A new drill bit can look impressive in a product photo. However, for professional drilling contractors, the more important question is: what does the bit look like after it has actually been in the hole?

In this field case, a 91 mm flat-top PDC drill bit was used for about 100 meters of rotary drilling. After the planned interval, the bit was pulled from the borehole and checked to review:

• PDC cutter wear
• Cutter retention
• Blade condition
• Gauge wear
• Overall cutting-structure condition

Because of this, the post-drilling condition gives a much clearer picture of bit performance than looks alone.

Project Overview

Exact drilling settings depend on the rock, rig, connection, flushing and operator. Because of this, RPM and weight on bit should always be adjusted to real field response.

What Is a Flat-Top PDC Drill Bit?

A flat-top PDC drill bit is a fixed-cutter, non-coring tool. Unlike a core bit, which cuts an annular path and leaves a central rock core, a flat-top PDC bit removes rock across the full bottom of the hole.

Because of this fairly simple cutting profile, this type of bit works well in many engineering jobs where the main goal is:

Fast hole advance, rather than core recovery.

The 91 mm version used in this case had 1308 PDC cutters set across the cutting face. These cutters use super petroleum-grade PDC, giving strong wear resistance and reliable cutting in soft-to-medium formations. As the drill string turns, the cutters shear the rock and keep removing material from the bottom of the hole.

Why a 91 mm PDC Bit Was Selected

A 91 mm diameter is common in a range of smaller engineering and exploration-related jobs. For this project, the chosen bit size gave a practical balance between:

• Borehole diameter
• Rig torque needs
• Cuttings removal
• Cutter load
• Drilling efficiency

In addition, the compact diameter keeps the cutting structure fairly simple while still giving enough strength around the body and gauge.

Why 1308 PDC Cutters Were Used

The bit in this case uses 1308 PDC cutting elements. This cutter spec is often used in smaller and medium-diameter PDC bits where a balance is needed between:

• Cutting aggressiveness
• Wear resistance
• Cutter support
• Compact cutting structure

For a 91 mm flat-top bit, the 7-cutter layout lets the cutters cover the full bottom of the hole while keeping enough space for cuttings to move away from the cutting face.

Even so, the key point is not only the cutter count. Their position and load spread decide how evenly the bit wears during drilling.

The 100 m Drilling Interval

During the roughly 100 m interval, the bit stayed in steady rotary cutting service. As the PDC cutters met the rock, they removed material mainly by shearing.

For PDC drilling, steady conditions matter a lot. Heavy vibration or impact can damage the diamond edges even when the rock itself is not very hard. Because of this, the operator needs to watch:

• Penetration rate
• Torque
• Drilling vibration
• Cuttings return
• Flushing condition
• Rock changes

If penetration suddenly drops, simply pushing more pressure is not always the right fix. The cause may instead be:

• Harder rock
• Poor hole cleaning
• Bit balling
• Cutter wear
• Rock transition
• Heavy vibration

What We Look for After Pulling the Bit

After about 100 m, the bit was pulled for checks. This step matters a lot, because a used bit holds data about what happened underground. Several areas should be looked at with care.

1. PDC Cutter Retention

The first question is whether all PDC cutters stay firmly in place. Cutter loss can quickly damage the rest of the cutting structure. Good retention means the pockets and brazed joints held steady during drilling.

2. Cutter Wear

The cutting edges should be checked for:

• Normal wear flats
• Chipping
• Cracking
• Odd rounding
• Local heat marks

Fairly even wear across the structure usually points to balanced load. If one cutter is heavily worn while its neighbors stay nearly intact, the load may have been uneven.

3. Cutter Chipping

PDC cutters resist wear very well, but they can still be damaged by impact. Chipping may come from:

• Fractured rock
• Hard inclusions
• Heavy vibration
• Sudden weight-on-bit changes
• Downhole debris

Because of this, cutter condition after 100 m is a key sign of whether the settings and bit design matched the job.

4. Blade and Body Wear

The bit body should also be checked. Even if the cutters stay usable, heavy wear on the support structure can still shorten bit life. Key areas include:

• Blade surfaces
• Shoulder
• Waterways
• Gauge
• Areas exposed to cuttings flow

Steel-body wear matters most in abrasive rock.

5. Gauge Condition

The outer diameter of the bit must keep the required hole size. Gauge wear can make the bit undersize, which may affect:

• Hole quality
• Later drilling tools
• Rod movement
• Reaming needs

For a 91 mm bit, keeping the gauge matters most if more drilling or casing work will follow.

What the Used Bit Can Tell Us

A used drill bit is not waste. It is field data. By studying the condition after 100 m, engineers can decide whether the next bit should keep the same structure or be changed.

This kind of check helps improve the next bit choice.

Flat-Top PDC Bits in Limestone and Dolomite

Flat-top PDC bits can work well in selected limestone and dolomite when the rock is fairly uniform and suits shearing. However, limestone and dolomite should never be treated as the same rock. Their behavior can change with:

• Compressive strength
• Silica content
• Abrasiveness
• Fractures
• Bedding
• Rock density

Fairly clean, solid rock may allow steady PDC drilling. A highly fractured or abrasive formation may need a stronger cutting structure or a different bit type.

Flat-Top PDC Bits in Sandstone

Sandstone deserves special attention, because its abrasiveness varies a lot. Some sandstone drills fairly easily. Quartz-rich sandstone can be very abrasive and may quickly wear:

• PDC cutting edges
• Gauge protection
• Steel body
• Shoulder areas

Because of this, the right PDC cutter grade and wear protection matter more and more in abrasive sandstone.

Applications of 91 mm Flat-Top PDC Bits

This type of bit can be used in many engineering drilling jobs. Common examples include:

Grouting Holes

Used in foundation work, slope support and underground engineering.

Gas Drainage Holes

Common in coal-mine work, where rotary PDC drilling gives fast penetration in suitable coal-bearing rock.

Water-Well Drilling

Useful for smaller pilot or drilling sections in the right sedimentary rock.

Geothermal Drilling

Can be used in suitable intervals where PDC shearing gives fast penetration.

Engineering Drilling

Suitable for many construction and geological jobs where full-face drilling is needed.

Why a Used-Bit Photo Builds Customer Confidence

For global customers, a new bit shows production quality. A used bit shows field performance. That gap matters.

A field photo taken after 100 m of real drilling lets customers see:

• How the cutters wear
• Whether cutters stay in place
• How the body holds up
• Whether gauge protection stays effective
• How the structure behaves after real use

This is far more useful than simply saying “our PDC bits are high quality.” In short, real field proof beats a marketing slogan.

How to Improve the Next Bit

If a customer’s current flat-top PDC bit is not working as expected, the used bit should be checked before making the replacement. Useful data includes:

• Clear front-view photo
• Side-view photo
• Close-up photos of damaged cutters
• Total meters drilled
• Main rock types
• Drill rig model
• Approximate RPM
• Flushing method
• Connection

From this data, it may be possible to adjust:

• PDC cutter grade
• Cutter quantity
• Cutter position
• Cutter exposure
• Gauge protection
• Waterway structure
• Body reinforcement

In short, the goal is not just to replace the worn bit. The goal is to understand why it wore the way it did.

Available Flat-Top PDC Bit Sizes

Chenghui flat-top PDC drill bits can be supplied in several diameters:

Connections can be customized to the drilling system and customer needs. Actual cutter count and structure may also be adjusted to the rock and job.

Planning a Similar 100 m Drilling Project?

If you are choosing a 91 mm PDC drill bit, please provide the following before ordering:

1. Required diameter
2. Target drilling depth
3. Rock type
4. Drill rig model
5. Required thread connection
6. Flushing method
7. Current drill-bit performance

If you already have a worn bit, send us photos. A worn bit can often give useful data about the rock and drilling conditions.

Frequently Asked Questions