A drill bit accounts for a tiny proportion of the cost of your overall drilling operation but failing to use the right drill bit can add substantial costs to the overall project. So, drill bit design is critical to the success of your drilling operation. Our recent article, Drill Bit Design for Beginners, guides on the general importance of drill bit design and the different types in the market. This article outlines the key considerations you need to make when selecting the appropriate drill bit for your operations.

Understanding the drilling environment

Selecting the correct product for your project will depend heavily on the lithology to be drilled. Drill Bit vendors for the project must accurately evaluate the wide range of drilling environments based on solid knowledge of geographical classification and lithology principles. Different drilling environments will influence the drill bit design characteristics that are chosen.

The main rock classes in petroleum drilling are sedimentary rocks. Sedimentary rocks are formed from deposits of pre-existing rocks or organisms that accumulate on the Earth’s surface. This rock class covers over 75% of the Earth’s surface and accounts for 90% of the formation drilled for petroleum.

When the rock formations have been classified, the drill bit vendors will evaluate these characteristics; 

• Compressive strength of the rock, the formation hardness, 
• Degree of porosity and permeability of the rock, 
• Degree of sorting (how rivers, streams, oceanic forces have reshaped the rock), 
• The shape of the rock and the cementation and compaction. 
• The Rate of Penetration (ROP is the speed at which a drill bit breaks the rock to deepen the borehole) and bit life (expected longevity). These guides measure the relative ease with which a formation can be drilled.

Understanding the drilling environment will ultimately help you design the appropriate drill bit. At Aurora, we have developed C3 (Cutting Structure Durability Indexing) and BE3 (Cutting Structure Efficiency Indexing) software to streamline the process. You can learn more about how these products can help you make accurate and risk-free predictions for operational performance here.

The building blocks of PDC drill bit design

Global interest in diamond drill bits grew in significance in the 1950s. Oil companies had begun research programs on diamond bits to understand better their ability to stay longer in rock formations than roller cone bits. Since then, Polycrystalline Diamond Compact (PDC) bits have continued to dominate the market. The building blocks for fixed cutter drill PDC bit design include bit profile, cutting structure, material, and cleaning. 

Bit profile

When choosing a fixed cutter PDC product, you will have to consider its profile, i.e. the ‘face’ of your drill bit. The bit profile will influence:

• Durability: the flatter the profile, the less durable the product will be on the shoulder, where work rates are highest. 
• Steerability: This is the opposite of durability. The more parabolic the product profile, the less steerable it becomes. 
• Inner-cone angle: A compromise is required on the inner-cone angle of a product. The deeper the inner cone angle, the more stable the product. However, it becomes less steerable. 

Cutting structure

The primary concerns of the bit designer when dealing with cutting structures is to deliver the optimum combination of a higher ROP and durability. When choosing a fixed cutter PDC product, the cutting structure will have a strong influence on, 

• Durability: Cutter size and redundancy will determine longevity. 
• Efficiency: Similar to durability cutter size and redundancy will influence the efficiency of the product. 
• Cutter shape: The employed cutter shapes and geometry influence the product’s efficiency.
• Cutter Quality: And of course, cutter quality will impact the product’s longevity. 

Body Material

With PDC products, the bit body has two material choices: Matrix or Steel. Here are the pros and cons of both.

Matrix 

Pros: Erosion resistant so that these products can work in more abrasive lithologies and with high to very high jet velocity / HSI (Bit Hydraulic Horsepower).

Cons: Relatively brittle material; therefore, the blade standoff must be limited to avoid blade breakage. Matrix can also be less reliable as bits can only be safely repaired between 3 to 4 times, as repeated heat cycles have shown to have a negative influence on the matrix material. Finally, matrix consists of a blend of heavy metals, so at the end of its life cycle, it will be more challenging to dispose of.

Steel 

Pros: Very reliable, steel body PDC bits can be repaired more often due to their composition. Steel is also more ductile than matrix, as steel body products can be designed with a much higher blade standoff, so it delivers advantages in many drilling applications. At the end of its life cycle, the steel is also far easier to dispose of.

Cleaning

Cleaning is a vital part to evacuate the cuttings from the face of the bit. The cleaning of a PDC product is influenced primarily by the following:

• Open face volume: The more open face volume, the better the cuttings evacuation will be at the bit face.  
• Nozzle position and layout: For efficient bit cleaning, the flow volume delivered at the bit face is directed to the required exit area.
• Junk Slot Area (JSA): Bit cleanability is measured by JSA, the cross-sectional area of the junk slot measured at the zero gage plane. The perception in the industry is that the larger the JSA, the better the products’ cleanability. However, our opinion at Aurora is that JSA has little influence on true open-face bit cleanability. 
• Mud type and properties: Oil-based mud and water-based mud have pros and cons in aiding with bit cleaning, hole cleaning and borehole stability. Therefore, selecting the right system is a critical part of planning.

Contact our team of drilling experts at abc@aurorabc.com to learn more.