Deep Bedrock Wells for Municipal Use: A Series on the Process - #1) Drilling
Drill rig for municipal well.
LWS is beginning a well drilling project in the Town of Monument with drilling company Layne, a Granite Company, to drill an Arapahoe aquifer well for municipal use. During this project we will be documenting the process and producing blogs regarding seven distinct steps of well drilling as they occur. The processes being covered in this series are:
Drilling
Geophysical Logging & Well Design
Well Installation
Gravel Packing & Grouting
Well Development
Well Testing
Water Quality Sampling
DRILLING
For this blog we will cover the drilling operations. The well being installed is rather large because it is for municipal use. Most residential wells for a single home are 4-inch diameter wells in an 8-inch diameter hole. This municipal well is a 12-inch diameter well in a 20-inch diameter hole, and the size of the drill rig matches the need to be able to cut this large a borehole.
The first step in the process is to install a surface casing. The surface casing is a 30-inch diameter, 40-foot-long steel casing that is drilled with a caisson rig prior to the drilling rig being set up on site. The surface casing is placed into the ground concentric with the planned borehole. The first 40 feet of soil is often the most unstable and the surface casing keeps the soil from collapsing into the hole and also stabilizes the ground surface so the drilling rig isn’t susceptible to movement during drilling. We will cover how the deeper parts of the borehole are stabilized in just a bit.
Speaking of bits, the drill bit for this drilling operation is called a tri-cone bit; it doesn’t cut straight through rock, it spins as the rounded, hardened steel teeth crush the rock using the massive weight of the drill pipe above it. The entire drill string is rotated from the surface, causing the bit to rotate at the bottom of the hole. As the bit works down, more pipe is added to the top of the drill string until the final depth of the hole is reached.
Back to how the deeper portion of the borehole is kept stable. As anyone who has dug a hole in the sand at the beach can imagine, keeping a 1,720-foot deep and 20-inch-wide hole from collapsing on itself is a rather difficult task. By the way, that is approximately 3,710 cubic feet of material that is removed from the earth during the drilling of this borehole. To maintain the borehole’s integrity we use what is called “drilling mud:” Drilling mud is water mixed with compounds to make it heavier than water, as determined by the drilling mud engineer. This heavier-than-water mixture is maintained at the top of the borehole throughout the drilling process and puts outward pressure on the inside walls of the borehole due to its density and weight, stabilizing the borehole.
Tri-cone drill bit
Schematic of reverse rotary drilling.
The drilling mud is maintained at a viscosity to help remove the cuttings from the borehole as the bit is advanced downward, and is circulated through the borehole with pumps that pull the drilling mud down around the outside of the drill string, and back up to the surface through the inside of the drill string. This method of drilling is known as “reverse rotary” drilling. Reverse, because the standard way is to inject the drilling mud down the center of the drill string and let it return outside of the drill string; and rotary, because the bit is driven downward through rotation.
The drilling mud does more than just keep the borehole stable though. By circulating the drilling mud up through smaller space inside the drilling string, the drilling mud must move faster than it did traveling down to the bottom in the larger cross section area of the borehole. The increased velocity of the drilling mud causes the cuttings from the bit to be carried to the surface in the mud, where they can be filtered out. Without the circulation of the drill mud, there would be nothing to remove the cuttings from the borehole. The drilling mud flowing through the bit also keeps the bit cool as it spins and crushes the rock under massive pressure. The drilling mud is the unsung hero of deep well drilling.
Not only is drilling mud a critical component of the drilling process, but the type of drilling mud used is equally important: Bentonite-based muds have historically been used as they are relatively inexpensive and provide a good “mud cake” to stabilize the borehole. However, bentonite-based muds can be very difficult to be fully removed from the borehole after well completion during the development process (watch for the upcoming LWS blog on Well Development).
LWS recommends the use of inorganic polymer muds in all Denver Basin deep bedrock aquifer wells because these muds are also effective in stabilizing the borehole and are much easier to remove during the development process. When drilling these deep wells, there is a drilling mud engineer who designs the properties of the drilling fluid to meet the necessary specifications of mud, based on the site-specific data.
This drilling will continue 24 hours a day until the final depth of the well is reached. During this time geologists and engineers from LWS will observe and document the progress of the operation and log the cuttings that are strained from the drilling mud to assess the properties of the materials that are being drilled.
After the drilling is complete comes the geophysical logging, which will give us more information about the aquifer conditions at depth, the demarcation between aquifers, and assist with the design of the well. Stay tuned for our next blog in this series!
If you have a need for a water resources firm for well drilling, groundwater modeling, water rights cases, or more, please reach out to any of the LWS personnel below and we will be happy to put together a plan for your needs. We help with projects big and small.
Bruce Lytle, President of LWS
Chris Fehn, Senior Project Engineer
Anna Elgqvist, Senior Engineer
