Date of Award

Spring 2026

Abstract

Heat flow governs the performance of geothermal energy systems, and thermal conductivity is the primary material property controlling subsurface heat transfer. This study investigates heat flow through media with spatially variable thermal conductivity within the heterogeneous Eliot Formation, a metamorphic unit extending across the coastal region of northeast New England. The objective of this work is to develop an approach for scaling thermal conductivity from core-scale measurements to field-scale values representative of geothermal borefield systems.Divided bar thermal conductivity testing, petrographic analyses, geological observations, and existing literature were integrated to construct a MODFLOW model that captures the heterogeneous thermal behavior of the Eliot Formation. Core samples of the Eliot Formation, 5 cm in diameter, were available from the Burley-Demeritt Farm in Lee, New Hampshire, to characterize thermal behavior at the centimeter scale. Divided bar measurements of 2 cm long core segments yielded an average thermal conductivity of 2.94 W/mK with a standard deviation of 0.69 W/mK. The measurements range from 1.48 to 4.20 W/mK. Petrographic analysis shows that elevated conductivity correlates with the presence of quartz and calcite, and that quartz veins increase conductivity depending on their orientation relative to heat flow. Geologic observations from core and outcrop exposures informed the layered structure of the numerical model. The model was run using varying fold wavelengths to evaluate how heterogeneity influences effective thermal conductivity. Three methods for calculating thermal conductivity were tested and evaluated based on the effective conductivity result of the model. The geometric mean was found to best represent field-scale behavior. Model results show that heterogeneity, driven by fold frequency, vein structures, and subfacies-scale variability, affects the effective thermal conductivity. In other words, rock fabric can help determine thermal conductivity at different scales. Backed by prior research on specific rock thermal conductivity (Dalla Santa et al., 2020) and the thermal conductivity tested at the Portland Jetport of 3.75 W/mK (Roy, 2010), these findings demonstrate how small-scale measurements, combined with geologic characterization, can be upscaled to produce a representative field-scale representation of the heterogeneous Eliot Formation. The methods used within this study can be replicated for other heterogeneous formations. The findings of this paper can contribute to thermal conductivity databases on metamorphosed phyllites that are quartz-rich, calcite-rich, and mica-rich.

Document Type

Master's Thesis

First Advisor

Matthew J Davis

Second Advisor

Sophie Coulson

Third Advisor

John Hogan

Degree Name

Master of Science

Download

Share

COinS