Date of Thesis

5-8-2014

Thesis Type

Masters Thesis

Degree Type

Master of Science in Civil Engineering

First Advisor

Jeffrey C. Evans

Abstract

Geothermal wells are commonly used in the USA for heating and cooling of both commercial and residential spaces. In these systems, the annulus between the ground and the pipe is filled with a sealant, frequently a bentonite grout which acts as a heat exchange pathway between the earth and the piping material for the thermal conductivity and limits water flow vertically along the well annulus. Due to the grout being subjected to an enormous number of cycles of heating and cooling, it is necessary to understand how, if at all, the thermal and hydraulic conductivity of the bentonite seal changes with cycles of heating and cooling through the life of the geothermal well. A 150 mm diameter polyvinyl chloride (PVC) that contained a bentonite seal with a 25 mm diameter high-density polyethylene (HDPE) pipe to circulate the fluid was constructed in order to study the thermal and hydraulic properties of bentonite-based seals. After 18 heating and cooling cycles, the bentonite grout used in this research had an average thermal conductivity of 0.64 W/m-K and 0.092 W/m-K for a heating and cooling cycle, respectively. The flow pump permeability results yielded an average hydraulic conductivity of 2.6 x 10-6 cm/s and 1.29 x 10-6 cm/s for a heating and cooling cycle, respectively. The average intrinsic permeability for a heating and cooling cycle was 1.25 x 10-11 cm2 and 1.84 x 10-11 cm2, respectively. The hypothesis suggested by the data is that the potential formation of an air gap on the grout/pipe interface increases the hydraulic conductivity of the bentonite grout from ~10-7 cm/s to ~10-6 cm/s. Numerical modeling of a geothermal borehole was performed using a computer software package, COMSOL. The three cases that were modeled in COMSOL were: (1) experimental thermal conductivity values from the lab geometry, (2) standard design thermal conductivity values in a model field geometry, and (3) experimental thermal conductivity values considering field conditions. The solutions from COMSOL showed that for a heating cycle, the published thermal conductivity had a heat transfer rate of 27 Watts whereas the experimental thermal conductivity yielded a heat transfer rate of 24 Watts. For a cooling cycle, the heat transfer rate for the published thermal conductivity was nine Watts and the heat transfer rate for the experimental thermal conductivity was one Watt. The conclusion made from the numerical heat flux solutions and analytical solutions was that it is more efficient to use the ground heat exchanging technique for heating rather than cooling. The analysis performed to determine the amount of downward seepage that could be experienced in a geothermal borehole showed that with an increase in hydraulic gradient, the potential for downward seepage also increases. Also, the potential for leakage is the highest during a heating cycle in comparison to a cooling cycle but the quantity of flow is, under all conditions evaluated, rather small.

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