Enhanced geothermal systems (EGSs) in this study are classified as fracturing-EGS (F-EGS), pipe-EGS (P-EGS) and excavation-EGS (E-EGS) according to reservoir stimulation strategies. However, the heat ...extraction performances of three EGSs employing different stimulation strategies are not fully understood. Here, we define the region where the pore pressure increment calculated by a hydraulic fracturing process is higher than rock tensile strength as the stimulation region for establishing a more accurate F-EGS model, and then compare three geothermal systems to select a appropriate reservoir stimulation strategy. We find that the F-EGS model assuming an entire stimulated region significantly exaggerates the heat extraction results. The optimal conditions for P-EGS are low injection rates and short operation times, which is suiTablefor seasonal heating or multi-energy co-generation projects including a thermal recovery phase. Theoretically, E-EGS has better geothermal extraction performance than F-EGS based on existing model assumptions, but its construction feasibility and economics need further exploration. H2O is more suiTableas a heat exchange fluid in E-EGS than supercritical CO2. This study provides a reference for geothermal mining simulation and reservoir stimulation strategy selection.
•F-EGS model with an entire stimulated region exaggerates heat extraction.•The optimal conditions for P-EGS are low injection rates and short operation times.•E-EGS has better heat extraction effect than F-EGS based on existing assumptions.•H2O is more suiTableas a heat exchange fluid in E-EGS than supercritical CO2.
Summary
Hydraulic‐fracturing treatments have become an essential technology for the development of deep hot dry rocks (HDRs). The deep rock formation often contains natural fractures (NFs) at micro ...and macroscales. In the presence of the NF, the hydraulic‐fracturing process may form a complex fracture network caused by the interaction between hydraulic fractures and NF. In this study, analysis of carbon dioxide (CO2)‐based enhanced geothermal system (EGS) and water‐based EGS in complex fracture network was performed based on the thermo‐hydro‐mechanical (THM) coupling method, with various rock constitutive models. The complexity of the fracture geometry influences the fluid flow path and heat transfer efficiency of the thermal reservoir. Compared with CO2‐based EGS, water‐based EGS had an earlier thermal breakthrough with a rapid decline in production temperature. CO2 can easily gain heat rising its temperature thus reducing the effect of a premature thermal breakthrough. Both CO2‐based EGS and water‐based EGS are affected by in‐situ stress; the increase in stress ratio improved the fracture permeability but resulted in an early cold thermal breakthrough. When the same injection rate is applied to water‐based EGS and CO2‐based EGS, water‐based EGS displayed higher injection pressure buildup. Water‐based EGS had higher reservoir deformation area than CO2‐based EGS, and thermoelastic constitutive model for water‐based EGS showed larger deformed area ratio than thermo‐poroelastic rock model. Furthermore, higher values of rock modulus accelerated the reservoir deformation for water‐based EGS. This study established a novel discussion investigating the performance of CO2‐based EGS and water‐based EGS in a complex fractured reservoir. The findings from this study will help in deepening the understanding of the mechanisms involved when using CO2 or water as a working fluid in EGS.
This paper showed that water‐ based enhanced geothermal system (EGS) had significant injection pressure buildup and higher reservoir deformation due to the flow behavior and properties of water in the thermal reservoir. CO2‐based EGS had a delayed thermal breakthrough than water‐based EGS, CO2 can easily gain heat thus reducing the effect of a premature thermal breakthrough. The thermoelastic or thermo‐poroelastic constitutive models influence the level of simulated results and they should be applied in accordance to the rock mineral constituents.
Enhanced (or engineered) geothermal systems (EGS) have evolved from the hot dry rock concept, implemented for the first time at Fenton Hill in 1977. This paper systematically reviews all of the EGS ...projects worldwide, based on the information available in the public domain. The projects are classified by country, reservoir type, depth, reservoir temperature, stimulation methods, associated seismicity, plant capacity and current status. Thirty five years on from the first EGS implementation, the geothermal community can benefit from the lessons learnt and take a more objective approach to the pros and cons of ‘conventional’ EGS systems.
Fractures play an important role in the geothermal energy recovery from enhanced geothermal systems (EGS) involving interacted multi-physical fields. A strongly coupled thermo-hydro-mechanical (THM) ...model is proposed to simulate the process of the long-term geothermal production in three-dimensional geothermal reservoirs containing arbitrary discrete fracture networks (DFNs). By introducing a strong discontinuity concept into the DFN model, the aperture variation of each fracture induced by the fluid pressure, external stresses and thermal expansion in the period of production can be captured. Non-isothermal fluid flow in DFN and the local thermal non-equilibrium (LTNE) between fluid and rock matrix are formulated and coupled with the fracture deformation model. Verification is carried out against an analytical solution, followed by a sensitivity and convergence analysis concerning time step and mesh size. This approach is then applied to Habanero EGS project in Australia to evaluate the geothermal productivity and efficiency for a period of 20 years with different injection and production pressures. The results demonstrate that the proposed model is robust and effective to simulate the THM coupled process in 3D fractured reservoirs.
Failure to rescue (FTR) is considered as an index of quality of care provided by a hospital. However, the role of frailty in FTR remains unclear. We hypothesized that the FTR rate is higher for frail ...geriatric emergency general surgery (EGS) patients than nonfrail geriatric EGS patients.
We performed a 3-y (2015-2017) prospective cohort study of all geriatric patients (age ≥ 65 y) requiring EGS. Frailty was calculated by using the EGS-specific Frailty Index (EGSFI) within 24 h of admission. Patients were divided into two groups: frail (FI ≥ 0.325) and nonfrail (FI < 0.325). We defined FTR as death from a major complication. Regression analysis was performed to control for demographics, type of operative intervention, admission vitals, and admission laboratory values.
Three hundred twenty-six geriatric EGS patients were included, of which 38.9% were frail. Frail patients were more likely to be white (P < 0.01) and, on admission, had a higher American Association of Anesthesiologist class (P = 0.03) and lower serum albumin (P < 0.01). However, there was no difference between the groups regarding age (P = 0.54), gender (P = 0.56), admission vitals, and WBC count (P = 0.35). Overall, 26.7% (n = 85) of patients developed in-hospital complications; and mortality occurred in 30% (n = 26) of those patients (i.e., the FTR group). Frail patients had higher rates of FTR (14% vs. 4%, P < 0.001) than nonfrail patients. On regression analysis, after controlling for confounders, frail status was an independent predictor of FTR (OR: 3.4 2.3-4.6) in geriatric EGS patients.
Our study demonstrates that in geriatric EGS patients, a frail status independently contributes to FTR and increases the odds of FTR threefold compared with nonfrail status. Thus, it should be included in quality metrics for geriatric EGS patients.
This study determines the theoretical, technical, optimal economic and sustainable potential of enhanced geothermal systems (EGS) on a global scale. The global potential ranges from 256 GWe to 108 ...TWe by 2050, depending on the selected constraints.
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•Potential of enhanced geothermal systems is estimated globally.•The results indicate that EGS can contribute to growing demand of renewable energy.•Around 4600 GWe of EGS capacity can be built at a cost of 50 €/MWh or lower.•Sustainable potential of EGS is estimated at 256 GWe in 2050.•All the input data and the results are provided on 1°×1° spatial resolution.
This study demonstrates the theoretical, technical, optimal economic and sustainable potential of enhanced geothermal systems (EGS) globally. A global estimate of EGS is presented in a 1°×1° spatial resolution. Constructed temperature at depth maps are computed for every 1 km thick layer, from 1 to 10 km. Multiple factors such as surface heat flow, thermal conductivity, radioactive heat production, and surface temperature are involved, and obtained from various sources and assumptions. The global EGS theoretical potential is assessed. Available heat content is then estimated using technical constraints for the temperature equal to or higher than 150 °C for any 1 km depth, and presented as thermal energy and electrical power capacity. The EGS optimal economic potential is derived from the optimum depth and the corresponding minimum levelised cost of electricity. The global optimal economic potential in terms of power capacity is found to be about 6 and 108 TWe for the cost years of 2030 and 2050, respectively. If economic and water stress constraints are excluded, the global EGS potential can be as much as 200 TWe. Further, an industrial cost curve is developed for the levelised cost of electricity as a function of EGS technical power capacity. The findings indicate that around 4600 GWe of EGS capacity can be built at a cost of 50 €/MWh or lower. A method is applied to measure the sustainable geothermal resource base. The obtained sustainable potential is found to be 256 GWe in 2050. Results are presented on a country basis and globally.
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•The low-velocity layer in the thickened crust provides the most important heat source in Tibetan Plateau.•Heat production of highly radiogenic rock is the main heat source of shallow ...intracrustal heat in southeastern China.•Heat upwelling along fractures is the main heat source of EGS in the north China sedimentary basin.
A heat source is the basis for Enhanced Geothermal Systems (EGS) and is the most important factor for the site selection for EGS development. In this paper, we analyze regional variation patterns of mantle heat flow, study crustal thermal states, construct models of crustal steady-state temperature fields, and discuss the heat source mechanism of EGS in different zones, including the strongly tectonic activity zone of the Tibetan Plateau, North China sedimentary basin, and the highly radiogenic rock distribution zone of southeastern China. The results show that differences in the crustal thermal structure of different regions control their deep thermal conditions and EGS heat sources. The intense tectonic activity zone of the Tibetan Plateau, represented by the Gonghe Basin, shows an anomalously heated crustal thermal structure, and the low-velocity layer in its thickened crust provides the most important intracrustal heat source in the region. The local area in southeastern China is influenced by both highly radiogenic rocks and good sedimentary cover insulation, indicating a hot crust-cold mantle type of crustal thermal structure, thus reflecting the role of heat production from highly radiogenic rocks in controlling the shallow crustal temperature field in the area. The North China Plain has a typical cold crust-hot mantle type of crustal thermal structure, and the development of EGS in the area must consider the undulating bedrock pattern in the sedimentary basin and whether there is a deep fracture that connects the nearby mantle heat source. The results are of scientific significance for guiding future target site selection and EGS development in different regions.
The DErisking Exploration for multiple geothermal Plays in magmatic ENvironments (DEEPEN) project aims to de-risk exploration of geothermal plays in magmatic systems, with a focus on superhot and ...supercritical geothermal systems. This article presents advancements in geoscientific assessment of superhot enhanced geothermal system (EGS) resources as part of the DEEPEN project, particularly in exploration and identification of these systems. While there exists a general consensus on key components of conventional hydrothermal systems, there has not been a prior consensus on such components for EGS, let alone superhot EGS. The DEEPEN project identified these key components for superhot EGS plays, and used them to modify the traditional geothermal play fairway analysis (PFA) approach to identify favorable areas in superhot EGS plays in 3D. This was done through modeling both favorability and uncertainty separately for each key component of a superhot EGS play, incorporating diverse 3D geoscientific datasets, including geologic features, models produced from direct observations, single inversions, and joint inversions. The PFA is applied to Newberry Volcano as a form of validation. Finally, the results appear to validate the methodology, aligning with expectations derived from conceptual modeling, and highlighting the area targeted for EGS stimulation well NWG 55-29 as favorable, and suggesting potential additional areas worthy of exploration at Newberry.
•Thermal cycling effects on granite fracture characteristics were investigated.•The number of thermal cycle in the present study were 1, 5, 10, 15, and 20.•Crack characteristics in granite was ...quantitative discussed.•Thermal cycling decreases Keff, U, Pv, and increases K.•Thermal cycling has effect on the predictive accuracy of MMTS.
The fracture characteristics of fine-grained granite were examined for a potential geothermal-energy reservoir. The granite was thermally cycled in a furnace between 100 °C and 300 °C and its mechanical behavior and meso-crack characteristics were analyzed. The results indicate that thermal cycling leads to decreased fracture toughness (Keff), absorbed energy (U), longitudinal wave velocity (Pv), and increased permeability (K) in granite. These changes can be explained using the thermal fatigue accumulated damage. The ability of granite to resist fracturing is greatly reduced in the first five thermal cycles. Thermal cycling is more conducive to inducing intergranular cracks. The interconnection of intragranular and intergranular cracks causes the structure of granite to become fragmented and more likely to fail. Thus, thermal cycling deteriorates the mechanical stability of fine-grained dense granite, and allows crack networks to form more easily. The correction of crack propagation critical radius (rc) can improve the accuracy of the modified maximum tangential stress (MMTS) predictive fracture.
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