DETERMINING THERMAL PROPERTIES FROM VISUAL IDENTIFICATION OF DRILL CUTTINGS AND THE LIFE CYCLE ASSESSMENT OF A DISTRICT-SCALE GEOTHERMAL EXCHANGE SYSTEM

Abstract

Geothermal exchange systems consist of underground HDPE piping that is used to pump an exchanger fluid into the ground to either extract heat from or expel heat into the ground. When properly designed, geothermal exchange systems can be more efficient than conventional heating and cooling systems. In this thesis, the thermal conductivity of four borings at two different proposed geothermal exchange system site locations, in Warren, Michigan, and in the Hudson Highlands area of New York were determined through rock and mineral cuttings identifications and thermal response tests (TRTs). In addition to this, an Environmental Life Cycle Assessment was conducted on a district-scale geothermal exchange (GHX) system located near Madison, WI to determine the energy payback time required for this system.

The thermal properties of the subsurface at a site must be accurately determined in order to design the system for the required load. This is typically done using a Thermal Response Test (TRT), where constant heat is injected into the borehole for a set amount of time, and the difference between the inflow and outflow temperatures is used to determine the overall thermal conductivity of the geothermal exchange system. The use of fiber optic cables installed inside and outside of the loop provides additional temperature data and can be used to determine the specific thermal conductivity of specific rock layers in the borehole. Drill cuttings collected during the drilling can be used to further identify the rocks and minerals being drilled through. The identification of drill cuttings is used to provide both a range of thermal conductivity values in specific stratigraphic layers, as well as specific layer conductivity values found through mineral identification of the cutting’s samples. The mineral identification of drill cuttings to calculate the thermal conductivity

of specific layers within the borehole can be used to determine the overall borehole conductivity value in a location where a TRT could not be conducted.

The Environmental Life Cycle Assessment (LCA) completed for the district-scale geothermal exchange system in Madison, WI finds that the energy payback time of the system is highly dependent on the drilling impacts. SimaPro, a software program commonly used for LCA studies, was used to calculate the fossil fuel depletion impacts of all components considered: the construction of the system, the transportation of the materials, the mechanical components, and the operation and maintenance of the system. The impact of drilling the borefield required for geothermal energy exchange was represented by the operation of a diesel machine per hour of usage in SimaPro. SimaPro also provided impacts for deep well geothermal drilling per meter drilled, which were much more energy intensive than the shallow drilling conducted for the installation of this borefield. Due to the large environmental impact difference between shallow and deep drilling, and the use shallow drilling conducted at this site, the diesel machine operation per hour was used to represent the drilling impact. The energy payback time was determined to be just over 1 year after the start of operations. Previous environmental LCA efforts completed at this site that focused only on the construction impacts found an energy payback time of 14.5 months (Tinjum et al., 2023). The update of the drilling method, as well as the addition of the mechanical components and the operation and maintenance of the system changed this payback period. Future studies on this field including the end-of-life impacts of this system, such as the decommissioning of the field, as well as the waste processes for the mechanical components, will not necessarily affect the energy payback time but it will affect the overall energy balance of the system.