If you are considering or just starting a career in the environmental services industry, you may feel overwhelmed by all the different technologies and applications you’re trying to learn and understand. In this blog post, we’ll cover the basics about each of the main types of environmental and geotechnical drilling.
You can also watch our on-demand webinars, “Drilling 101” and “Drilling 201: Sonic Methodologies and Best Practices.”
The first type of drilling is Direct Push Technology or DPT. This is one of the most basic methods, as it consists of pushing a series of small drill rods directly into the ground, using sheer weight or a percussion hammer to advance it deeper. As it moves into the ground, the cuttings are displaced into the outside of the borehole, unless a sample is being recovered. In this case, the cuttings (or sample) are assimilated into a sample barrel.
What are cuttings? Cuttings are the bits of in situ soil or debris that are displaced during the drilling process. What happens to them depends on the drill method being used.
Hole stabilization is achieved using small diameter flush-threaded drill rods or casings.
PROS: DPT is cost-effective and versatile when used in shallow, unconsolidated softer subsurface applications.
CONS: DPT is susceptible to depth refusal. However, there are sometimes ways to overcome DPT refusal depending on the project objectives. In the case of equipment refusal, other drilling technologies may prove to be safer, efficient, and more cost effective for the project as a whole.
The second type of drilling is hollow stem auger, which uses force and torque applied to an aggressive drill bit and flighted augers (it looks similar to a screw) to claw through formation. As the bit rotates, cuttings are lifted by the auger flights (like the threads on a screw) to the surface to create a borehole. As the name implies, the flighted augers are hollow, which is how the boring is stabilized while samples are taken or a well (or an instrument) is installed. This technology is also widely used in the geotechnical market to collect standard penetration test (SPT) data to understand soil density characteristics.
PROS: Hollow stem auger is a relatively inexpensive technology and is well suited for use in environments with sands, silts or clay. Because there are no additives used to remove cuttings, it is also a great option for environmental sampling.
CONS: Not all formations are suited for this equipment as refusal can occur in rocky and denser formations. Also, this technology can also create a lot of cuttings depending on borehole diameter, which must then be dealt with.
Sonic drilling is a method of advancing into the subsurface by the mechanics of resonating a series of connected steel drill pipes. The sonic energy is generated using a highly specialized sonic drilling head at between 50 to 150 Hz (cycles/second). The resonating drill pipe fluidizes the adjacent formation to overcome the borehole friction. Rotation of the drill pipe is not always required. Stabilization is achieved during drilling—as drilling, coring, and casing happen nearly simultaneously.
PROS: Sonic drilling can provide a continuous core sample of in situ materials (unconsolidated and consolidated) with limited refusal, which is important when trying to understand the finer details of the underlying geology. It is an extremely versatile and fast method of drilling usable in nearly all lithologies and fill materials. Investigation derived waste (IDW) is also reduced by up to 70-80% compared to technologies like auger or rotary drilling. In addition, a sonic borehole can be advanced on almost any angle as low as 20-degree off horizontal.
CONS: Sonic is an advanced technology and the driller requires additional training and experience for it to be effective. Also, fill materials such as rubber and wood tend to absorb the sonic energy making drilling difficult. In some cases, bits can be engineered to facilitate drilling in this type of fill.
“Rotary” actually refers to many different drilling methods, but they all share a few things in common. They penetrate using a rotating bit suspended by drill pipe, grinding down the geology as bit weight is applied to advance the borehole. Cuttings must be removed using a drilling medium, such as an engineered fluid or compressed air.
If fluid is used, it is applied to the borehole and debris is captured and suspended in it. Thanks to the rotation of the bit, the fluid is circulated back to the surface carrying the cuttings. The fluid can also help stabilize the borehole so it doesn’t collapse.
If compressed air is used, the sheer velocity of the air in the borehole pushes all the cuttings to the surface and out of the hole.
When engineered fluid is not being used to stabilize the borehole, there are two other options—a driven casing, or in the case of bedrock, self-stabilization.
PROS: Rotary drilling is capable of achieving greater depths and borehole widths than other drilling methods, such as auger or DPT. It is also a versatile option, as there are several different types of rotary methods to fit project needs.
CONS: Rotary drilling, unfortunately, generates a lot of IDW, which can be costly and inconvenient to manage. When additives or mud are used to remove cuttings, rotary is a less than ideal option for environmental sampling as IDW disposal can be expensive.
Wireline coring and diamond coring are nearly identical methods, and the names are used interchangeably. Both use a rotating core barrel bit (with diamond coring, the bit is diamond-impregnated) to penetrate formations. Core samples can be recovered without removing the drill rods, which makes this an efficient sampling method. Cuttings that are not part of the sample are ground and flushed out of the space with fluid (typically water). The drill pipe remains in place throughout drilling and sampling process which is how hole stabilization is achieved. With wireline coring, the barrel is brought to the surface using a wire, while conventional diamond coring uses drill rods to bring it to the surface.
Depending on lithology hardness, different bit designs can be selected to make drilling more efficient.
PROS: Wireline coring is a fast, efficient way to obtain geotechnically acceptable (RQD) rock core samples. Drilling can be advanced on an angle.
CONS: Wireline coring is limited by the maximum core sample it can recover to approximately 3.27 inches. Softer and karstic formations can pose challenges to the drilling process. Fluid circulation is required to lift cuttings, creating IDW that must be managed.
Hopefully, you now have an understanding about the five major drilling methods: direct push technology (DPT), hollow stem auger, sonic, rotary and wireline coring.