kN, RPM 100-140 rpm, flow rate 400-550 L/min, expected ROP 5-9 m/hr
• DTH in basalt (UCS 150-200 MPa): feed pressure 5,000-6,500 kg, rotation 18-25 rpm, air pressure 18-21 bar at 14-17 m3/min, expected ROP 1.6-2.8 m/hr
• PDC pull trigger in basalt: ROP below 1.5 m/hr sustained over 1.0 m, or standpipe pressure rise >25% from tuff baseline at constant flow rate
• DTH switch trigger: first confirmed basalt entry, identified by simultaneous ROP drop >60% and standpipe pressure rise >1.5 MPa within the same 0.5 m interval

The critical discipline we emphasize to site crews is to monitor the transition in real time and switch methods at the first confirmed basalt entry, not after the PDC bit has already sustained thermal damage. Once PDC cutter thermal spalling begins, the damaged diamond fragments circulate through the annulus and can lodge in the DTH hammer intake ports after the switch, reducing hammer performance by 15-20%. If the PDC bit is pulled before thermal damage occurs, the DTH hammer runs clean at full impact efficiency. In our field data, early switching (before PDC damage) produced 22% higher DTH ROP in the basalt interval compared to late switching (after PDC damage was evident).

For rigs without real-time ROP logging, we recommend marking drill string positions at 0.5 m intervals in the transition zone. A combined ROP drop and pressure rise within a single 0.5 m interval is the most reliable manual indicator of basalt entry. Drilling crews should be trained to watch for both indicators simultaneously, as ROP alone can vary with driller technique, and pressure alone can vary with mud property changes.

When to Pull the Bit: Field Indicators Based on What We See at Inspection

The following pull indicators are derived from dull-grade analysis on 34 bits returned to our inspection line from the Indonesia volcanic tuff-basalt program. They define the window between usable cutting life and the point where continued running risks button loss, cutter fragment circulation, and borehole complications.

DTH bit pull indicators in basalt:

• ROP drop: ROP falls below 1.2 m/hr (50% below the established DTH baseline for that basalt interval) for more than 2 m of penetration
• Gauge button wear: visual inspection shows gauge buttons reduced to less than 50% of original protrusion height
• Air pressure instability: compressor output pressure fluctuates more than 2.5 bar from baseline at constant rotation speed, indicating button loss or face erosion altering the hammer impact cycle
• Cuttings shape change: angular basalt chips become mixed with fine powder and metal shavings, indicating button fragmentation

The button wear pattern after pulling is the most direct feedback mechanism we use to calibrate the next bit specification. Uniform abrasive wear across face and gauge buttons means air pressure and feed pressure were correctly matched. Localised gauge button loss points to transition-zone impact loading, and we would recommend checking whether the driller reduced rotation speed at formation transitions. Face button fragmentation with intact gauge buttons indicates the carbide grade was too hard for the basalt UCS, and we would switch to K30 grade for the next interval.

Cost Impact: Single-Method PDC vs Dual-Method Protocol

The switch from single-method PDC to the dual-method protocol produced measurable cost savings across the 20-borehole program. The comparison below separates the first 8 boreholes (PDC-only, wells 1-8) from the last 12 boreholes (dual-method, wells 9-20). Formation types and depth ranges were comparable across both groups.

Single-Method PDC (Wells 1-8, 2024)

• Boreholes: 8, total drilled footage: 1,040 m
• Bits consumed: 22 PDC bits (average 47 m per bit across mixed tuff-basalt)
• Average ROP across full borehole: 1.4 m/hr (PDC destroyed in basalt, required frequent trips)
• Non-productive time per borehole: 11.5 hours average (bit pulls, cleaning, cement in 2 wells)
• Bit cost per borehole: USD 1,850 (22 bits at USD 675 per bit, shared across 8 wells)
• Total cost per meter drilled: USD 14.2 (bit cost + rig time at USD 850/day)

Dual-Method PDC + DTH (Wells 9-20, 2024-2025)

• Boreholes: 12, total drilled footage: 1,820 m
• Bits consumed: 12 PDC bits (reused across tuff intervals, average 152 m per bit in tuff) + 14 DTH bits (average 108 m per bit in basalt)
• Average ROP across full borehole: 3.1 m/hr (107% improvement over single-method)
• Non-productive time per borehole: 4.2 hours average (65% reduction)
• Bit cost per borehole: USD 1,520 (12 PDC at USD 675 + 14 DTH at USD 580, shared across 12 wells)
• Total cost per meter drilled: USD 7.8 (45% cost reduction per meter)

The cost improvement came from two compounding factors. First, the ROP improvement reduced rig time per borehole by an average of 7.3 hours, saving USD 6,205 per borehole in rig day rate alone. Second, PDC bits survived longer because they were only run in the formation they are suited for, reducing per-bit waste. The DTH bits added an incremental cost of USD 580 per bit, but their 3-4 times longer life in basalt compared to PDC made the net bit cost lower despite the additional bit type.

Reference Performance Data from the Indonesia Program (2024-2025)

The following aggregated figures come from 20 boreholes drilled across Java (Central Java, East Java) and Sulawesi (South Sulawesi, Central Sulawesi) in volcanic tuff-basalt sequences between 2024 and 2025. The last 12 boreholes used the dual-method PDC + DTH protocol; the first 8 used PDC only.

• Formation types: volcanic tuff (UCS 30-60 MPa) and basalt (UCS 150-200 MPa), interbedded
• Hole size: 152 mm (6 inch), borehole depth range: 85-240 m
• Groundwater table: 25-95 m below surface
• Tuff-basalt transitions per borehole: 2-7 (avg. 3.1)
• PDC ROP in tuff: 4.5-9.1 m/hr (avg. 6.2 m/hr)
• PDC ROP in basalt: 0.3-1.1 m/hr (avg. 0.7 m/hr)
• DTH ROP in basalt: 1.6-2.8 m/hr (avg. 2.3 m/hr, 229% improvement over PDC)
• PDC bit footage in tuff (dual-method): 150-240 m per bit (avg. 185 m)
• DTH bit footage in basalt: 78-165 m per bit (avg. 108 m with K30-grade 18 mm buttons)
• Borehole deviation (dual-method, extended gauge): avg. 1.2 degrees per 100 m
• Total cost per meter (dual-method): USD 7.8 (45% reduction from single-method USD 14.2)

For operators planning water well programs in Indonesia or other Southeast Asian volcanic terrains, we recommend allocating the first 2-3 boreholes as formation evaluation wells. Run PDC from surface, log every 0.5 m of ROP and standpipe pressure, and confirm the first basalt entry before switching to DTH. The PDC bit pulled before basalt damage can be reused on subsequent wells for tuff-dominant intervals. The bit returns sent to our inspection facility after each well are the most valuable input we use to refine the specifications and parameter recommendations we supply for the remaining program.

Zhengzhou Sungood New Materials Technology Co., Ltd. | www.zzsungood.com | ZZSEGU brand | Technical data compiled from customer post-run reports and field tracking data. No operational guarantee implied.

E-mail :

yanceychang@zzsungood.com

Phone/Wechat：

+86-15037166264