Accurate modeling of the deep crustal temperature field is critical for geothermal resource assessment and understanding lithospheric thermal evolution. Traditional approaches relying on interpolation-based terrestrial heat flow (THF) map and average radiogenic heat production often fail to capture the geological heterogeneity of the crust, particularly in data-sparse regions. In this study, we present a refined framework that integrates a machine learning-predicted THF map with vertically stratified radiogenic heat production data, applied within a one-dimensional steady-state heat conduction model. This approach allows us to compute deep temperature profiles across mainland China at multiple depths (3-10km) with improved resolution and geological consistency. The results align well with borehole observations and reveal extensive high-temperature zones exceeding 150°C in regions such as the Tibet-Sanjiang Orogen, North China Craton, and Yangtze Craton. Moreover, based on modeled temperatures at 3km and 10km, we derived a geothermal gradient map and identified eight favorable geothermal belts characterized by high vertical gradients and deep temperatures suitable for enhanced geothermal system (EGS) deployment. This study offers a scalable framework for geothermal prospecting in data-sparse regions and provides strategic insights for future deep energy exploration.

Stochastic Fracture Generation and Thermo-Hydro-Mechanical Modeling in an Equivalent Continuum Framework for Enhanced Geothermal Systems

The formation and exploitation of geothermal reservoirs in hot dry rock (HDR) primarily rely on microseismic methods, but seismic techniques lack sufficient sensitivity to fluids. The electromagnetic method, however, demonstrates sensitivity to fluid movements during the monitoring of fracturing processes that form geothermal reservoirs in HDR. This study examines the role of electromagnetic methods in HDR development, taking China’s first Enhanced Geothermal System (EGS) demonstration site in the Qinghai Gonghe Basin as a case study. Based on the Gonghe HDR development site, a frequency-domain 3D borehole-to-surface electromagnetic forward modeling method with unstructured-grid discretization was employed to simulate the complex electromagnetic field responses induced by fracturing fluid injection and dynamic changes in fractures during HDR reservoir development. To enhance computational efficiency, a supercomputer was employed to perform 3D borehole-to-surface electromagnetic data inversion under conditions of massive multi-source and multi-frequency data. This quantitatively revealed the electrical characteristics at different depth intervals within the study area. The research demonstrates the feasibility of borehole-to-surface electromagnetic methods for determining the spatial distribution of fracturing injection, dynamically monitoring fracture development, and tracking fluid migration, thereby providing crucial technical support for monitoring HDR resources development.

Enhanced geothermal systems (EGS) rely on hydraulic fracturingto create permeable pathways in hot dry rock formations, yet the influenceof fracturing fluid rheology on proppant transport and fracture geometryremains poorly constrained under field conditions. This study presentsa field comparative analysis of cross-linked gel versus slickwaterfracturing fluids in deep granitic rock at the Utah FORGE site. Twoconsecutive stages in well 16A(78)-32 Stage 8 (cross-linked gel, ∼200cP) and Stage 9 (slickwater, ∼5 cP) were pumped at equivalenthigh rates (∼80 bpm), with fluid rheology as the primary designdifference between stages. Integrated diagnostics including real-timetreating pressure analysis, poststimulation production logging (PLT),and distributed fiber-optic strain sensing (DSS) on an offset wellwere employed. Results show that the cross-linked gel produced a fracturespan, defined here as the cross-well fiber-optic response depth range(i.e., the interwell disturbance depth range inferred from fiber-opticresponse), of 575 ft compared to 145 ft for slickwater, suggestingapproximately 4-fold greater vertical extent of fracture-driven interactions.The slickwater treatment exhibited pronounced near-wellbore pressureescalation indicative of screen-out risk, coupled with nonuniformcluster injection efficiency (Gini coefficient = 0.27) including completeexclusion of one perforation cluster. Fiber-optic strain profilesrevealed distributed, multipeak fracture signatures for the gel treatmentversus concentrated near-wellbore responses for slickwater. Whileconfounding factors including sequential stress shadowing effectscannot be fully excluded, the observed performance differences areconsistent with the expected role of fluid rheology in governing proppanttransport and fracture geometry. These findings provide field evidencethat high-viscosity fracturing fluids may offer advantages for proppanttransport and fracture network development in EGS applications incrystalline basement formations.

Enhanced geothermal systems (EGS) is an environmentally friendly technology to effectively extract geothermal energy from hot dry rock (HDR). The fracture geometry of EGS is crucial for the utilization of geothermal reservoirs. This paper proposed a multiple stage fracturing wells (MSFW) EGS and evaluated its geothermal energy extraction performance by three-dimensional thermal-hydraulic-mechanic (THM) coupled numerical modeling. The MSFW EGS increases heat exchange area, and thus enhances heat extraction. The temperature and flow fields in MSFW EGS are compared with the multiple fracturing horizontal wells (MFHW) EGS, showing a higher temperature and power outputs. The effect of fracturing parameters (including fracture spacing, number of fractures, fracture width, fracture permeability and distribution) on heat extraction was investigated. The MSFW significantly improves the efficiency of EGS thermal extraction, thus opening up new perspectives for geothermal energy utilization.

Enhanced geothermal systems (EGS) are a promising technology for exploiting deep hot dry rock resources. In this study, a numerical model of a dual-horizontal-well EGS incorporating full-wellbore heat transfer was developed to investigate thermal evolution, wellbore heat loss, and the effects of key parameters on heat extraction performance. Orthogonal experimental design combined with a normalized weighted scoring method was further applied for multiparameter optimization. The results show that during a 50-year operation period, the reservoir cooling rate decreases from 0.47 to 0.39 °C/year, indicating a transition from rapid thermal depletion to a slower heat decay stage. Parameter analysis reveals clear trade-offs among thermal power, outlet temperature, and long-term thermal sustainability. Increasing the flow rate to 0.035 m3/s improves thermal power by 56.7% but accelerates reservoir cooling, whereas reducing the injection temperature to 20 °C increases thermal power by 26.5%. Increasing well spacing effectively delays thermal breakthrough, while a higher geothermal gradient further enhances heat extraction performance. Range analysis indicates that flow rate is the most sensitive factor affecting cumulative heat production, whereas well spacing plays the dominant role in outlet temperature stability. The optimal balanced parameter combination is a flow rate of 0.030 m3/s, an injection temperature of 20 °C, a well spacing of 300 m, and three fractures. Under this scheme, the cumulative heat production reaches 22.57 PJ and the final outlet temperature is 113.42 °C after 50 years of operation.

The fracture characteristics and dynamic response to hydraulic fracturing in the Gonghe enhanced geothermal system (EGS) field, Qinghai Province, Northwest China

Enhanced Geothermal Systems (EGSs), developed to extract heat from hot dry rock (HDR) formations, are promising renewable heat resources; however, their long-term performance is governed by coupled thermo-hydro-mechanical (THM) processes. This study introduces a novel L-shaped injection-production wellbore configuration designed to enhance reservoir contact while reducing drilling complexity compared to conventional vertical, multilateral, and closed-loop systems. A fully coupled three-dimensional THM numerical model is developed to evaluate its performance, accounting for conductive-convective heat transfer, pore-pressure evolution, and thermally induced deformation. A sensitivity analysis investigates the effects of horizontal wellbore length, injection pressure, injection temperature, rock thermal expansion coefficient, and fluid compressibility. Extending the horizontal section increases the stable production period from 2.6 to 6years and doubles reservoir lifetime from 6.3 to 12.4years compared to a quintuplet vertical system, while delivering more than twice the thermal power output. Reducing injection pressure from 10 to 5MPa suppresses cold-front migration and extends sustainability beyond 20years. Increasing injection temperature from 323.15 to 343.15K raises lifetime from 11 to 12.4years but reduces power by 20%. The results demonstrate that the proposed configuration provides a balanced solution between thermal efficiency, mechanical response, and operational simplicity for HDR development.

Enhanced geothermal systems (EGS) are a key technology for developing deep geothermal resources, yet they face significant challenges in constructing efficient thermal reservoirs within high-stress, high-strength, and low-permeability crystalline rock formations. Traditional hydraulic fracturing (HF) techniques encounter deep challenges in these environments, including excessively high fracturing pressures, limited fracture network patterns, and the risk of induced seismicity. This paper reviews the multi-scale thermal-mechanical mechanisms, fracture evolution patterns, and control strategies associated with thermal stimulation and permeability enhancement in the modification of deep geothermal reservoirs. Research indicates that thermally induced fracturing triggers intergranular and transgranular cracks at the microscopic scale due to mineral thermal expansion mismatches, which macroscopically manifests as nonlinear degradation of rock strength and modulus. The redistribution of the thermal elastic stress field significantly lowers the breakdown pressure, while matrix thermal contraction increases fracture aperture, leading to an exponential enhancement of permeability following a cubic law. However, the high confining pressure constraints, true triaxial stress anisotropy, and thermal short-circuiting risks present substantial suppression and challenges to the effectiveness of thermal stimulation in deep in situ environments. Different fracturing media, such as water, liquid nitrogen (LN2), and supercritical CO2, exhibit varying advantages in thermal stimulation efficiency due to their unique thermal-flow characteristics. Future research should focus on the thermal-mechanical coupling mechanisms under true triaxial stress conditions, and develop intelligent control strategies for permeability enhancement and thermal short-circuiting risk mitigation. This study synthesizes existing analyses and proposes potential engineering strategies for stimulating deep EGS reservoirs, offering significant strategic value for the development of geothermal energy as a baseload renewable resource.

Thermo-hydro-mechanical-chemical controls on fracture and fault responses to fluid injection in Enhanced Geothermal Systems: Current understanding and future directions

A critical review of the techno-economic feasibility and sustainability of enhanced geothermal systems (EGS): Global insights and strategic pathways

CT-based study on microscopic damage mechanisms of high-temperature granite under different cooling methods

Quantifying Genesis of the Matouying Enhanced Geothermal System Anomaly

Full moment tensor (MT) inversion of induced microseismic events provides insight into stimulation processes during Enhanced Geothermal System (EGS) development. This study resolves the mechanisms of >180 microseismic events (local magnitudes 0.0-1.9) induced in two fracture zones during the 2024 stimulations at the Utah FORGE EGS test site. The remarkably homogeneous strike-slip mechanisms are generally consistent with the local stress field, but small rotations are observed between fracture zones of different stimulation stages. A significant proportion of events exhibit positive isotropic components, indicating simultaneous tensile opening. Within each fracture zone, the maximum isotropic component increases with injected volume. Interestingly, tensile opening associated with microseismic events is more prominent in fault reactivation than in zones where a hydraulic macrofracture is dominant. Non-double-couple MT components, particularly positive isotropic components, prove to be a powerful tool for characterizing reservoir development and differentiating between complex fracture networks and hydraulic macrofractures. These MT components may serve as an indicator of fault reactivation and a proxy for increased conductivity, highlighting the potential for improved reservoir characterization and management.

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 228078, “Enhanced Geothermal System Propped Stimulation Greater Than 300°C: From Design to Implementation Phase I,” by Gabrijel Grubac, SPE, Mazama Energy; Geoffrey Gullickson, SPE, Halliburton; and Alain Bonneville, Mazama Energy, et al. The paper has not been peer-reviewed. _ The complete paper provides an account of the design, implementation, and operational insights from a first-of-its-kind enhanced geothermal system (EGS) proppant stimulation targeting a dry rock setting with a bottomhole temperature of approximately 330°C. It highlights the unique challenges of creating a high-temperature EGS and the strategies employed to mitigate them, with the goal of advancing the scalability and viability of geothermal energy as a sustainable resource. Newberry Volcano Geological and Geophysical Evidence for High-Temperature EGS Development. More than 40 years of geological and geophysical investigations of the Newberry Volcano, a large active volcanic system in central Oregon, have established the presence of a substantial conductive thermal anomaly, with temperatures exceeding 320°C at 3,000-m depth and projections of over 400°C below 4,000 m. Importantly, this high-temperature resource occurs at relatively shallow depths and in a relatively well-characterized volcanic setting. Subsurface characterization at the site incorporates lithologic data from well logs and cuttings, which reveal variable rock properties relevant to EGS development. Several deep exploration wells have been drilled on the northwestern flank of the volcano. Davenport Newberry Holdings’ Well NWG 55-29 displays a conductive thermal regime from an approximately 1,700-m elevation to total depth at approximately 1,300 m. Bottomhole temperatures appear to reach 350°C. CalEnergy wells encountered temperatures greater than 315°C below 2,740 m, although insufficient permeability prevented classification as a conventional hydrothermal resource. A 3D conceptual geologic model was originally developed during Phase 1 of the US Department of Energy’s Frontier Observatory for Research in Geothermal Energy initiative. In 2022, new gravity and wideband magnetotelluric data collection focused on undercharacterized southern regions of the caldera and south flank. The resulting data set was merged with legacy data in a joint inversion to update subsurface resistivity and density models. Well 55-29. NWG 55-29 was drilled in an area of the Newberry Volcano west flank away from any surface expressions of the caldera ring fractures mapped elsewhere. The openhole interval of the well extends between 1,903 to 3,066 m total measured depth. The well first intersects microcrystalline granodiorite at a measured depth of 2,627 m. The completion approach of the well included a comprehensive design phase wherein each facet of the stimulation approach was analyzed and checked. The stimulation of this EGS well has been completed successfully, and a twin producer well has been drilled and recently stimulated with the goal of connecting the wells and enabling heat harvesting. From the application of propped stimulation greater than 300°C to running the world’s first sleeve and successfully pumping through it in the producer-well stimulation, the findings and results are an encouraging step to making superhot-rock (SHR) geothermal a scalable reality.

ABSTRACT Recent advances in enhanced geothermal system (EGS) development have opened new frontiers for geothermal energy production. However, static geophysical imaging and time-lapse monitoring of these systems can be challenging, with recent pilots located in areas with attenuating near-surface sediments and generally low-quality surface seismic data. These same problems are relevant to achieving accurate subsurface characterization, which is essential for optimizing drilling and reservoir development and enhancing the economic viability of EGS by ensuring sustainable energy extraction. The applicability of source-independent converted wave imaging was demonstrated using microseismic energy to passively image key geologic structures at an active EGS pilot site. To test this imaging concept, a dense linear nodal (3C) data set was acquired at the Frontier Observatory for Research in Geothermal Energy (FORGE) facility located in Milford, UT. This acquisition campaign, FORGE Observation Array Linear (FOAL) 1, was conducted during the April 2022 stimulation of a deep EGS injection well (FORGE well 16A). Despite a short observation period (approximately 1 month) and a limited number of seismic events, the granite–alluvium interface — the major velocity contrast at the FORGE site — was successfully imaged. The results suggest the potential of our approach for site characterization and real-time monitoring of subsurface changes. Moreover, this illuminates the prospects of using related large-N technologies, including distributed acoustic sensing for source-independent imaging.

Abstract Mapping fracture networks in Enhanced Geothermal Systems (EGS) is essential for optimizing reservoir performance, yet complex fracture evolution during stimulation remains difficult to resolve. This study examines the evolution of microseismicity and fracture networks during stage 3 of the 2022 EGS stimulation at the Utah Frontier Observatory for Research in Geothermal Energy site. We map the fracture network represented by 20 clusters of seismic events identified by waveform similarities with cross‐correlation. We characterize their geometric properties such as strike, dip, length, and width, and analyze the time evolution of activated fractures. The results reveal a systematic fracture evolution: early activation of pre‐existing natural fractures, complex network development during peak injection, and continued activation of less favorably oriented fractures post‐injection. Magnitude calibration using the Principal Component Analysis of cross‐correlated waveforms improves relative amplitude measurements, refining estimations of the Gutenberg‐Richter b‐values with spatial variations in b‐values suggesting stress re‐distribution across the stimulated area. Analysis of the stress state of selected fractures further shows that fractures requiring higher excess pore pressure primarily activate at the end of injection and post‐injection, highlighting stress transfer due to pore pressure as a dominant triggering mechanism. These findings provide insights into fracture propagation, stress evolution, and seismic hazard assessment in EGS reservoirs.

In enhanced geothermal systems (EGS), in situ stress substantially influences reservoir rock properties. We employed nuclear magnetic resonance (NMR) to examine pore-structure evolution, damage, and permeability in red sandstone exposed to 200 °C, cyclic loading at 0.3 times peak stress, and subsequent water cooling. We refined the damage-coefficient calculation using MRI bitmap data, enabling visualization and quantitative analysis of the two-dimensional spatial distribution of damaged microns. Results demonstrated that prestress reduced the threshold cycles for porosity and damage changes (15 for Group W, 10 for Group WF) and increased their rates. This effect altered the medium-pore distribution: in Group WF, 50–200 nm pores decreased while other pores expanded; in Group W, only pores >2000 nm increased. The pore fractal dimension increased before 15 cycles—reflecting greater complexity—and declined thereafter, indicating reorganization and simplification. Prestress accelerated homogeneous pore damage and promoted clustered microcrack growth; beyond 15 cycles, it fostered pore redistribution and uniform microcrack clustering. Overall permeability—including macropores—increased by over 65%. Ten cycles marked the permeability transition threshold for micropores and mesopores, with macropores contributing more substantially. Additionally, determining the damage coefficient via MRI bitmap data circumvented over-limit calculations based on the T 2 spectrum peak-area weighting. This study elucidates macro-micro damage mechanisms under thermo-hydro-mechanical coupling in EGS reservoirs and provides a theoretical basis for reservoir optimization.

During reservoir stimulation and long-term operation of Enhanced Geothermal Systems (EGSs), repeated injection of cold fluids induces cyclic thermal shock in the surrounding rock mass, leading to progressive modification of mechanical properties and fracture behavior. However, the combined effects of cyclic thermal shock and true-triaxial stress conditions on granite strength and failure characteristics remain inadequately quantified. In this study, a series of true-triaxial compression tests were conducted on granite specimens subjected to cyclic thermal shock at 400 °C. Thermal shock cycles of 0, 1, 5, 10, and 15 were considered in conjunction with intermediate principal stress levels of 5, 20, 30, and 50 MPa to systematically evaluate their coupled influence on characteristic stresses and macroscopic failure behavior. The results show that the peak intensity increases with the rise of the intermediate principal stress, but with the increase in the number of thermal shocks, it first increases and then decreases. Macroscopic failure is dominated by asymmetric V-shaped fracture surfaces, roughly oriented along the σ2 direction. As the intermediate principal stress increases, the failure mode transitions from tensile–shear mixed failure to shear-dominated failure, whereas thermal cycling promotes the persistence of tensile–shear cracking even under relatively high σ2 conditions. Based on these observations, a modified Mogi–Coulomb strength criterion that accounts for thermal shock-induced damage is proposed to describe granite strength under true-triaxial stress conditions. The research results can provide a theoretical basis for optimizing the design of hydraulic fracturing in hot dry rock and evaluating reservoir stability.

Rock damage and fault reactivation mechanisms under initial perturbation in enhanced geothermal systems