01 Roller Cone Bits: Versatile Tools for Adapting to Formations
Since their advent in 1909, roller cone bits have become the most widely used bit type in rotary drilling. Their unique multi-cone structural design enables them to adapt to various formation conditions, from soft to extremely hard.
Structural Composition and Core Technologies
A roller cone bit consists of five core components:
- Bit Body: Three cone legs welded together, with connection threads on the upper part.
- Roller Cones: Conical metal bodies with milled teeth or insert teeth on the surface.
- Bearing System: Comprising four sets of bearings: large, medium, small, and thrust bearings.
- Nozzles: Usually 3-4 in number, with a diameter of 7-14 mm.
- Oil Storage and Sealing System: Rubber or metal sealing rings combined with pressure compensation devices.
Among these, bearing sealing technology represents a key technological breakthrough for roller cone bits. Modern bits adopt an oil storage pressure compensation system, which maintains dynamic balance between the oil pressure in the bearing cavity and the downhole drilling fluid column pressure through components such as pressure transmission holes, pressure compensation diaphragms, and oil cups.
Classification System and IADC Coding
The International Association of Drilling Contractors (IADC) has developed a globally unified classification standard for roller cone bits, using a three-digit coding system:
First Digit: Tooth type and applicable formation
1: Milled teeth, soft formations
2: Milled teeth, medium to medium-hard formations
3: Milled teeth, hard and abrasive formations
5: Insert teeth, soft to medium formations
6: Insert teeth, medium-hard formations
7: Insert teeth, hard and abrasive formations
8: Insert teeth, extremely hard and highly abrasive formations
Second Digit: Subdivision of formation hardness (Grade 1-4; higher numbers indicate harder formations)
Third Digit: Bit structural characteristics
4: Sealed roller bearings
6: Sealed sliding bearings
7: Sealed sliding bearings + insert gage protection
8: Deflecting bits for directional drilling
Simplified IADC Classification System for Roller Cone Bits
First Digit |
Tooth Type |
Applicable Formation |
Second Digit |
Hardness Grade |
1 |
Milled |
Soft formations |
1 |
Extremely soft |
2 |
Milled |
Medium to medium-hard formations |
2 |
Soft |
3 |
Milled |
Hard formations |
3 |
Medium-hard |
5 |
Insert |
Soft to medium formations |
4 |
Hard |
6 |
Insert |
Medium-hard formations |
||
7 |
Insert |
Hard formations |
||
8 |
Insert |
Extremely hard formations |
Rock-Breaking Mechanism and Motion Characteristics
During operation, a roller cone bit exhibits three types of composite motion:
- Revolution: The cones rotate clockwise with the entire bit.
- Rotation: The teeth rotate counterclockwise around the cone axis.
- Sliding: Including radial and tangential sliding.
This composite motion achieves a dual rock-breaking effect:
1.Impact Crushing Effect: Alternating contact of single and double teeth with the bottom hole generates longitudinal vibration, creating impact loads.
2.Sliding Shear Effect: Rock shearing is achieved through the design of overhang, double cone, and offset structures.
Selection Strategy and Formation Matching
Basic principles for selecting roller cone bits based on rock properties:
- Soft Formations: Select bits with offset, overhang, and double-cone structures, equipped with tall, wide, and sparse milled or insert teeth.
- Medium-Hard Formations: Reduce the values of offset, overhang, and double cone; use short, narrow, and dense teeth.
- Hard and Abrasive Formations: Adopt single-cone rollers with no overhang or offset, equipped with ball teeth or conical-ball teeth.
- Deflection-Prone Formations: Select short-tooth bits with no offset or small offset, and no gage teeth.
- Interbedded Soft and Hard Formations: Select bit types based on the harder rock, and dynamically adjust parameters during drilling.
Response to Special Working Conditions
- Small Boreholes (<177 mm): Use single-roller cone bits, which have larger cones, teeth, and bearings for higher strength.
- Directional Drilling: Select special deflecting bits with the third digit of IADC code being 8.
02 Diamond Bits: Powerful Tools for Hard Formations
As the hardest material in nature, diamond has a Mohs hardness of Grade 10, a compressive strength of up to 8800 MPa, and a wear resistance 9000 times that of steel. Leveraging this property, diamond bits have become the ultimate tool for tackling hard formations.
Classification System and Technological Evolution
Modern diamond bits are mainly divided into three types:
1.Surface-Set Diamond Bits
- Diamond particles are exposed on the matrix surface.
- Suitable for medium-hard to hard formations.
- Diamond particle size classification:
- Soft formations: 2 grains per carat (gpc) (particle size approx. 4 mm)
- Medium-hard formations: 3-4 gpc (particle size approx. 3.6 mm)
- Hard formations: 10-15 gpc (particle size approx. 2.0 mm)
2.Impregnated Diamond Bits
- Diamonds are impregnated inside the matrix (60-400 gpc).
- Suitable for hard and abrasive formations (e.g., flint, siliceous dolomite).
- Diamond self-sharpening is achieved through matrix wear.
3.PDC Bits (Polycrystalline Diamond Compact Bits)
- First introduced by General Electric (GE) of the United States in 1973.
- Compact structure: Diamond layer + cemented carbide substrate.
- Suitable for soft to medium-hard homogeneous formations.
Structural Design and Core Parameters
Diamond bits adopt an integral design with no moving parts, mainly including:
- Steel Body: Made of medium carbon steel, with connection threads on the upper part.
- Matrix: Tungsten carbide powder + copper-based bonding metal, with a hardness grade of HRC 30-45.
- Cutting Elements: Natural/synthetic diamonds or PDC compacts.
- Hydraulic Structure: Water courses and flow channels (radial, spiral, etc.).
Key Design Parameters
- Diamond Concentration: Adjusted based on formation abrasiveness; higher concentration for more abrasive formations.
- Protrusion Height:
- Soft formations: 1/3 of the particle size
- Hard formations: 1/6-1/10 of the particle size
- Lip Shape:
- Flat-bottomed: For homogeneous formations
- Curved: For hard formations
- Serrated: For abrasive formations
Rock-Breaking Mechanism and Formation Response
The rock-breaking method of diamond bits varies with formation properties:
Plastic Formations (e.g., mudstone, gypsum): Similar to a “plowing” process; diamonds penetrate the formation to cause plastic flow of rock.
Brittle Formations (e.g., quartz sandstone): Volumetric crushing pits are formed, with cutting sizes 2-4 times the protrusion of diamonds, resulting in high efficiency.
Hard Rocks (e.g., flint, siliceous rock): Impregnated bits are used, and rock breaking is achieved through micro-cutting and scratching, similar to grinding wheel abrasion.
Technical Advantages and Limitations of PDC Bits
As a revolutionary product in the diamond bit family, PDC bits have unique advantages:
Structural Features
Steel-Body PDC Bits: Integral medium carbon steel with surface hardening treatment.
Matrix-Body PDC Bits: Upper steel body + lower tungsten carbide matrix, offering better performance.
Profile Design
Parabolic: For soft formations, enabling high footage and drilling rate.
Round: For rotary table drilling, facilitating penetration of hard interbeds.
Conical: For high-speed drilling, with good penetration performance.
Application Limitations
Not suitable for gravel formations and interbedded soft-hard formations.
Temperature resistance limit (wear intensifies above 350℃; strength failure occurs at 700℃).
Poor impact resistance; edge chipping of new bits is prone to occur.
Comparison Table of Applicable Formations for Diamond Bits
Bit Type |
Optimal Applicable Formations |
Abrasion Resistance |
Impact Resistance |
Temperature Limit |
Drilling Parameter Characteristics |
Surface-Set Diamond |
Medium-hard to hard formations |
High |
Medium |
860℃ |
Low WOB, high RPM |
Impregnated Diamond |
Hard and abrasive formations |
Extremely High |
Medium |
860℃ |
Low WOB, high RPM |
PDC |
Soft to medium-hard homogeneous formations |
Medium |
Low |
350℃ |
Low WOB, high RPM |
03 Scientific Selection Guide: Matching Formations and Operational Requirements
Golden Rules for Roller Cone Bit Selection
Formation Hardness Matching
Soft formations: Select bits with high offset, overhang, and double-cone structures, equipped with wedge-shaped or spoon-shaped teeth.
Hard formations: Adopt single-cone, non-offset structures, equipped with ball teeth or conical-ball teeth.
Response to Abrasiveness
For abrasive formations, use insert bits with gage teeth.
If outer-row teeth are rounded while middle teeth show little wear, the next bit should be designed with enhanced gage protection.
Countermeasures for Special Working Conditions
Deflection-prone formations: Select short-tooth bits with no offset or small offset; the formation adaptability should be slightly softer than the actual formation.
Interbedded soft-hard formations: Select bits based on the harder rock, and dynamically adjust parameters during drilling.
Deep well section drilling: Use bits with high total footage to compensate for tripping time loss.
Selection Strategy for Diamond Bits
Applicable Conditions for PDC Bits
Optimal scenarios: Long homogeneous soft to medium-hard formations (e.g., shale, mudstone, gypsum).
Prohibited scenarios: Gravel formations, flint interbeds, interbedded soft-hard formations.
Parameter setting: Low WOB (30-60 kN), high RPM (100-300 rpm), large flow rate.
Applicable Conditions for Natural/Synthetic Diamond Bits
Hard to extremely hard formations (e.g., granite, quartz sandstone).
Highly abrasive formations (e.g., flint, siliceous dolomite).
Turbine drilling, deep/ultra-deep wells, and coring operations.
Special Requirements for Core Bits
Roller cone core bits: Four-roller cone (conical/cylindrical) or six-roller cone (full cylindrical) design.
Diamond core bits: Require symmetrically distributed cutting edges with consistent wear resistance.
Key indicator: Concentricity between the inner hole and outer circle to avoid core ovalization.
Downhole Anomaly Judgment and Handling
Roller Cone Bit Working Condition Identification
Bearing Damage: Periodic jolting of the rotary table, worsening under high WOB, reduced drilling rate but normal pump pressure.
Cone Loss: Severe drilling sticking, violent fluctuation of the weight indicator, change in trip-in depth when pulling up the drill string.
Tooth Wear: Reduced rotary table load, no jolting, significant decrease in drilling rate.
Usage Taboos for Diamond Bits
The bottom hole must be cleaned before running the bit downhole to ensure no metal debris remains.
Start drilling with small WOB and low RPM for “break-in” (0.5 m bottom hole conditioning).
Avoid reaming operations; if necessary, use low WOB and low RPM for uniform operation.
Contact :Jessie Zhou
Mobile/Whatsapp:+0086-18109206861
Web: www.landrilltools.com
Post time: Sep-24-2025