Geothermal Well Drilling—Demystified Part 2: Installing the Test Well

By David Brown, President of Yadkin Well Co, Inc.

A well test is prescribed for any vertical geothermal loop field in a commercial-sized application. This test provides an opportunity to analyze the heat transfer properties of geological formation at the well field site. Once analyzed, this information helps the design engineer determine if the current loop field design needs adjustment. First things first, we need a well to test.

The “test well” will become a functional part of the well field, so it should be drilled within the confines of the loop field design. The well driller constructs this well according to the exact design parameters of the rest of the field--same depth, piping, grout, etc. We simply test this first well to confirm its heat transfer capacity before the owner goes to the expense of drilling additional wells. Therefore, it must be installed precisely as designed.

Bore Hole and Casing Installation

Soft soil drilling begins! Notice the sand being expelled from the PVC Pipe at the bottom right of the picture.

Unlike horizontal well fields, vertical well drilling requires a lot of heavy equipment, including the drill rig, several hundred feet of drill pipe and well casings, drill bits, piston grout pump, and support trucks filled with cement bags, fuel, and more. The drilling method and equipment can vary somewhat, depending on the geological formation of the site.

Air drilling with what is known as a down-the-hole hammer drill is the preferred method in areas where you are likely to encounter solid rock.

Our drilling rigs are equipped with compressed air that not only drives the drill into the earth but also expels all the drill cuttings from the full depth of the borehole.

Initially, we use an air-driven blade to cut through the topsoil and then switch to a hammer bit when we encounter rock. Once we hit solid rock, we pause drilling to install the borehole casing. Using the drill rig, we systematically connect 21 ft. sections of steel pipe and gradually lower them into the hole.

Preparing to install casing and hammer bit

These casings line the borehole from the bedrock to the ground surface, keeping the soft soil portion of the hole from collapsing onto itself. Since the casings are steel, which conducts heat well, they do not interfere with heat transfer and may be left in permanently.

After we set the casings, we continue drilling into the bedrock until we reach the target depth. Casings are not required in bedrock.

An air drilling rig can typically power through 500 feet of earth a day, assuming we don't encounter excessive amounts of rock, water, or sand, which may require us to modify our drill strategy. Throughout the drilling process, we periodically monitor the drilling debris and record what type of matter we encounter and at what depths. We will share this log with the company that provides the equipment and data analysis for the well test.

Pipe installation and Grouting

High-density polyethylene pipe (black) in a u-bend formation. Before lowering the pipe into the bore hole, we tape an 8 ft. section of metal pipe (gray) to weight and straighten the pipe as it is inserted.

After the borehole is complete, it’s time to insert the geothermal piping. We use prefabricated and pre-tested U-bend pipe assemblies made of high-density polyethylene pipe. We typically tape an approximately 8 ft. section of metal pipe to the end of the polyethylene pipe assembly to help weight and straighten the pipe as we lower it into the hole. When the pipe is fully inserted, we cut the supply and return ends of the pipe, leaving approximately three feet of excess above the surface.

Now, it's time to grout the hole.

Lowering the polyethylene pipe assembly into the bore hole. This U-bend pipe will serve as the supply and return for this geothermal well.

Grouting is required for vertical wells to conduct heat between the water pipe and the formation. Drillers use a mixture of water, sand, and bentonite that is tailored to the geology. Our grout trucks are equipped with a piston grout pump, water tank, sand hopper, and a large reel of 1-1/4 in. "tremie" pipe, which dispenses grout into the hole.

We want to begin dispensing grout at the bottom of the hole, so we lower the tremie as close to the bottom as possible. This prevents standing water in the hole from thinning the grout from the bottom.

We continue pumping grout from the bottom to the top, raising the column approximately 30-35 feet per mixed batch until the hole is filled. After the grout cures, we may or may not remove the steel case from the hole, depending on time. As stated, the casing does not impact heat transfer if left in.

We mix the water, sand, and bentonite on site and dispense it into the hole directly from the grout truck.

Cooling-Off Period

Air drilling introduces approximately 230°F of compressed air into the borehole. This heat must be allowed to dissipate before we start the 48-hour thermal geothermal conductivity test (TGCT). The International Ground Source Heat Pump Association standard requires a five-day cooling period between the well drill and the TGCT test. This ensures that the data collected during the test is based on real-world conditions and will yield a more accurate analysis of the loop’s heat transfer properties and performance.

Next up—the test!

David Brown is the Co-owner and vice President of Yadkin Well Co, Inc. in Hamptonville, NC. He and his technicians drill water and geothermal wells throughout North and South Carolina. The business, purchased by David's father in 1964, is now owned by David, his son, Matthew, and his brother, Vaughn. The Brown family has been drilling wells for geothermal installations since 1985.

David is a Certified Well Driller/Pump Installer (C.W.D./P.I.), a Certified Vertical Closed Loop Driller (Geothermal) (CVCLD), and a Geothermal Accredited Installer (AI) and inspector (CGI). He is also a North Carolina Level A contractor. For more information, click Yadkin Well Co.