A formation thermal conductivity (in situ) test is typically recommended for commercial projects when considering implementing a geothermal system or a system is in the design phase. This test provides the engineer with valuable information about the earth’s thermal properties, which are necessary to size the ground heat exchanger (GHX) correctly. Drilling a test bore also provides great insight into the constructability of a vertical GHX at the proposed location. However, in order to maximize the benefit of the test bore and conductivity test, certain factors should be considered before mobilizing to the site to drill. Even once drill plans are developed, some flexibility may still be required during the test bore installation due to unforeseen drilling conditions.   

Test bore depth is one of the first things a designer should consider when planning a thermal conductivity test. It is important to note the test results are valid for only the tested depth, so the test bore depth should match the planned depth of the rest of the bores. Site plans should be reviewed to determine if the available land area for the loopfield is sufficient, based on estimated loads and bore footage requirements. Limited land area may require deeper bores.  

In addition to available land, local geology may influence test bore depth. For example, if local drill logs indicate 100 feet of clay on top of limestone that must be drilled with air, it may be more economical to drill deeper bores—in the 400-500 feet range. This is due to a reduction in the total amount of casing compared to shallower bores (250-300 feet). In addition, the bedrock will typically be more thermally conductive than the clay on top so it is desirable to drill deeper into the bedrock to reduce total system length requirements. If the depth to rock is 200-250 feet, mud drilling to a shallower depth is typically the lower-cost solution, provided there is room to accommodate shallower bores.

Another important consideration for the designer is how many tests must be completed to properly represent the site conditions. Like bore depth, local geology may affect the number of test bores necessary to adequately characterize a site. For example, drilling conditions in Nashville, Tennessee, are typically consistent so only one test bore and in situ test may be necessary for a small school project.

Move east towards Knoxville and the limestone becomes karst, with voids, mud seams and more groundwater. The same project may require two or three test bores scattered across the potential loopfield to get a representative sample of what drilling conditions might be like, as conditions can change significantly over short distances in that terrain.

Once the target test bore depth and number of test bores necessary for proper testing have been established, any regulatory permits and/or forms should be completed. Drilling regulations differ from state to state and often even from county to county, so it is important to know the rules and ensure any required paperwork is complete before sending the drill crew out. Local health department regulations on well construction may be more stringent than state regulations, as well.

Now, with paperwork in hand and a drill plan in place, it is time to drill. Casing has been set to 110 feet and the drill is hammering through the limestone. A water-bearing fracture at 440 feet produces groundwater at a rate of 200 gallons per minute, which reduces the drill penetration rate significantly. Options include continuing to drill to 500 feet, the planned depth, or shortening the bore depth to 400-425 feet. Drilling the remaining bores to 500 feet will require containment ponds to handle cuttings and groundwater. Shortening the bore depth will increase the number of bores (and casing amount) but may be more economical considering the time and cost savings of not diverting large quantities of groundwater.

This article has focused on planning for a test bore installation, and there are plenty of considerations before even discussing the details of the conductivity test. While most loopfield planning is done on a computer, it should be kept in mind that these are field tests, and even with proper planning there is some inherent unpredictability with drilling. It is important for the designer to properly plan for the test bore, while also maintaining some flexibility to make changes on the fly if necessary, based on geology and the driller’s recommendations. With a properly planned and executed test bore, a loopfield can be designed that best balances system performance, installation cost, and environmental constraints.

Chad Martin is the regional managing engineer for Geothermal Resource Technologies, Inc., whose products/services include thermal conductivity test equipment and testing, commercial system design, and quality assurance (QA) during construction. Chad is a professional engineer, a member of IGSHPA and ASHRAE, and has been involved with geothermal heat pumps for 20+ years.