Understanding how to match bits and power to drilling conditions to help maximize jobsite effectiveness.
The structural integrity of a bore and the productivity of rock drilling operations depend on knowing the conditions and pairing the right machine and tooling to those conditions. For example, when drilling in rock up to 10,000 lbs per square inch in hardness, rock drilling systems for single-rod machines are qualified for most horizontal directional drilling (HDD) operations. However, the demands on the machine and tooling call for a different kind of downhole power when rock hardness exceeds 10,000 lbs per square inch.
It comes down to this — different projects are made up of different materials that can challenge both the effectiveness and steerability of a bore.
The soil, sand and rock formations of projects that a drill operator will encounter can be highly variable. These different formations call for different tooling: A bit designed for compacted soil may not perform as well in cobbled or solid rock formations, and bits made for hard rock may not handle soil very well.
Conditions can change quickly, so it is important to be attentive to the conditions in which you’re drilling, and to apply the right amount of power to the right material with the right tooling and the right fluids for the job, according to Vermeer Lifecycle Product Manager Jason Zylstra.
“A big challenge can be transitioning out of dirt into rock, then back into dirt,” Zylstra said. “Once you put certain rock tooling, like an air hammer, into dirt, you may have a difficult time because an air hammer is not meant to drill through that kind of soil. It’s like pushing a telephone pole through the dirt.”
Planning your Bore
To better understand the specific characteristics of the downhole material of a project — whether that’s rock, clay, sand or a combination — it can be helpful to have core samples lab tested for composition and abrasivity, helping you to select the right tooling. Past experience with a specific region’s subsoil conditions and composition is also important to account for in selecting the right tooling.
“Knowledge of the conditions based off of past work done in an area can also be helpful in finding the right tooling for a job,” said Zylstra. “Sometimes, you can get an idea of what your job will involve by understanding the conditions and area where a drilling has been done before.”
Different Tooling Options
Boring through soil and sand, as well as rock — whether solid or cobbled — requires different tooling, which also can dictate the required pressure and cutting force of the selected bit. Some rock formations, like sandstone, are softer but have higher abrasivity, while others are harder but have lower abrasivity.
With rock up to 10,000 lbs per square inch, a conventional drill combined with different tooling options can offer flexibility. The Armor drilling system from Vermeer includes different bit options that are specifically designed for a range of common conditions an operator may encounter. The Lance bit features a scoop design but has carbide buttons and hardfacing that help it perform in both hard soils and cobble. On the other end of the spectrum is the Gauntlet bit. With carbide hardfacing and buttons to help minimize wear resistance, and replaceable carbide-tipped teeth for cutting, the Gauntlet bit is more suited to drilling in medium rock, shale or caliche ground conditions.
Medium and harder rock formations, 10,000 lbs per square inch (68.948 MPa) and up, may require one of several options beyond conventional drills and tooling. Air-hammer, mud-motor and dual-rod machines like a Vermeer D40x55DR S3 Navigator HDD offer high-pressure capacity and tooling that can handle both hard formations and those with high abrasivity.
Air hammers are effective in HDD applications in rock with a compression strength of 18,000 lbs per square inch and above. However, one drawback with air hammers is the availability of adequate power equipment; a large compressor is required to power an air hammer, and they’re sometimes in short supply in many areas. And large compressors can add to the cost of a job completed with an air hammer.
Mud motors use high-pressure drilling fluid to independently power a downhole tri-cone or polycrystalline diamond compact (PDC) bit. Because mud motors tend to require a larger amount of drilling fluid than other types of machines, they’re often paired with a reclaimer on the jobsite. The drawback of mud motors is that they are mostly limited to larger rigs, 60,000 lbs and above.
Dual-rod systems use two independently controlled drill rods, one within the other, during operation. When in pilot bore mode, an outer rod provides rotation for steering, while an inner rod provides rotational torque to the tri-cone or PDC bit. Among rock-focused drilling systems, dual-rod machines can be a very versatile option. They are qualified for conditions ranging from mixed rock to solid rock. It will have some limits to its productivity in extreme environments, but there’s a huge swath of conditions in which a dual-rod machine is going to be the most suited for the job.
Matching the right bit type to your material and conditions is important, but so, too, is operating with the right speed and pressure to enable the selected bit to function optimally. “Regardless of the bit type you choose to operate, it is very important to know how to manage rotation speed and torque, as well as thrust, to help maximize your productivity on the jobsite,” Zylstra says.