Geothermal Resources Research Articles

Advancing Geothermal Resource Assessment Using 3D Gravity Modeling: A Case Study from Kamojang, West Java

The research was conducted using the gravity method in the Kamojang geothermal manifestation area, West Java. Data was obtained through the Topex website which contains information on latitude, longitude, elevation, and FAA value. The purpose of this research is to determine the density value of subsurface constituent rocks to determine the location of reservoirs and heat sources in the research area. The corrections made are Bouger correction and Terrain correction obtained from the CBA map which is sliced as many as 5 incisions. Calculation of subsurface average mass density using the Parasnis method obtained a density of 2.5882 mGal. Spectrum analysis was performed to determine the regional-residual depth using the moving average method. The regional anomaly contour map shows a range of 15 - 135 mGal. The residual anomaly contour map shows a range of -26 to +28 mGal with more complex results. Modeling results using Grav3D with a mesh value of 500 show a density range of 1.6752 - 3.1712 gr/cm³ with an altitude of 600 - 2000 m and a thickness of 1400 m. Zones with high density values between 2.9219 - 3.1712 gr/cm³ are indicated as heat source areas with volcanic rocks such as lava to basalt igneous rocks. Medium density values in the range of 2.4232 - 2.6722 gr/cm³ are indicated as boundary zones which are interpreted as basement in the caprock of the study area with igneous rocks surrounding the boundary zone. The low density zone with a value of 1.6752 - 2.1732 gr/cm³ is indicated as a reservoir zone with sedimentary rocks composed of pyroclastic rocks and breccias.

Mechanism of heat transfer and accumulation in sedimentary basin geothermal anomaly belts: a case study of Bohai Bay Basin, China

The mechanism of heat transfer and accumulation in sedimentary basin geothermal anomaly belts is very complicated, resulting in a lack of in-depth understanding. The Bohai Bay Basin is the largest Mesozoic–Cenozoic rifted basin in China and taken as the research object in the present work, the relationship between geothermal anomaly characteristics and influencing factors, including the structural geometry of the bedrock bulge, the bedrock thermal conductivity, and the caprock thickness was quantitatively discussed. Based on a typical measured geothermal geologic profile in the central Bohai Bay Basin, the formation mechanism, controlling factors, and the guiding significance for geothermal resources exploration of geothermal anomaly belts were analyzed. The formation of a geothermal anomaly in the sedimentary basin was driven by the rapid transfer of uniform deep heat flow in bedrock with the high thermal conductivity and accumulation of that in caprock with the low thermal conductivity of shallow bedrock bulge belts. As a result, heat flow depletion in the deep bedrock and heat flow enrichment in the shallow caprock were formed, with the characteristics of a nearly symmetrical coupling variation pattern. The bedrock bulge amplitude and the bedrock thermal conductivity were the main factors controlling geothermal anomaly characteristics, and the sufficient caprock thickness was vital to the formation of the geothermal anomaly. Simulation results of theoretical models showed that the top surface temperature of bedrock geothermal reservoirs with a depth of 2 km can increase by up to 31.3 ℃, and positive anomaly amplitudes were up to 41.8%. Based on the difference of tectonic deformation patterns in the rifted basin and their determining role in a geothermal gradient of caprock, geothermal fields developing bedrock geothermal reservoirs in the Bohai Bay Basin were divided into seven types, including the high bulge type, the bulge type, the low bulge type, the sag type, the gentle slope type, the basin margin fault-uplift type, and the shallow burial type, with a decrease in turn of the caprock geothermal gradient from > 5.0 ℃/100 m to < 2.0 ℃/100 m. The high bulge-type and the bulge-type geothermal fields were favorable targets for the shallow-burial bedrock geothermal reservoir exploration. The low bulge-type and the sag-type geothermal fields were the main targets for exploring deep-burial bedrock geothermal reservoirs with high temperatures above 150℃.

Exploration of Shallow Geothermal Resources Based on Gravity and Magnetic 3D Inversion in the Wudalianchi–Laoheishan Volcano and Surrounding Areas

Geothermal resources represent one of the most vital renewable energy sources, offering substantial development potential within the energy sector. Wudalianchi, renowned as one of China’s prominent volcanic clusters, has undergone extensive underground volcanic activities, suggesting a promising capacity for geothermal resource accumulation. This paper is the first to apply the cross-gradient gravity-magnetic joint inversion method to study the shallow structures in the Laoheishan Volcano and surrounding areas of Wudalianchi, based on high-precision measured gravity and magnetic data. The inversion results indicate the presence of a rock body at a depth of approximately 2 km beneath the Laoheishan and Bijiashan regions, which simultaneously exhibits characteristics of low density, high magnetization, and low seismic velocity. Integrating previous research findings, the rock body is interpreted as basalt formed during magmatic activity, retaining remanent magnetism. Furthermore, the rock body contains fractures filled with fluids, thereby excluding the possibility of a shallow magma chamber or dry hot rocks beneath the Laoheishan area. These rock bodies are interconnected at depth and align with the NE and SE fault directions in the Wudalianchi area, confirming that these faults govern the region’s volcanic activities. The inversion results, from the perspectives of density and magnetic susceptibility, elucidate the material distribution in the shallow subsurface of the Laoheishan and surrounding areas, providing new evidence for further exploration of geothermal resources in the region.

Geothermal Genesis Mechanism of the Yinchuan Basin Based on Thermal Parameter Inversion

The Yinchuan Basin harbors significant geothermal resource potential and could be a clean energy source critical for transitioning to a low-carbon economy. However, the current research primarily focuses on the exploration and development of geothermal water in the sedimentary basins, with limited studies on the deep geothermal formation mechanisms and regional geothermal types. Although geophysical methods provide insights into the types and formation mechanisms of deep geothermal resources in the basin, there is still a lack of a connection between quantitative understanding and direct evidence. A series of algorithms based on thermal parameter characteristics can directly extract underground thermal features from raw geophysical signal data, offering a powerful tool for characterizing the structure and aggregation patterns of deep thermal sources. Therefore, this study employed a Bayesian thermal parameter inversion method based on interface information to obtain the spatial distribution of thermal conductivity, surface heat flow, and mantle heat parameters in the Ningxia Basin study area. Additionally, correlation analysis and global sensitivity analysis were conducted to further interpret the predicted results. A comprehensive analysis of the geophysical inversion results showed that the deep thermal anomalies in the basin are primarily controlled by fault activities and the lithospheres’ thermal structure, while shallow high-heat flow anomalies are closely related to convective circulation within faults and heat transfer from deep thermal sources. The established geothermal genesis mechanism and model of the Yinchuan Basin provide crucial support for sustainable regional geothermal development planning and the utilization of deep geothermal resources, contributing to energy security and emission reduction goals.

Numerical Simulation of Proppant Embedment Depth in Inhomogeneous Formation Based on Field Variables Method

ABSTRACTHydraulic fracturing, a deep formation resource extraction method, is widely used to extract oil, gas, and geothermal resources, where fracturing fluids and proppants are usually injected to support the hydraulic fractures that transport the resources. After fracturing fluid loss, there will be the embedding process of proppant in inhomogeneous formations. To understand the mechanism of this phenomenon, the study comparatively investigated the embedding process of proppant in homogeneous formations, layered formations, and continuously varying inhomogeneous formations with finite element methods. Specifically, formation properties, in terms of the inhomogeneous formations, are defined as the nonlinear function of a constant position with the field‐variable (FV) method. The results show that equating nonhomogeneous formation to homogeneous formation underestimates the depth of proppant embedment in actual hydraulic fractures, which varies with proppant size. Next, the underestimation of proppant embedment in homogeneous formation, in turn, results in an overestimation of hydraulic fracture permeability. The FV method can more accurately characterize the proppant embedding process in inhomogeneous formations and reflect the fracture permeability after proppant embedding. Further, the advantages, limitations, and future research directions of this study are discussed.

A Precise Prediction Method for Subsurface Temperatures Based on the Rock Resistivity–Temperature Coupling Model

The accuracy of deep temperature predictions is critical to the precision of geothermal resource exploration, assessment, and the effectiveness of their development and utilization. However, the existing methods encounter significant challenges in predicting the distribution characteristics of deep temperature fields with both efficiency and accuracy. Many of these methods rely on empirical formulas to approximate the relationship between geophysical parameters and temperature. Unfortunately, such approximations often introduce substantial errors, undermining the reliability and precision of the predictions. We present an advanced prediction methodology for deep temperature fields based on the rock resistivity–temperature coupling model (RRTCM). By converting the fixed parameters in the empirical formulas to variables dependent on the formation depth, we establish a dynamic model that correlates rock resistivity with temperature on the basis of limited constrained borehole data. We then input the 2D magnetotelluric inversion results into the model, and the subsurface temperature distribution can be predicted indirectly with high precision on the basis of the resistivity–temperature coupling relationship. We validated this method in the Xiong’an New Area, China, and the determination coefficient (R2) of maximum temperature prediction reached 98.88%. The sensitivity analysis indicates that the prediction accuracy is positively correlated with the number and depth of the constrained boreholes and negatively correlated with the sampling interval of the well logging data. This method robustly supports geothermal resource development and enhances the understanding of geothermal field formation mechanisms.

Mechanical property variation mechanisms of granite subjected to real‐time high temperatures and subsequent cooling treatment

Abstract During geothermal resource exploitation, the potential deterioration of mechanical properties in high‐temperature granite subjected to cooling poses a significant safety concern. To address this, the present study investigates the coupled thermo‐mechanical behavior of granite during heating and cooling through a combination of laboratory tests and finite difference method analysis. Initial investigations involve X‐ray diffraction, thermal expansion test, thermogravimetric analysis, and uniaxial compression test. Results show the significant variations of granite properties under different thermal conditions, attributed to temperature gradients, water evaporation, and mineral phase transitions. Subsequently, a model considering temperature‐dependent parameters and real‐time cooling rates was employed to simulate linear heating and nonlinear cooling processes. Simulation results indicate that the thermal cracking predominantly occurs during the heating stage, with tensile failure as the primary mode. Additionally, a faster real‐time cooling rate at higher temperatures intensifies the thermal cracking behavior in granite. This study effectively elucidates the thermo‐mechanical coupling behavior of granite during heating and cooling processes, providing insights into the mechanisms of mechanical property changes with rising or decreasing temperatures.

Audio-frequency magnetotelluric data inversion method based on an adaptive binary structure constraint and its application in geothermal exploration

Audio-frequency magnetotelluric (AMT) inversion technology plays a crucial role in the exploration of deep resources, especially in the detection of geothermal resources. Although AMT inversion technology has significant advantages in vertical resolution, the inversion process still faces serious challenges of nonuniqueness. To improve the accuracy and reliability of inversion, this paper develops an AMT data inversion method based on an adaptive binary structure constraint. This method incorporates adaptive extraction techniques to conduct a cluster analysis of the resistivity model generated in each iteration, thereby identifying and distinguishing the target and background areas in the model. These target areas are then used as prior structural information for the next iteration, guiding the refined reconstruction of the resistivity model. During the iteration process, the AMT inversion algorithm focuses on calculating the target areas while the resistivity values of the background areas remain unchanged. This strategy effectively reduces the degrees of freedom in the inversion problem and lowers the dimension of the solution, thereby improving the computational efficiency and accuracy of the inversion. Model tests show that compared with the traditional smoothing-constrained AMT (S-AMT) inversion method, the adaptive binary structure-constrained AMT inversion method (ABS-AMT) can reconstruct high-resolution refined resistivity models, significantly reducing the uncertainty of inversion. Furthermore, the ABS-AMT method not only reduces the dependence on prior knowledge but also enhances the algorithm’s adaptability to different geologic conditions. Practical application in the geothermal area of Laosandui in Linjiang City, Jilin Province, China, fully demonstrates the method’s effectiveness in known geothermal areas.

Evaluation of thermal tourism potential of Şırnak

The aim of this study is to identify the existing geothermal resources in Şırnak Province, determine their characteristics, and assess their potential for thermal tourism. Fieldwork was conducted on two different dates during the research, and samples were taken from 10 sources at 5 different locations. The parameters of ECI, pH, and temperature were measured at the sources. The collected samples were then analyzed in accordance with the provisions of the "Hot Springs Regulation" published in the Official Gazette dated 24.07.2001 and numbered 24472, at the Diyarbakır Public Health Laboratory under the Provincial Health Directorate. The results were evaluated in terms of public health. As a result of the research conducted at 5 locations, it was determined that the Beytüşşebap-Ilıcak geothermal area (43.3°C) and the Güçlükonak-Belkıs Main Geothermal Area (62.8°C) are suitable for thermal tourism. It was also found that the other sources could be used for wellness purposes. Recommendations regarding the measures to be taken for these locations to become thermal tourism centers were also presented.

Mapping the Crustal Magmatic System and Regional Deformation of the Toba Region by Multiparameter Ambient Noise Adjoint Tomography

Abstract In this study, we develop a multiparameter ambient noise adjoint tomography method, and apply it for the first time to image the crustal magmatic system and regional deformation of the Toba region. Using Rayleigh and Love waves at periods of 5–20 s extracted from ambient noise, we construct a new multiparameter 3D crustal model that includes shear‐wave velocity, radial and azimuthal anisotropy. The isotropic component of our model reveals (a) over 30% Vs reductions beneath the Toba caldera with a melt fraction ranging from to , and (b) two low Vs bodies located in the middle crust (10–20 km) beneath the Helatoba volcano and the upper crust beneath the Lubukraya volcano, suggesting a large transcrustal magmatic mush model beneath this volcanic arc region. Our anisotropic model shows positive radial anisotropy in the middle crust of the volcanic regions, indicating the presence of horizontally layered melt sills. In the upper crust, we find predominantly weak negative radial anisotropy and significant azimuthal anisotropy, suggesting subvertical rock fabrics dominate upper crustal anisotropy. The orientation of fast velocity directions (FVDs) mostly aligns with the Sumatran Fault due to fault fabrics resulting from shearing deformation along the plate boundary. In the Tarutung region with rich geothermal resources, FVDs shift to being fault‐perpendicular probably due to the alignment of stress‐induced, fluid‐rich microcracks. Our study provides new insights into crustal magmatic architecture and deformation regimes of the Toba region shaped by regional tectonics and magmatic processes.

Ensuring Baseload Power Delivery by Complementing Two Independent Energy Sources: Solar Energy and Wellbore Closed-Loop Thermal Energy

Summary For this study, we explored solutions to revive inactive wells for power generation. Accordingly, in this article, we show a method that involves circulating an efficient heat-preserving refrigerant down the well’s annulus and up through the tubing in actual field settings. This closed-loop process generates steam or hot water at the wellhead, enabling power production independent of the usual geothermal source. Among the working fluids analyzed, R-134a and R-1234yf refrigerants demonstrated superior performance over a 20-year production period compared with pentane and water. The method combines closed-loop fluid circulation to harness thermal heat at night with solar energy generation during the day for electricity production. Together, these two energy sources provide a reliable baseload power supply, addressing the intermittent nature of solar energy while extending the productive use of geothermal resources. In this system, geothermal and solar energy are used independently but complement each other to ensure continuous power delivery. At night, thermal energy generates electricity via a closed-loop system that circulates a working fluid through the wellbore, extracting heat from the surrounding formation. During the day, solar energy supplies electricity, reducing reliance on thermal resources and enabling heat recovery for subsequent nighttime operations. The geothermal wells in the studied area contain many noncondensable gases produced in the geothermal brine, ranging from 0.5% to 3% mass content. The proposed method is noncondensable gases-free and environmentally friendly. In addition, geothermal power plants exhibit higher efficiency during the nighttime due to the enhanced performance of the cooling system under nocturnal conditions. Key metrics, including net present value (NPV) and levelized cost of energy (LCOE), support the economic viability of this method. Specifically, four field examples from two geothermal fields provide the overall value proposition. We applied the hybrid thermal/solar approach to various well depths and geothermal gradient conditions and found that it produces between 52 MWh and 138 MWh of electricity per well over a 20-year period, which is sufficient to power 226–730 homes in Türkiye.

Technical initiatives to develop low-medium temperature geothermal resources in Indonesia: Lessons learned from the United States

Technical initiatives to develop low-medium temperature geothermal resources in Indonesia: Lessons learned from the United States

Research on deep geothermal resource exploration based on the wide field electromagnetic method: a case study in the Xuwen area

Research on deep geothermal resource exploration based on the wide field electromagnetic method: a case study in the Xuwen area

The Effect of Hydrothermal Alteration and Microcracks on Hydraulic Properties and Poroelastic Deformation: A Case Study of the Blue Mountain Geothermal Field

Abstract Geothermal energy plays a vital role in decarbonizing electricity and heat supply. Effective utilization of geothermal resources hinges on identifying or generating permeable reservoir zones and understanding how effective pressure variations affect fluid circulation and reservoir properties by poroelastic deformation. Hydrothermal alteration can modify the petrophysical properties of geothermal reservoir rocks, which may increase or decrease its productivity. Understanding these alteration effects is essential to predict and optimize long‐term sustainable geothermal operations. Here, we investigate the impact of hydrothermal alteration on poroelastic and hydraulic properties of diverse lithologies in a series of deformation tests performed at several confining (0–80 MPa) and pore pressure (10–30 MPa) levels. Experimental results of hydrothermally altered dikes and phyllites obtained from the Blue Mountain geothermal field (Nevada, USA) are compared to thermally cracked La Peyratte granite (France) and correlated with petrophysical properties, mineral composition, and microstructures. Argillic alteration of dikes increases porosity and storage capacity but lowers thermal conductivity and increases pore compressibility. Conversely, silicate precipitation in phyllites increases stiffness and thermal conductivity but also reduces porosity and permeability. Experimentally determined effective pressure coefficients range from 0.1 to 0.9, differ for permeability and volumetric strain and decrease with increasing effective pressure. The presence of compliant microcracks and crack‐like pores significantly increases the stress sensitivity of La Peyratte granite and silicified phyllites. This study demonstrates how thermal and chemical alteration impacts poromechanical and petrophysical characteristics of geothermal targets, which ultimately govern reservoir stability and subsidence, induced seismicity as well as fluid and heat extraction efficiency during geothermal operations.

Modeling of Mineral Efficiency in Geothermal Hot Waters with Data Envelopment Analysis

The Biga Peninsula is an important region of geothermal resources, heat mining in Western Anatolia. In this study, the modelling of mineral efficiency in hot waters was made with data envelopment analysis for the first time. Gold, silver, and lithium in the geothermal hot water were defined as the outputs, whereas physical properties of the geothermal resource such as temperature, pH level, electrical conductivity, and salinity were defined as the inputs. The output-oriented Charnes, Cooper, and Rhodes data envelopment analysis model, which measures the total efficiency, and the output-oriented Banker, Charnes, and Cooper data envelopment analysis model, which measures technical efficiency, were used in the study. A total of 50 models were created –25 with the first model and 25 with the second model – to analyse 21 geothermal resources in the Biga Peninsula. As a result of the analysis of the models, nine geothermal resources were found to have a relative efficiency of 100%. The average technical efficiency score in the Banker, Charnes, and Cooper model was 70%, whereas the average total efficiency score in the Charnes, Cooper, and Rhodes model was 68.5%. It was found that data envelopment analysis can be used to model geothermal resources in mineral operations.

High-Quality Heat Flow Determination from Coastal Areas of Fujian Province, China

The coastal region of Fujian is part of the southeastern hilly and coastal geothermal anomaly zone and is a significant high-heat-flow area in mainland China. However, existing heat flow data suffer from low quality due to insufficient calculation depth and inadequate corrections; this hinders theoretical research and geothermal resource exploration in the region. This study obtained high-quality geothermal heat flow data through steady-state temperature measurements in six boreholes and thermal conductivity tests on 79 core samples collected in situ. The results indicate that the average heat flow values from four newly analyzed representative boreholes (HDR1, LJSZ, JM-1, DS) are 65.2 mW/m2, exceeding the mainland Chinese average of 60.5 mW/m2, highlighting the region’s high heat flow characteristics. This finding corroborates previous high-heat-flow measurements in the coastal area of Fujian and aligns with earlier studies. The research fills a gap in providing high-quality heat flow data for the region, enhances our understanding of its thermal state, and is significant for studying the distribution and development of geothermal resources in the area.

Research Progress and Technical Challenges of Geothermal Energy Development from Hot Dry Rock: A Review

The reserves of hot dry rock (HDR) geothermal resources are huge. The main method used to develop HDR geothermal resources is called an enhanced geothermal system (EGS), and this generally uses hydraulic fracturing. After nearly 50 years of research and development, more and more countries have joined the ranks engaged in the exploration and development of HDR in the world. This paper summarizes the base technologies, key technologies, and game-changing technologies used to promote the commercialization of HDR geothermal resources. According to the present situation of the exploration, development, and utilization of HDR at home and abroad, the evaluation and site selection, efficient and low-cost drilling, and geothermal utilization of HDR geothermal resources are defined as the base technologies. Key technologies include the high-resolution exploration and characterization of HDR, efficient and complex fracture network reservoir creation, effective microseismic control, fracture network connectivity, and reservoir characterization. Game-changing technologies include downhole liquid explosion fracture creation, downhole in-situ efficient heat transfer and power generation, and the use of CO2 and other working fluids for high-efficient power generation. Most of the base technologies already have industrial applications, but future efforts must focus on reducing costs. The majority of key technologies are still in the site demonstration and validation phase and have not yet been applied on an industrial scale. However, breakthroughs in cost reduction and application effectiveness are urgently needed for these key technologies. Game-changing technologies remain at the laboratory research stage, but any breakthroughs in this area could significantly advance the efficient development of HDR geothermal resources. In addition, we conducted a comparative analysis of the respective advantages of China and the United States in some key technologies of HDR development. On this basis, we summarized the key challenges identified throughout the discussion and highlighted the most pressing research priorities. We hope these technologies can guide new breakthroughs in HDR geothermal development in China and other countries, helping to establish a batch of HDR exploitation demonstration areas. In addition, we look forward to fostering collaboration between China and the United States through technical comparisons, jointly promoting the commercial development of HDR geothermal resources.

Scale Treatment Planning Using Broaching Method in a Vapor-Dominated Geothermal Well X at Kamojang Geothermal Field

Scaling in geothermal production wells poses a critical challenge to sustainable energy production, particularly in vapor-dominated systems where scaling mechanisms are less understood. This study investigates scale treatment planning using the broaching method in Well X at Indonesia’s Kamojang geothermal field. Through well integrity testing, geochemical analysis, and XRD characterization, silica (quartz) scale formations were identified in the production casing. Performance monitoring revealed gradual decreases in steam production and wellhead pressure over a three-year period. The selection of the broaching method was validated through analysis of scale characteristics, well geometry, and economic feasibility, offering a significantly more cost-effective solution compared to conventional methods with a substantially shorter payback period. Broaching has effectively operated on multiple geothermal wells, restoring significant production capacity at approximately half the expense of conventional well workover methods. Our results challenge accepted assumptions on scaling in vapor-dominated systems and provide a methodical framework for scale treatment planning. This study demonstrates how strategic scale management can efficiently preserve well productivity while lowering operating costs, thus enabling sustainable geothermal resource development for operators worldwide.

Enhancing Geothermal Anomaly Detection with Multi-Source Thermal Infrared Data: A Case of the Yangbajing–Yangyi Basin, Tibet

Geothermal resources are crucial for sustainable energy development, yet accurately detecting geothermal anomalies in complex terrains remains a significant challenge. This study develops a multi-source thermal infrared approach to enhance geothermal anomaly detection using Landsat 8 and ASTER land surface temperature (LST) data. The Yangbajing–Yangyi Basin in Tibet, characterized by high altitude and rugged topography, serves as the study area. Landsat 8 winter time-series data from 2013 to 2023 were processed on the Google Earth Engine (GEE) platform to generate multi-year average LST images. After water body removal and altitude correction, a local block thresholding method was applied to extract daytime geothermal anomalies. For nighttime data, ASTER LST products were analyzed using global, local block, elevation zoning, and fault buffer strategies to extract anomalies, which were then fused using Dempster–Shafer (D–S) evidence theory. A joint daytime–nighttime analysis identified stable geothermal anomaly regions, with results closely aligning with known geothermal fields and borehole distributions while predicting new potential anomaly zones. Additionally, a 21-year time-series analysis of MODIS nighttime LST data identified four significant thermal anomaly areas, interpreted as potential magma chambers, whose spatial distributions align with the identified anomalies. This multi-source approach highlights the potential of integrating thermal infrared data for geothermal anomaly detection, providing valuable insights for exploration in geologically complex regions.

Effect of Working Fluid on Characteristics of Organic Rankine Cycle with Medium Temperature Geothermal Water

The total installed geothermal power plant capacity at year-end 2023 was 16,335 MW, while the forecast for the installed capacity in 2025 is 19,331 MW. In Croatia, several medium-temperature geothermal resources (geothermal water) with temperatures from 90 to 200 °C exist, by means of which it is possible to produce electricity in binary plants, with the Organic Rankine Cycle (ORC) or with the Kalina cycle. In earlier studies, the authors presented the results of an energy-exergy analysis of geothermal sources at Velika Ciglena (170 °C), Lunjkovec-Kutnjak (140 °C), Babina Greda (125 °C), and Rečica (120 °C), aiming to determine which binary plant is more suitable for the environmental conditions in Croatia. The calculations indicate that the plant with ORC is thermodynamically superior to the one with the Kalina cycle for all geothermal sources. Taking into account the typical challenges faced by new technologies during their initial implementation, the authors recommend using the ORC plant for all medium-temperature geothermal sources. Literature on ORC applications mainly addresses working fluid selection, unit and plant optimization, and modifications to enhance thermodynamic efficiency or net power output. While many studies on working fluid selection exist, each geothermal source is unique due to its specific temperature and local cooling fluid (water or air). As a result, this paper presents the findings of an analysis on the influence of working fluids on the thermodynamic performance of an ORC system, focusing on the Lunjkovec-Kutnjak Geothermal Power Plant with a geothermal water temperature of 140 °C. As the working fluid, the next are analyzed: isopentane (C5H12), isobutene (C4H10), isohexane (C6H14), R114 (C2Cl2F4), R141B (C2H3Cl2F), and R142B (C2H3Cl2F2). In respect to cycle efficiency and net power, all working fluids are equally favorable, but R601a (isopentane) with low ALT, ODP, and GWP, favorable upper and lower pressure, is the most suitable fluid for ORC with a medium-temperature geothermal source.