THERMAL PERFORMANCE EVALUATIONS OF FRACTURED AND CLOSED-LOOP GEOTHERMAL RESERVOIRS

Abstract

Earth’s interior contains an enormous amount of heat that can be exploited for carbon-free direct-use or electricity generation. Even though numerous studies have predicted that geothermal power will become an important contributor to the world’s energy mix, the use of these resources is still growing at a notably slow speed compared to other renewable energy alternatives. This thesis uses computational models to explore the technical challenges that two kinds of geothermal resources face to reach full commercialization. In particular, the temporal evolution of heat production of several fractured and closed-loop geothermal reservoirs is investigated. Thermal-hydraulic simulations are conducted for a fractured meso-scale geothermal reservoir in northern New York, USA. The modeling parameters considered here are constrained by empirical data related to lithology, hydrogeology, and thermal behavior measurements collected on site. This work shows how the addition of realistic complexities, that are well-constrained by field data and often disregarded, can significantly improve the thermal performance predictions compared to overly simplified models. Additionally, the results presented here highlight the importance of characterizing subsurface permeability distributions in order to optimize thermal efficiency and devise appropriate reservoir management strategies that extend the lifespan of geothermal reservoirs. To evaluate how closed-loop or advanced geothermal systems (AGS) compare to alternative ways of extracting geothermal energy, several AGS designs displaying varying reservoir and operating conditions are evaluated to estimate their heat and temperature generating potential. Our findings indicate that the thermal efficiency of AGS is characterized by a considerable exergy loss. Sensitivity analyses show that varying different parameters have slight and moderate improvements on thermal performance, however, AGS designs appear to present multiple technical challenges making them less cost-competitive than both conventional hydrothermal systems and enhanced geothermal systems (EGS). The following key findings summarize the results of these two studies: 1) if well-constrained, computational models are a good tool to assess, manage and intervene geothermal reservoirs to ensure their long-term sustainability, 2) non-uniform permeability can drastically modify fluid flow and heat transport processes in geothermal reservoirs compared to theoretical models that consider homogenous reservoir properties, 3) prospecting adequate subsurface properties is of critical importance to develop geothermal reservoirs, and 4) despite their recent popularity, closed-loop systems are expected to be considerably less productive than other types of geothermal resources at a similar scale.