Common Questions About Rotary Drilling, Part 2
Sep 8, 2020 -
Operations Managers Bryan Nydoske and Mike Mottet are known for their rotary drilling expertise, having been in the drilling industry for a combined total of 81 years. Recently, they presented the most popular Cascade webinar to date, Drilling 104: An Introduction to Rotary Drilling. In it, they provided industry newcomers with an introduction to and seasoned professionals with a refresher about rotary drilling technology.
If you weren’t able to catch the live webinar, it’s now available to watch on-demand.
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Participants submitted a lot of great questions covering a variety of applications and scenarios. Bryan and Mike shared their expertise in our two-part blog series. You can read Part 1 here. [LINK] Today’s post is Part 2.
What’s the typical productivity (feet per day) of rotary drilling in hard rock lithology?
The nature of the rock can make a big difference, but if it’s hard and competent, I’d expect between 200 and 350 feet per day could be achieved.
If you’re running dual tube, however, it’s not uncommon to hit 600 to 800 feet per shift. In one project discussed in the webinar, we drilled 750 feet of 12.75 inch-diameter boring in a single shift.
There’s a pretty wide variety of production that can be expected overall, but with hard rock in good conditions, 200 to 350 feet per shift. And of course, that depends on the diameter of the hole.
What is the greatest depth you have drilled with a rotary rig?
In the environmental services industry, we don’t typically have a need for really deep drilling. At Cascade, the deepest I’m aware we’ve drilled using rotary was about 2,200 feet. When I worked for a previous company, we went as deep as 3,000 or 3,500 feet.
It’s uncommon to need a deeper borehole, but when we do, we have the equipment to do so.
What is the preferred well development method(s) when utilizing mud rotary?
Well development is the most important component of ensuring a productive well.
The best method, in my opinion, is to complete drilling of the well, cement it and let it cure, and evacuate the heavy drilling fluids out of the well via airlifting. That’s usually done with an open-ended string of drill pipe. Next, use a double swab and work your way down from the top of the screen to the bottom, aggressively swabbing and airlifting. Six minutes per foot of screen is a good rule of thumb. Once you’re at the bottom of the screen, add a dispersant and then blind swab (no airlifting) from the bottom of the screen to the top. Some people believe you need to let it sit for 24 hours, but I do not. You want to remove that wall cake from the borehole wall as soon as possible. Airlift and swab again from the top to the bottom of the screen. Once you get all the way to the bottom, pull out of the hole and enjoy your productive well.
This process is extremely important. A well could be installed perfectly, but shortcuts during the development process could limit the productivity of the well.
When using a drilling mud during monitoring well installations, what steps can be taken to ensure representative groundwater samples?
Hydropunch is an older method, but is still used today. We typically use a SIMULPROBE. Once the bit reaches just above the depth where someone wants to take a water sample, pull our drill out of the hole, and then run a SIMULPROBE sampler and drive it into the undisturbed formation to obtain a sample of both the water and formation.
If you’re interested in learning more about groundwater sampling specifically, my colleague Greg Zekoff wrote a great introduction to it: Sampling 102: 4 Ways to Collect Deep Groundwater Samples During Drilling.
What are typical ranges for air/fluid pressures during operation?
The range for air or fluid pressure depends on what method you’re using, whether it’s dual tube reverse, flooded reverse or direct air.
If we’re using pounds per square inch (psi), we use a higher psi the deeper we go.
If we install an air sub and allow it to assist with airlifting to the surface, we can run a lower pressure air system.
Some folks use foam and/or polymer when drilling with air. Anything like foam or water that occupies space in a borehole allows you to drill with lower volume and lower pressure as there is less annular volume.
Are there any types of refuse that you can’t get through using air rotary casing hammer (ARCH) drilling?
Sure, if you’re using ARCH drilling in a landfill and encounter an engine block, you may not be able to get through it. Same with railroad steel. These are quick ways to wipe out a $15,000+ drill bit. Drilling in landfills is a hit or miss thing. Numerous times we’ve been asked to scoot 10 or 20 feet over to reach the required depth, and were able to achieve the required depth with that small change.
How can you best identify the water table/saturated zone while drilling?
Depending on the drilling method and if you’re drilling where the borehole isn’t already full of fluids, it’s actually very easy to determine when you encounter the water table. You’ll see saturated cuttings come to the surface, and sometimes even the water comes to the surface, too.
It’s more difficult when you’re drilling a hole that’s full of fluids. In these cases, you should review historical data, such as logs from any surrounding wells in the vicinity. You’ll also want to keep an eye on drill cuttings that are coming to the surface—look for sands, gravels and rocks, as they may indicate there’s water nearby. Mud temperature is another indicator of when you might encounter the water table, because the temperature increases from the pressure of being circulated. Typically, the ground water temperature will be around 72 degrees, but if your mud starts cooling while circulating, that’s an indication you are below the water table.
Other options include using Hydropunch or SIMULPROBE tools. With both of these, you’d need to drill to the approximate area of suspected water, then drive the tool into the undisturbed formation below the drill bit. Either option will be able to determine if there is water present.
If you need to be absolutely certain about the depth of the water table, a surefire way to find it is to set a test well. To do this, you’d need to drill to the desired depth, run a PVC or steel test well to where you run the screen into the area, then gravel pack around it. Next, you’d put a bentonite seal above the gravel pack, develop the temporary well, and then run a water level indicator down to find the water level. Of course, this costs money and time, but it’s a sure way to get a definite answer.
When would I need/use temporary or permanent steel casing when drilling?
Anytime you have a formation that won’t stand open with direct air and you don’t want to use mud, you should use casing.
What are the primary factors that determine what rotary rig size is used?
The primary factors that determine the size of the drilling rig used on a project are the formation, the required borehole depth, and the borehole diameter. Other considerations include project goals and the available budget. These criteria affect your rig selection no matter the drilling technology you use.
If you’d like to learn more about rotary drilling, be sure to watch Bryan and Mike’s webinar, Drilling 104: An Introduction to Rotary Drilling.
WATCH NOW >>