Drill Bit Selection: Simplifying the Process

With many choices available today, selecting the proper bit for a particular application can become quite confusing. There are spade, drag and shear type bits, point attack bits, PDC bits, milled tooth and TCI (tungsten carbide insert) roller cone bits. There are IADC codes, soft, medium, and hard categories for each bit, sealed, non-sealed, roller, and friction bearings. The list goes on forever. What is a contractor to do when the success of the drilling operation is directly related to what is screwed on the end of the drill pipe? In an effort to simplify the process, I am going to suggest a simple, systematic approach. This process involves a few simple steps and will get you in the ballpark every time:

• Identify – Soil/Rock type(s)
• Classify – Soil/Rock category
• Choose – Bit type within the category
• Optimize and Adjust – Operating parameters/subsequent selection

Identify – Obtain geotechnical information. This part of the process is the most important. When done correctly, it will ensure proper tool selection and application through reaming. On larger pipeline jobs, owners often supply contractors with accurate formation properties including rock compressive strengths. In many cases where detailed information is not provided, I suggest the following:

• Perform a site (or sight) survey. Sometimes actual geology is exposed from pre-existing road work, other construction, or through onsite potholing.
• Contact the U.S. Geological Survey for geology specific to a location.
• Contact the Natural Resources Conservation Service (formerly Soil Conservation Service) for soil surveys in a specific area.
• Contact local water well drillers.
• Contact local construction companies.
• Contact tooling suppliers/manufacturers, they may provide information related to past experiences in specific areas.

Remember, you are interested in the geology that exists within 100 ft (or less) of the surface. The more information you obtain the better equipped you’ll be to make the right choice.

Compressive Strength/Hardness	Range Soil & Rock Types
0 to 10,000 psi/Low	Dirt, Clay, Cobble, Talc, Shale, Siltstone, Unconsolidated Sandstone, Slate
10,001 to 30,000/Mid	Limestone, Dolomite, Sandstone, Coarsely Grained Granites
30,000 +/High	Finely Grained Granites, Trap Rock, Chert, Iron Formations, Quartzite

Classify – Define soil/rock category. To simplify, I suggest the following categories.

Choose – Choose a bit within the category. Bit choices progress from softer to harder in descending order. Choices listed within each category should produce cost-effective results. However, there can be overlap between hardness categories. Use of a downhole motor can influence bit choice. Blade and point attack type bits are commonly used without a downhole motor.

Compressive Strength/Hardness Range	Suggested Bit Types
0 to 10,000 psi/Low	Drag or Blage Type (duckbill, spade, etc.) Point Attack, Shear Type (PDC) Milled Tooth & TCI Roller Cone, Single Cone TCI
10,001 to 30,000/Mid	Milled Tooth Rollar Cone
30,000 +/High	TCI Roller Cone with conical or round top (dome) inserts

Optimize – Utilization and adjust subsequent selection if required.

• Talk to suppliers and manufacturers of these products. They can provide recommended operating parameters and guidelines for optimal use.
• Noting and identifying cuttings return characteristics while drilling can help in refining ongoing utilization and selection.

Other General Considerations:

• Rig operating limits can influence bit selection.
• Torque requirements become greater in softer
• materials.
• WOB (weight on bit) requirements become greater in harder materials.
• Sealed, friction bearing, roller cone bits are preferred for fluid applications. However, open bearing roller cone bits are used successfully on shorter bores of less than 200 ft.
• TCI roller cone bits with round top shaped inserts will produce the slowest ROP (rate of penetration).
• Maximum gage, shirttail, (skirt) and leg protection is a must for roller cone bits.
• Your rig operating cost investment outweighs the cost of a drill bit. Choose a bit that produces the most cost effective balance of ROP and total run time. Trip time is costly – improved ROP equals less drilling hours and money spent.
• Conventional “petroleum” roller cone bits are not always best suited for HDD.
• Certain bits, designed specifically for HDD, can prove to be more cost effective.
• Re-run roller cone bits are “used bits.” No one can tell you how used they are. These bits are considered a risk and discarded by petroleum operators. Consider the investment in your bore and weigh the risks.

Tim Lahay is responsible for product and business development for Sandvik HDD Products.