Air drilling methods have been around since the 1870’s when Simon Ingersoll, the founder of Ingersoll-Rand partnered up with the Rand Brothers. Simon brought the two methods together to create a new concept of drilling with air. Jack-Legs were widely already in use on many drilling and infrastructure projects. It was not until much later that air drilling became an effective method for deeper drilling applications for water wells and oil & gas drilling. As companies advanced the technology for drill bit designs and compressed air capabilities, these two areas became catalysts for mobile drilling rig development.
Drill Bit Designs
Drill bit designs have proliferated over the past 120 years. The original rotary drag bit design is still being used today. This fishtail drag bit is one of the oldest bit designs of what is commonly known as a rotary bit. These bits were used in the 1890s to drill for oil. They can be manufactured with a multiple number of cutting blades and shapes.
Some of them are arranged in a step bit design, or a blade bit. Some are more of a traditional cross bit. Still others have diamond cutters fixed to them in place of hardened steel or carbide inserts. These bits will work both with compressed air or mud fluid while drilling.
Advantages of these type of bits is the relatively low cost and speed of drilling achieved in in soft and gummy or plastic type formations. The drilling mechanism or fracture mechanic at work here is shearing of the formation through scraping. Despite the improvement in the design and manufacture of drag bits, their use has continued to decline. The advantages of fast penetration rate and a long life in soft formations has been minimized by vast improvements in rolling cuter bits. These are commonly known as bi-cone or tricone rotary bits.
In the 1920s, Howard Hughes significantly advanced the development of a rolling cutter rock bit. Prior designs did not allow the teeth of the rolling cones to “mesh up”. This caused material and drill cuttings to “ball up” in softer materials. The Hughes development was named a “self cleaning” drill bit. This caused the roller once drill bit to proliferate in its applications. Both steel tooth and tungsten carbide inserts.
A standardized IADC (International Association of Drilling Contractors) coding system has been developed to fully describe all the options for these type of drill bits. For softer formations, drilling contractors tend to use steel tooth or “mill tooth” drilling bits. These bits are more cost effective and more aggressive in drilling higher penetration rates.
When you are drilling harder rock conditions, you should choose a tungsten carbide insert rather than a mill tooth drill bit. These TCI inserts are manufactured in a variety of shapes. The shapes include:
- dome (round button)
- semi ballistic (cross between round and cone shaped)
The other features that distinguish different types of roller cone bits are several types of bearings and nozzles for flushing. The key to selecting the best drill bit for your application is knowing the type of ground conditions / formation that you are drilling through.
The industry has come up with a clear and standard classification for formation type. These basic formation types are soft, medium, and hard with corresponding rock bit types. In the most simple definition, the major differences between bits designed for a given formation type is the length and the configuration of the cutting teeth.
Soft formation bits are characterized by their long, widely spaced teeth. While drilling a soft formation, the bit T easily penetrates the formation. If they are longer, they can penetrate deeper. In addition, the teeth are made as slim as possible. This allows the maximum tooth penetration to be maintained even after the tooth is worn down.
Medium formation bits, for somewhat harder formations, feature teeth that are wider and stubbier. Penetration into the formation is still significant, but limited to the extent of the hardness of the material being drilled. Because of this, extended tooth length is not required. The shorter teeth offer more resistance to the increased forces on the teeth is the result of the gouging and scraping action in the firmer formations.
Hard formation bits have shorter and stubbier teeth that act to crush and chip the rock instead of scraping and gouging. These teeth are usually Tungsten Carbide Inserts or buttons that are made from an extremely hard material. These buttons are formed and then pressed into the cones of the drill bits. As they wear down, the buttons can be replaced individually or regrinded to sharpen the button after it has been flattened from drilling extremely hard rock.
This last classification of drill bits has a chief advantage over steel tooth bits. There is virtually no wear or change to the configuration of the cutting structure due to wear.