Session 2D: Low-Temperature/Direct Use

The reliability of heat flow maps and techno-economic models for geothermal energy projects is significantly influenced by the accuracy of bottom hole temperatures (BHTs) derived from oil and gas drilling operations. However, BHTs are often compromised by the rapid cooling effect of drilling fluids, necessitating the application of correction methods to approximate the rock's equilibrium temperature. Previous methodologies, including those developed by Harrison, Förster, and Kehle, rely on correlating unadjusted BHTs with high-quality, equilibrated temperature logs within specific basins in the United States to determine a generalized offset suitable for each study area.
This research evaluates the efficacy of these established correction methods by applying them to raw BHT data from Presidio County, Texas—a region lacking high-quality equilibrated temperature logs. Utilizing the GEOPHIRES tool, we conducted an in-depth analysis of the thermal and economic feasibility of developing a geothermal plant based on corrected BHT values. Our findings indicate a significant discrepancy of up to 15% between the Harrison method and the other two correction methods, translating to over a kilometer difference in the estimated depth to reach temperatures of 200°C. This variability becomes even more pronounced at higher temperatures, impacting the depth estimation and, consequently, the project's feasibility. Moreover, the techno-economic analysis reveals that the choice of correction method can substantially affect the project's economic outlook, with a variance exceeding $10 million in Net Present Value among the methods for an enhanced geothermal systems project scenario in Presidio County. These results underscore the critical need for high-quality temperature logs from abandoned wells to validate temperature corrections or to develop a new correction method that is more tailored to regional conditions.

Underground thermal energy storage (UTES) systems behave like a thermal battery which preheats or precools the ground to later heat/ cool buildings. Borehole thermal energy storage (BTES) is a UTES system in which fluid is circulated in closed-loop pipes installed in a closely spaced borehole array. BTES is gaining popularity over the world due to its wide adaptability in various climates, limited surface area, lower total borehole length compared to traditional geothermal heat pump (GHP) installations, and its ability to efficiently provide seasonal energy storage. To maximize the economic value of the cost of drilling and operating a BTES it is critical to model and optimize the BTES for the site, climate, building energy loads, available energy sources, and the time-of-day pricing from the electric utility. The round-trip efficiency of a BTES is influenced by factors such as borehole spacing, the way boreholes are connected, and the operating schedules. In this work, we optimize the design of BTES with the help of numerical simulator FEFLOW coupled with various charge/discharge algorithms.

Coal plants are more efficient than geothermal plants since thermal efficiency is proportional to temperature. When geothermal steam comes out of the well, the temperature is around 180°C compared to coal plants with steam temperatures greater than 500°C. This is why typical coal plants have a thermal efficiency of 36%, whereas geothermal has about 18%. In this work, a novel hybrid system was designed to control the temperature and pressure of geothermal steam to approximate the steam conditions and thermal efficiency of coal plants using solar thermal technology. The thermodynamic model of the hybrid system was developed wherein heat transfer was modified by directly using steam as HTF instead of air to eliminate heat loss in the heat exchanger resulting in more efficient use of solar energy. The results showed that the hybrid plant produced a net additional power output of 24% compared to a standalone geothermal plant due to increased thermal efficiency from 18% to 35%. The hybrid plant also performed better than geothermal and solar thermal plants operating separately. The novel design proved technically feasible and more efficient than all current hybrid designs. Detailed engineering design and economic feasibility of the proposed system are recommended for future studies.

Community Geothermal Coalitions across the United States are nearing completion of their first phase of work focused on expanding community-scale geothermal systems. The work began on October 1, 2023, and is funded through U.S. Department of Energy (DOE), Geothermal Technologies Office (GTO) Community Geothermal Heating and Cooling Design and Deployment Initiative Cooperative Agreements.

A main goal of the initiative is to support the formation of U.S.-based community coalitions that will develop, design, and install community geothermal heating and cooling systems that supply at least 25% of the heating and cooling load in communities. The coalitions describe how switching to a geothermal district heating and cooling system would result in greenhouse gas emission reductions for the community where the system is installed.

In total, the selected coalitions comprise more than 60 partners across the U.S. and include one or more partners that: represent the community, are expert geothermal designers, focus on geothermal workforce development and training needs and include a deployment focused coalition partner such as a gas utility.

The eleven coalitions are designing their heating and cooling systems in close partnership with, and many led by, the communities in which the geothermal projects would be deployed. The coalitions are based in urban, suburban, rural, and remote communities each developing innovative community scale systems.

Anticipated community benefits include addressing: environmental justice conditions, such as cumulative environmental pollution and other hazards; underserved and disadvantaged communities; and community members who have historically experienced vulnerability due to climate change impacts.

Preliminary case studies developed in the first phase will help illustrate how Community Geothermal Coalitions can be replicated throughout the U.S. followed by the development of more complete case studies by coalitions whose projects move to the deployment phase.

This project is part of a national initiative to showcase the benefits of incorporating low-temperature geothermal resource assessment into the deployment of geothermal heating, combined heat and power (CHP), and geothermal direct-use (GDU) technologies. The initiative was established to accelerate the country's decarbonization efforts by identifying potential for low-temperature geothermal resource utilization ( < 150°C, e.g., CHP and GDU) in selected sedimentary basins with numerous population centers.
The play fairway analysis (PFA) methodologies in this study were adapted from previous PFA investigations of sedimentary basin geothermal play types (SBGPTs) that evaluated the potential for low-temperature resources ( < 150°C). Workflows, relevant datasets, a new Python library, and common and composite geological criteria maps are utilized to develop low-temperature geothermal resource favorability maps for the Denver Basin, a sedimentary basin spanning Colorado, Nebraska, and Wyoming. The replication of these methodologies in other SBGPTs can evaluate potential for low-temperature resources. To facilitate future assessment of low-temperature geothermal resources in SBGPTs, this project provides PFA workflows, data, tools, and favorability maps that will ultimately support the utilization of low-temperature geothermal resources in sedimentary basins.

With the increasing focus on sustainable energy solutions and national net-zero emissions goals, the repurposing of abandoned underground mines for thermal energy storage has the potential to make significant impacts. The international consortium, Galleries-2-Calories (G2C), is investigating the potential for storing waste heat from a supercomputing facility in abandoned flooded coal mines southeast of Edinburgh, Scotland. The system termed the GeoBattery or GeoTES involves injecting heated water into mine workings, conveying it using regional groundwater flow and constant groundwater temperatures, and then using the stored thermal energy for district heating and cooling via heat pumps. The Edinburgh site is unique due to the connection of three individual collieries which are linked by underground roadways and hydraulically conductive coal seams, thus enabling heat to be transported over several kilometers.
The internal structure of collieries poses challenges due to their partially unknown nature, involving fully excavated seams, partially mined pillars, and unmined rock formations. To address these uncertainties, we propose to use a multi-level stochastic modeling approach using the open-source Multiphysics Object-Oriented Simulation Environment (MOOSE). Our team has developed a preliminary numerical model that focuses on the main coal seams with simplified geometrical properties. These seams are further subdivided into multiple subdomains so that they can be discretized individually. Initially, a stochastic analysis involving many thermo-hydraulic simulations is conducted to identify promising parameter combinations that replicate field observations. Once these parameter combinations are determined, the model is extended in a second phase to incorporate the geomechanical effects associated with various operational conditions of the Geobattery. This extension facilitates the necessary evaluation of heat plume migration and stress field changes, critical for assessing the mechanical stability of the collieries during the operation of the GeoBattery.