Deep Bedrock Wells for Municipal Use: A Series on the Process - #1) Drilling

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:

1. Drilling

2. Geophysical Logging & Well Design

3. Well Installation

4. Gravel Packing & Grouting

5. Well Development

6. Well Testing

7. 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.