Drill Bit Selection by Rock Hardness Coefficient
Compare alloy, PDC, tricone and DTH bits for loose soil, medium-hard rock and hard rock while avoiding selection by hardness coefficient alone.
Understand the Ground Before Choosing the Machine
A rock hardness coefficient is a useful screening reference, not a complete drilling design. Two formations with the same approximate value can differ in abrasiveness, fractures, stickiness, water sensitivity and chip size.
Main Drilling Risks
Wrong selection causes balling in clay, broken PDC cutters in impact ground, worn teeth in abrasive rock, low penetration and poor hole cleaning. The bit also has to match rotary speed, torque, feed, rod thread and circulation system.
Recommended Starting Method
Classify the formation as loose, cohesive, granular, uniform rock, fractured rock or competent hard rock. Then compare cutting action, return size and available machine energy. Keep a second tool ready when the borehole crosses a transition.
Match the Bit and Cleaning System to the Formation
Drill-Bit Selection
Use alloy or drag bits for many soft soils; PDC for selected uniform medium-hard formations; tricone for variable, broken or harder rotary drilling; and DTH hammer bits for competent hard rock with a matched air system. These are starting points, not universal rules.
Mud or Air Cleaning
Every bit needs a cleaning path. Mud ports must remain open and pump flow must lift cuttings. DTH holes need enough air volume and pressure to operate the hammer and return chips at depth. A good bit fails when cleaning is inadequate.
Casing, Screen and Pump Planning
Bit diameter must follow finished well design. Borehole diameter exceeds casing outside diameter, and casing inside diameter exceeds pump outside diameter. Choosing an oversized bit without need increases torque, fluid or air demand.
Practical Construction Sequence
Use this sequence as a planning checklist. The operator must still adjust feed, speed, fluid or air and casing to actual returns and local well-construction requirements.
Step 1
Identify hardness, abrasiveness, fractures, clay content and water response.
Step 2
Check bit diameter, connection and operating range against the rig and rods.
Step 3
Begin with conservative feed and inspect the first returned cuttings.
Step 4
Measure wear and penetration after each major formation change.
Step 5
Change tools when geology changes instead of forcing one bit through the entire hole.
What Changes With Geology?
| Decision | What to Check | Why It Matters |
|---|---|---|
| Depth and diameter | Finished depth, bit diameter, casing OD and pump OD | Larger holes require more torque, cleaning capacity and practical depth margin |
| Formation | Layer sequence, hardness, fractures, abrasiveness and stability | Determines drilling method, bit, casing and expected penetration |
| Cuttings removal | Mud-pump flow and pits, or compressor pressure and air volume | Cuttings left in the annulus cause sticking, wear and false depth limits |
| Rig capacity | Machine mass, torque, feed, pullback, rods and mast | A catalog depth cannot compensate for an under-sized working system |
| Operator control | Logs, returns, penetration, torque and cleaning cycles | Early formation recognition prevents tool damage and borehole loss |
Responsible Compact-Rig Limitation
Do not use the labels alloy, PDC, tricone or DTH as proof that a compact machine can drill any rock. Rig mass, torque, feed, pullback and cleaning capacity remain the operating limits.
LIYO compact rigs are built mainly for portability, easy operation and lower investment. A responsible recommendation uses real geology and borehole dimensions instead of promising maximum rated depth in every rock.
LIYO Models to Compare
Final configuration depends on depth, diameter, geology, drilling method, terrain and transport.
Continue the Drilling Selection Process
Questions From Water Well Drilling Buyers
What information is needed before selecting the rig?
Provide the application, target depth, required borehole diameter, casing and pump size, geological layers, terrain, drilling method and whether an air compressor is already available.
Which drilling method is recommended for this formation?
Classify the formation as loose, cohesive, granular, uniform rock, fractured rock or competent hard rock. Then compare cutting action, return size and available machine energy. Keep a second tool ready when the borehole crosses a transition.
Which drill bit should be used?
Use alloy or drag bits for many soft soils; PDC for selected uniform medium-hard formations; tricone for variable, broken or harder rotary drilling; and DTH hammer bits for competent hard rock with a matched air system. These are starting points, not universal rules.
Can a compact rig guarantee maximum rated depth?
No. Do not use the labels alloy, PDC, tricone or DTH as proof that a compact machine can drill any rock. Rig mass, torque, feed, pullback and cleaning capacity remain the operating limits.
How should the finished well diameter be planned?
Use the rule: borehole diameter must exceed casing outside diameter, and casing inside diameter must exceed submersible pump outside diameter. Also allow space for gravel pack and installation tolerance where required.
Is Drill Bit Selection by Rock Hardness Coefficient the only geology expected in the hole?
Do not assume so. Ask for nearby well logs and monitor returns, torque and penetration because several layers commonly occur in one borehole.
Send LIYO Your Real Geological Conditions
Tell us the application, depth, borehole diameter, casing and pump size, terrain, soil or rock layers, destination and compressor specifications if available.