Fracture flow channeling stemming from heterogeneous aperture distribution is a widely observed phenomenon in enhanced geothermal systems (EGSs) and has been considered a main cause of unsatisfying thermal extraction performance. Many numerical studies have been performed to quantify the impact of flow channeling on thermal performance, while the dynamic evolution of flow channeling under complex thermo-hydro-mechanical-chemical (THMC) coupled processes remains underexplored. This study develops a 3D field-scale THMC coupled EGS model with heterogeneous fracture apertures to systematically investigate the individual and joint effects of thermoelastic process and mineral reaction on fracture flow channeling and long-term thermal performance. The results demonstrate that during long-term injection of undersaturated water, the thermoelastic process leads to aperture enlargement in low-temperature zones, intensifying flow channeling, whereas the mineral dissolution preferentially enlarges fracture aperture in high-temperature zones, leading to flow dispersion. These two mechanisms exhibit strong physicochemical feedbacks: the mineral dissolution counteracts thermoelastic-induced flow channeling by enlarging heat exchange zones and homogenizing thermal stress distributions, while the thermoelastic process enhances the effect of mineral dissolution by narrowing heat exchange zones. Parametric analyses further reveal that reservoirs with higher rock elastic modulus and lower fracture stiffness are more susceptible to severe thermoelastic-induced flow channeling, whereas higher injection temperatures, lower injection concentrations, and greater reactive mineral content enhance the mitigating effect of mineral dissolution. These findings suggest that long-term thermal performance of EGSs can be optimized by selecting reservoirs with low elastic modulus, high fracture stiffness, and abundant reactive minerals, combined with high-temperature, undersaturated injection strategies.

The development of hot dry rocks (HDRs) is of great significance to adjusting energy structure, alleviating energy shortage, reducing pollution, etc. Low-permeability granite is the predominant rock type in deep HDRs, making fractures the primary pathways for fluid circulation and heat extraction. The production of HDRs is significantly influenced by variable fracture conductivity, but current conductivity characterization primarily relies on the elastic deformation of the matrix, neglecting the impact of damage. Accordingly, we propose an experimental method and a supporting apparatus, which is used to unveil the conductivity evolution characteristics resulting from the comprehensive effects of damage and elastic deformation. The experimental results demonstrate that when subjected to confining force squeezing inward, the fracture conductivity experiences varying degrees of decrease compared to its initial state before the experiment. By utilizing the conductivity evolution rate as the evaluation criterion and conducting grey correlation analysis, it has been determined that temperature exerts the most significant influence on the conductivity evolution, followed by injection flow, and lastly, confining pressure. Moreover, rock particle types and production cycles also have different degrees of effect. After considering the comprehensive effects of damage-elastic deformation at the field-scale, the damage has a positive effect on conductivity enhancement. Our study provides a new approach for the characterization of fracture conductivity evolution for deep geothermal projects.

In the long-term mining of geothermal resources in hot dry rock (HDR), the change of thermal stress and pore pressure will increase fracture conductivity evolution, further improving production performance. The optimization and decision-making of the development scheme based on the impact of damage from fractures have yet to be reported. The damage to fractures is essential in designing and adjusting geothermal resource development schemes, particularly in selecting optimal schemes. Therefore, the production performances of HDR resources under different parameters are analyzed to establish a database. Then, minimizing flow resistance, maximizing net power, and maximizing economic benefits are set as optimization goals. Various injection-mining parameters and fracture characteristics are treated as decision variables. Multi-objective optimization and multi-attribute decision analysis is conducted to obtain optimal schemes. Finally, optimal schemes are evaluated and compared, considering damage and non-damage scenarios. Results show that the NSGA-II algorithm is more suitable for optimizing geothermal development questions. Net power and economic benefits of the optimal scheme considering damage increase by 45.84 % and 21.35 % compared to the control scheme with damage. For the non-damage scenario, the above values increased by 31.55 % and 5.15 %, respectively. Compared to not considering the damage, higher mass flow and well spacing of optimal scheme can be selected for the case when damaged. Moreover, the parametric design of the optimal scheme becomes more conservative as the production cycle increases.

Fractures and caves are the main flow and storage channels for the karst geothermal reservoirs, and the water-rock reaction within them significantly affects the thermal performance. Most previous studies concentrated on the fractures, disregarding the impact of the pore water-rock reaction. The objective of this study is to explore the importance of pore water-rock reactions and identify the influence of various parameters when considering pore and fracture water-rock reactions. A 3D thermal-hydraulic-chemical coupling model considering dual media of pores and fractures was developed. The importance of pore water-rock reactions is demonstrated, and quantitatively characterize the effect of injection temperature (T_in), injection rate (Q_in), injection concentration (c_in), and ratio of the reaction-specific surface area between pore and fracture (A_p/A_f) on the thermal performance. Results indicate that the pore water-rock reaction drastically affects the hydraulic conductivity and pressure difference, even leading to an opposite trend. The influence of water-rock reaction in pores on fracture deformation is regulated by A_p/A_f, which augments with A_p/A_f. The relative contribution of A_p/A_f to production temperature, net thermal power, pressure difference, and hydraulic conductivity are 12.8%, 4.1%, 6.8%, and 13.7%, respectively. This study provides a significant guide for accurate production prediction and exploitation of karst-based geothermal reservoirs.

The chemical reaction in the reservoir causes fracture deformation during the heat extraction of enhanced geothermal systems (EGS), affecting thermal performance. The reaction rate is sensitive to temperature, concentration, and reaction-specific surface area. While previous research mainly focuses on the influence of temperature and concentration on fracture deformation, conversely, the effect of fracture morphology(aperture and tortuosity) is ignored. In this study, the deformation characteristics of rough and flat fractures are compared, and the influences of aperture and tortuosity on fracture deformation are analyzed. According to the influence law, the fracture deformation relationship equation between the aperture deformation rate with tortuosity and aperture is fitted. Results show that the deformation of rough fracture is significantly higher than that of flat fracture, and the variations of fracture aperture increase with the aperture and tortuosity. Furthermore, the influence of tortuosity (the variation of aperture increased by 47.97% when the tortuosity increased from 1 to 1.5) is greater than the aperture (that increased by 4.8% when the aperture increased from 0.2 to 1). The rate of aperture change is a logarithmic function of tortuosity and a power function of the aperture. These results provide significant references for the study of EGS, subsurface karst et al.

The fractures are the main flow and heat transfer channel for fluids in deep high-temperature enhanced geothermal systems (EGS). The deformation of the fracture controlled by reactive flow is a common phenomenon during geothermal development, which might lead to a reduction in the system's thermal performance and operating life. While most previous research focuses on the influence of fracture deformation on system heat extraction performance, conversely the fracture deformation mechanism caused by the reactive flow is ignored. In this paper, a coupled thermal-hydraulic-chemical-deformation (THCD) model is established to investigate the fracture deformation mechanism. The deformation behavior of quartz and anorthite is compared, and the Damkohler number (Da) is adopted to explore the reaction mechanism. Meanwhile, the influence mechanism of concentration and temperature on fracture deformation is also analyzed. Results show that the Da at the fracture surface is 10⁻¹² -10⁻⁸, which means that the deformation of the fracture is controlled by the reaction rate. Compared with the concentration, the influence of temperature on fracture deformation is complex, and there is an inflection point. The inflection point is governed by the mineral reaction kinetics. These results provide significant references for the efficient heat extraction of the EGS.

The multi-physics coupling process during the heat extraction from enhanced geothermal system, encompassing thermo(T)-hydro(H)-mechanical(M)-chemical(C) interactions, plays a pivotal role in changing geothermal reservoir characteristics. However, a comprehensive quantitative assessment of these multi-physics behaviors has been lacking. In this study, a novel approach was proposed to calculate the magnitude of mechanical, chemical, strong mechanical-chemical coupling, and weak mechanical-chemical coupling effects on the variations of reservoir characteristics. In particular, mechanical-chemical coupling effects are quantified for the first time. They are obtained by the fracture aperture difference results across five distinct coupling models (thermo-hydro, thermo-hydro-chemical, thermo-hydro-mechanical, partially-coupled four-field, and fully-coupled four-field models). The findings indicate that mechanical effects lead to an increase in fracture aperture, while chemical effects contribute to its reduction under underbalanced injection conditions. Strong mechanical-chemical coupling effects, exhibiting a negative correlation with chemical effects, conversely result in a diminished fracture aperture. The influences of these effects are investigated from the temporal and spatial perspectives. Temporally, mechanical effects dominate early production while chemical effects become prominent in later stages. Spatially, there mainly exists two zones when stable production: a mechanical-controlled region surrounding injection wells, and a chemical-controlled area distant from the injection wells. Furthermore, sensitivity analysis of injection concentration indicates its alternation changes the reservoir traits and production performance by modifying the magnitudes of chemical and mechanical-chemical coupling effects. This quantification of multi-physics effects offers insights into optimizing injection strategies for better geothermal development. The approach could hold promising potential in other geo-energy scenarios like carbon and hydrogen storage in reservoirs.

Geothermal is an important renewable energy source, but the high cost of drilling limits its popularization. The oilfield area is rich in geothermal resources and has a large number of high-temperature abandoned wells. It is an economic and effective method to transform the abandoned wells into geothermal wells. At present, the heat extraction research of abandoned wells mostly focuses on single-well closed systems, while the most common in oilfields is the well patterns open system, which involves the flow and heat extraction of oil-water multiphase. More, the main utilization mode of oilfield geothermal is heating, which is an intermittent operation, and the existence of the heat recovery period makes the solution process more complicated compared to continuous production. Therefore, the numerical simulation of oil-water two-phase and the scheme optimization under intermittent operation are the key problems in the current oilfields geothermal research, that is, it should be made clear what the effects of operation parameters are and how to get the optimization design for the intermittent mining of heat-oil cogeneration from abandoned wells. For this reason, the oil-water two-phase heat-flow coupling model and economic evaluation model are established. Then, database is established through parameter sensitivity analysis, and multi-objective optimization research is carried out on the lifetime and economic benefits using Non Sorting Genetic Algorithm II (NSGA-II). The results show that economic benefit and end-point temperature of the optimized scheme are enhanced by 6.75 million US dollars and 7.10 K after 15-year production, respectively. The heat-oil cogeneration performance is superior, so the influence of oil phase cannot be ignored in the heating process of the well patterns system for oilfield geothermal production.

Fracture distribution plays a significant role in the behavior of subsurface environments, affecting such activities as geothermal production, exploitation and management of groundwater resources, and long-term storage of nuclear waste and carbon dioxide. A key challenge in these and other applications is to estimate the fracture network properties from sparse and noisy observations. We evaluate the utility of cross-borehole thermal experiments for this task, using both physics-based particle-tracking (PBPT) heat-transfer approach and its deep neural network (DNN) surrogates. Synthetic data are provided by the PBPT simulations and used to train and test the DNN surrogates over a full range of the fracture network properties. We propose regionalized and step-by-step training techniques to reduce the computational cost of expensive PBPT forward solves over large ranges of the (to-be-estimated) parameters. Our numerical experiments suggest the feasibility of training a regionalized DNN surrogate over parameter ranges for which the PBPT solves are fast and extrapolating its predictions to parameter ranges with few additional data. We analyze the balance between computational cost and model accuracy, and provide both PBPT and DNN models for applications to others kinds of data.

Oil shale in-situ conversion is an effective and promising exploitation method. The most concerned problem of oil shale in-situ conversion is how to exploit maximum oil and gas by injecting the least energy. However, the relationship between injection energy utilization efficiency and productivity under different operational conditions remain unclear. In this paper, based on a multiphase flow, heat transfer and chemical reaction numerical model, evolution of kerogen pyrolysis with reservoir temperature distribution is thoroughly analyzed. Aims at injection energy utilization efficiency and productivity, effects of injection energy rate, well shut-in measure, reservoir pressure and well spacing on the production performance of the oil shale in-situ exploitation are investigated. Results show that the useless heating region exists during kerogen pyrolysis, which significantly reduces the energy utilization efficiency. A shut-in measure can slightly improve the energy utilization efficiency but lower oil output, thus not a very effective measure to solve the useless heating problem. Under the same energy injection rate, a higher injection temperature and lower injection flow rate will simultaneously obtain higher oil production rate, oil output, and energy utilization efficiency. Furthermore, a larger reservoir pressure and well spacing of 40 m–50 m are recommended to obtain higher oil production rate and output. Results provide meaningful suggestions for optimizing operational parameters in view of injection energy utilization efficiency and oil output.

Hot dry rocks (HDRs), as an essential renewable energy source, its development has received widespread attention, especially for heat extraction. The fracture is the main seepage and heat transfer channel of circulating fluid in dense HDR reservoirs, and its conductivity evolution significantly affects the production performance. Most existing studies have focused on the change of fracture conductivity under elastic deformation without considering the additional conductivity induced by rock damage. However, the additional conductivity may have significant implications for rational design and timely adjustment of the production scheme. Therefore, a three-dimensional model at the field-scale is established, and it is used to analyze the effect of additional conductivity on production performance and economic efficiency. To simplify the calculation, the actual forms of damage are equivalent to the macroscopic physical evolution of the matrix. Results show that the rock is mainly tensile failure affected by thermal stress during production. The occurrence of damage will increase the reservoir permeability and porosity, reduce Young's modulus, and then reduce the differential pressure and production temperature, with a maximum reduction of 2.21 MPa and 14.21 °C in the control case, respectively. The effects of injection temperature, Young's modulus, and injection mass flow on the production performance are significant, followed by Poisson's ratio. In contrast, production pressure and fracture initial permeability had less influence. The maximum differential economic benefit of the control case is up to 2.289 million RMB. This research proves the necessity of damage study during the long-term production of HDRs.

Fracture networks, fluid flow and heat extraction within fractures constitute pivotal aspects of enhanced geothermal system advancement. Conventional hydraulic fracturing in dry hot rock reservoirs typically requires high breakdown pressure and only produces a single major fracture morphology. Thus, it is imperative to explore better fracturing methods and consider more reasonable coupling mechanisms to improve the prediction efficiency. Cyclic fracturing using liquid nitrogen instead of water can generate more complex fracture networks and improve the fracturing performance. The simulation of fluid flow and heat transfer processes in the fracture network is crucial for an enhanced geothermal system, which requires a more comprehensive coupled thermo-hydro-mechanical-chemical model for matching, especially the characterization of coupling mechanism between the chemical and mechanical field. Based on the results of field engineering, laboratory experiments and numerical simulation, the optimum engineering scheme can be obtained by a multi-objective optimization and decision-making method. Furthermore, combining it with the deep-learning-based proxy model to achieve dynamic optimization with time is a meaningful future research direction.