Session 1E: Closed Loop/Advanced Geothermal Systems

Flexible geothermal operations could boost project returns through the allocation of improved power purchase agreements and/or exploitation of power price arbitrage opportunities. In this study, we investigated the techno-economic feasibility of variable flow rate control and time-of-day pricing in closed-loop geothermal systems. We considered U-shaped multilateral system configurations and modeled a variety of technical system parameters. These designs were simulated using a slender-body theory (SBT) model for transient heat transfer and fluid flow. This subsurface model was integrated into the flexible geothermal economic model (FGEM) tool to evaluate the overall flexible geothermal system techno-economics. Future hourly ambient temperature conditions were based on the Sup3rCC dataset. Published datasets were used for future hourly wholesale electricity prices. We analyzed four operating strategies: 1) baseload operation, 2) seasonal dispatch (high flow rate during summer and nominal flow rate during the rest of the year), 3) net generation maximization by varying flow rate to maximize net power output, and 4) revenue maximization by varying flow rate to maximize revenue. We ran all four scenarios for a multiloop configuration with 12 lateral passes, 7-km vertical depth and 87-km total drilling length. Furthermore, we assumed a 60℃/km geothermal gradient and ambient temperature and wholesale electricity prices for New Mexico as a typical state location. The nominal flow rate was set to 80 kg/s. When considering drilling costs of $1,000/m and a discount rate of 7%, the generation maximization scenario resulted in the lowest levelized cost of electricity (LCOE) of ~$150/MWh. When considering project return on investment (ROI), defined as lifetime net income divided by upfront capital costs, all flexible operation scenarios performed better than the base case scenario. The highest ROI of 80% was obtained with the revenue maximization scenario. With drilling costs of $200/m and a discount rate of 5%, the generation maximization scenario resulted in LCOE of $49/MWh.

Eavor Technologies Inc. (Eavor) is currently drilling a complex, multi-lateral closed-loop geothermal system (an “Eavor-Loop (TM)”) in the Geretsried area, Bavarian Molasse Basin, southern Germany. A total of four Eavor-Loops are planned for the location, and spud for Loop 1 was in July 2023. The target for the Eavor-Loop development is the Jurassic Malm carbonate formation.

Geomechanics, including in-situ stress estimation, is important in Eavor-Loop designs to evaluate wellbore stability, mud weights, operating pressures, and to optimize well orientations. GEN-1 and GEN-1ST-A1 are nearby offset wells and were used to constrain the geomechanical model pre-drill. However, within the academic community, there is considerable uncertainty over the stress regime in the Bavarian Molasse Basin. Normal faulting, strike slip and reverse faulting regimes have all been predicted in relatively close proximity.

As part of the completion of the first loop, a series of Formation Integrity Tests (FITs) were completed by Eavor. These data points provide new insights into the possible stress regime. This paper will present the pre-drill and current estimates for the stress regime for the Geretsried area, and the planned future data collection. The understanding of the stress regime will impact the chosen mud weights while drilling, and the optimal drilling direction for future loops, to reduce the likelihood of wellbore instability.

Vacuum insulated Tubulars (VITs) can be employed as tubing for geothermal wells, particularly for Advanced Geothermal Systems (AGS), where the VIT greatly reduces heat transfer between the cold fluid typically being injected through the annulus and hot fluid being produced through the VIT. The vacuum-insulated design ensures minimal heat loss from the geothermal fluid as it travels from the reservoir to the surface, maximizing energy extraction efficiency.

Accurately determining the K-values of vacuum insulated tubulars involves numerous uncertainties stemming from both theoretical considerations and practical challenges. One significant uncertainty arises from the complex heat transfer mechanisms within the tubulars, influenced by factors such as gas composition within the vacuum space, material properties, and construction quality. Additionally, variations in manufacturing processes and assembly techniques can introduce inconsistencies in insulation performance across different tubulars. Moreover, the dynamic nature of operating conditions in real-world geothermal systems, including temperature fluctuations and mechanical stresses, further complicates accurate K-value estimation. Furthermore, limitations in measurement techniques and equipment sensitivity contribute to uncertainties in experimental assessments. Addressing these uncertainties requires comprehensive modeling approaches, rigorous experimental validation, and ongoing refinement of testing methodologies to ensure reliable determination of K-values and ultimately optimize the efficiency and performance of closed-loop geothermal systems.

This paper seeks to outline laboratory research conducted to analyze the k-values of two joints of a vacuum insulated tubular of a geothermal setup in a lab environment. Both cold and hot fluid will be flowed through the VIT system in a coaxial design in order to mimic an AGS closed loop system and temperatures will be measured throughout the entire system that will then be used to model and determine the K-value. This VIT test bench that will be deployed for this testing has two main advantages: 1) k-values can be measured at not only ambient conditions but at multiple temperatures and flowrates, allowing for much greater accuracy of the entire system and 2) the test bench allows us to assess performance of new products (connections, insulators, etc) in simulated working conditions during the design phase to more accurately predict thermal performance and overall well efficiencies. Optimizing the VIT design allows customers to have the best performing geothermal system possible; whether to maximize production temperatures, extracting the most heat/power possible or to optimize LCOE and installation costs for a given performance level.

Over the past few years, advancements in closed-loop geothermal systems (CLGS), also called advanced geothermal systems (AGS), have sparked a renewed interest in these types of designs. CLGS have certain advantages over traditional and enhanced geothermal systems (EGS), including not requiring in-situ reservoir permeability, conservation of the circulating fluid and allowing for different fluids, including working fluids directly driving a turbine at the surface. CLGS may be attractive in environments where water resources are limited, rock contaminants must be avoided, and stimulation treatments are not available (e.g., due to regulatory or technical reasons). Despite these advantages, CLGS have some challenges, including limited surface area for heat transfer and requiring long wellbores and laterals to obtain multi-MW output in conduction-only reservoirs.

CLGS have been investigated in conduction-only systems. In this paper, we explore the impact of both forced and natural convection on the levels of heat extraction with a CLGS deployed in a hot wet rock reservoir. We bound potential benefits of convection by investigating liquid reservoirs over a range of natural and forced convective coefficients. Additionally, we investigate the effects of permeability, porosity, and geothermal temperature gradient in the reservoir on CLGS outputs. Reservoir simulations indicate that reservoir permeabilities of at least ~100 mD are required for natural convection to increase the heat output with respect to a conduction-only scenario. The impact increases with increasing reservoir temperature. When subject to a forced convection flow field, Darcy velocities of at least 10-7 m/s are required to obtain an increase in heat output.

*Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy National Nuclear Security Administration under contract DE-NA0003525. SAND2024-02210A

As the demand for sustainable energy sources intensifies, geothermal energy stands out as a promising solution. Efficient utilization of geothermal reservoirs relies on optimal borehole placement and density, a complex optimization problem. This paper introduces a novel algorithm leveraging simulated annealing to address the challenges associated with optimizing both the total number of boreholes and their spatial distribution in geothermal applications.

The proposed algorithm employs simulated annealing, inspired by the annealing process in metallurgy, to iteratively explore and refine the solution space. By systematically adjusting the temperature parameter, the algorithm balances exploration and exploitation, allowing it to escape local minima and converge towards a globally optimal solution. The objective is to minimize the total number of boreholes while maximizing their effectiveness in harnessing geothermal energy.

The algorithm's performance is evaluated through simulations on various geothermal reservoir scenarios. Results demonstrate its ability to adapt to diverse geological conditions and provide solutions that optimize the utilization of geothermal resources. This paper concludes with a discussion of the algorithm's potential applications, scalability, and contributions to advancing sustainable geothermal energy solutions.

While long (e.g., tens of kilometers) closed-loop geothermal wells are under consideration for the extraction of subsurface heat, long wells might also serve as an efficient energy storage mechanism for electricity generation. Using a semi-analytic solution, we evaluate the potential for electric-grade heat storage as a function of ambient temperature (e.g., corresponds to depth of loop), well length, well diameter, and flow rate. We consider the following simplified cases for comparison of standard closed loop operation with energy storage operation: [1] constant flow rate and constant temperature (90 °C) injection to represent the standard closed-loop base-case; and [2] constant flow rate and annual cycle sinusoidal temperature (90-150 °C) to represent seasonal (summer) charging while solar resources are peak. Initial temperatures for all scenarios considered herein are a uniform 175 °C. Electric-grade heat is assumed to be delivered whenever temperatures at the extraction point exceed 90 °C, and for calculation purposes only, we assume that if temperature falls below 100 °C, that heat delivered is sub-economic, so no electricity would be produced. For all scenarios, temperatures at the extraction well asymptotically approach the flow-weighted average injection temperature, but energy storage scenarios exhibited a damped time-varying signal that diminishes in magnitude with length of the loop. The asymptotic approach depends on initial temperatures in the rock and the heat extraction rate (a function of well diameter and flowrate). We demonstrate that shorter closed loops can produce more electricity over time than longer closed-loops previously proposed for electricity production over typical engineering design lifetimes (e.g., 30 years). While only a high-temperature scenario is considered herein, rock that is initially below boiling temperature would not host a standard closed-loop resource, but injection of hot water seasonally would asymptotically heat this low-temperature system to temperatures capable of electricity production. In other words, regardless of initial temperatures, closed loops could be used to store electricity with no critical minerals in the geothermal battery.