Although wells have been constructed for more than 8,000 years, the concept of reverse circulation drilling was invented in the mid-20th century. It took another few decades until the technology was developed to take reverse circulation drilling from theory to reality. Since the 1990s, however, reverse circulation drilling has been the process of choice for obtaining rock samples in soft to medium consolidated formations.
The benefits of reverse circulation drilling are well documented. Samples are protected from cross-contamination since they are pulled up in sequence as the reverse circulation drill bit cuts new rock. Reverse circulation drilling consumes less water than other drilling techniques, such as diamond drilling.
However, not all reverse circulation bits are the same. Here are five characteristics to understand when choosing reverse circulation bits:
Outer Diameter
As with any drilling operation, reverse circulation drilling can produce a range of bore size diameters from a few inches to 24 inches. The size of the bore will depend on the application and the geology of the well site.
However, keep in mind that larger bore sizes will typically require more powerful downhole hammer equipment since a larger bit will distribute drilling energy over a larger area and larger bits will take more energy to move than smaller bits. Moreover, larger wells will require more water and larger air compressors to push a larger volume of samples up the sample tube.
Inner Diameter
In addition to the pin diameter that determines the size of the interface between the hammer and reverse circulation bit, the inside diameter of the bit’s sample collection openings will determine the size of the rock samples that will be transported to the sample tube.
If the inside diameter of the sample collection opening is on the small end of the range, your rock samples will be relatively small and you may have difficulty obtaining a representative range of samples. For example, the only samples that may be small enough to pass through a small sample collection opening will be rock that is easily pulverized while harder rock may become trapped against the sample openings or clog in the sample tube.
Type of Sample System
Some reverse circulation bits have sample collection openings on their ends (also called a center sample system) while others have sample collection openings above the cutting surface (also called a crossover system).
As a practical matter, center sample systems are less likely to produce cross-contamination since the rock samples are extracted immediately after being broken. However, since the cutting surface has holes for sample collection, these drill bits may require more careful drilling since the drill bit is hollow.
Conversely, crossover systems can be pushed harder because the drill bit can be solid. Since samples are not collected until they work around the bit to the sample collection opening, the drawback of crossover systems is that they are more prone to deliver samples cross-contaminated with debris from higher in the well that has mixed with the newly cut samples.
Bit Material
Reverse circulation bits are typically made from steel or tungsten matrix. These materials have very different properties and choosing the wrong material may result in less efficient and more costly drilling as you replace broken or worn bits.
Steel is more ductile than tungsten. This means tungsten is more brittle than steel. This difference in material property produces two tangible effects:
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- Ductile materials tolerate larger impact loads than brittle materials. This means you can use more powerful hammer drilling loads for steel bits than tungsten bits. Under heavy impact loads, tungsten bits will crack and break.
- Hard, brittle materials are less prone to abrasive wear than soft, ductile materials. This means the drilling action that accompanies each impact will wear out steel bits quicker than it wears out tungsten bits.
Cutter Treatments
PDC bits, such as Silver Bullet PDC bits, have been treated with polycrystalline diamond materials. Diamonds, due to their hardness and thermal transfer properties, make excellent abrasives on cutting surfaces. The coating on steel PDC drilling tools, therefore, can ameliorate some of the wear issues that arise from steel reverse circulation bits.
Selecting reverse circulation drill bits depends heavily on your drilling equipment and the geology of your drill site. However, the composition, treatments, and size of the drill bit can complicate or ease your drilling project.
