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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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A two-dimensional particle-based heat transfer model is used to train a deep neural network. The latter provides a highly efficient surrogate that can be used in standard inversion methods, such as grid search algorithms. The resulting inversion strategy is utilized to infer statistical properties of fracture networks (fracture density and fractal dimension) from synthetic thermal experimental data. The (to-be-estimated) fracture density is well constrained by this method, whereas the fractal dimension is harder to determine and requires adding prior information on the fracture network connectivity. The method is tested on several fracture networks and hydraulic conditions.@en

Geothermal energy, a kind of clean and environmentally friendly energy source, is an important object of future natural resource development and utilization, among which, hot dry rock is one of the important deep geothermal resources. In the current multi-objective optimization of heat extraction performance, reservoir production models are less considered and the effects of different optimization ideas are not compared comprehensively. To improve the heat extraction efficiency and prolong the exploitation life of geothermal reservoirs, this paper determines the appropriate operating parameters of geothermal system (injection temperature, injection rate, production pressure and injection-production well spacing) based on the coupled thermal-hydraulic-mechanical model of hot dry rock exploitation in the Gonghe area of Qinghai and three heat extraction optimization methods. In addition, the heat extraction performances of different schemes are comparatively evaluated. And the following research results are obtained. First, the sensitivity analysis of injection and production parameters shows that power generation and recovery factor are in a reverse relation with injection-production pressure difference, which is the direct reason for the adoption of multi-objective optimization. Second, the optimization scheme prepared on the basis of parametric study indicates that the shortest life of a geothermal reservoir is 10 years, the injection-production pressure difference is up to 67 MPa, there is a significant thermal breakthrough phenomenon and the reservoir safety faces challenges. Third, by virtue of multi-objective optimization and decision making integration, the optimal operation parameter combination of hot dry rock system is determined, the life of geothermal reservoirs can exceed 20 years and balanced optimization is achieved. In conclusion, the idea of multi-objective optimization is feasible and applicable to geothermal energy exploitation and this method provides a reference for the efficient geothermal energy development and utilization and is helpful to the realization of “double carbon” goal in China.

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Horizontal ground heat exchanger (HGHE) is a low-cost shallow geothermal exploitation method compared with vertical ones, but it needs more land area. Optimizing geometrical configurations and arrangements of ground heat exchangers is desirable to enhance heat exchange per unit land area. However, a comprehensive study on different HGHE configurations and arrangements considering atmosphere-soil-HGHE interaction is still inadequate. To this end, thermal performances of four configurations, including slinky, spiral, multi-tube with serial connection and tiled tubes, are compared based on a 3D numerical model for HGHE considering atmosphere conditions variation and humidity migration. Effects of main geometrical parameters on HGHE performance and economical efficiency are investigated for these four configurations. Finally, different tube arrangements of multi-tube HGHE are designed and their thermal performances are analyzed. Results indicated that longer HGHE tube length has better thermal performance but lower economical efficiency, thus being not advisable for the HGHE design. Among these four configurations, multi-tube with serial connection shows the best thermal performance. Increasing the tube spacing is beneficial for reducing the tube mutual interference and enhance the heat exchange per unit land area, which is recommendable. Results are expected to provide meaningful suggestions for the HGHE design.

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As a kind of clean renewable energy, the production and utilization of geothermal resources can make a great contribution to optimizing the energy structure and energy conservation and emission reduction. The circulating heat extraction process of working fluid will disturb the equilibrium state of physical and chemical fields inside the reservoir, and involve the mutual coupling of heat transfer, flow, stress, and chemical reaction. Revealing the coupling mechanism of flow and heat transfer inside the reservoir during geothermal exploitation can provide important theoretical support for the efficient exploitation of geothermal resources. This paper reviews the research advances of the multi-field coupling model in the reservoir during geothermal production over the past 40 years. The thrust of this paper is on objective analysis and evaluation of the importance of each coupling process and its influence on reservoir heat extraction performance. Finally, we discuss the existing challenges and perspectives to promote the future development of the geothermal reservoir multi-field coupling model. An accurate understanding of the multi-field coupling mechanism, an efficient cross-scale modeling method, as well as the accurate characterization of reservoir fracture morphology, are crucial for the multi-field coupling model of geothermal production.@en

Geothermal reservoir scaling controlled by reactive flow is a common phenomenon during geothermal development, which might lead to the decline of geothermal productivity and abandonment of geothermal projects. It is significant to reveal chemical influence on the geothermal reservoir characteristics and production performance to provide the reference for geothermal sustainable development. In this work, a thermal-hydraulic-chemical coupling model is developed for a triple-well fractured enhanced geothermal system considering the silica dissolution and precipitation at the fracture surface. It is focused on the evolution of fracture aperture and geothermal productivity. The results indicate that undersaturated injection will cause mineral dissolution which leads to the rise of fracture transmissibility and a thermal break. The fracture width is enhanced by 0.093 mm, and the permeability is 8 times the initial value after 20-year production. The oversaturated injection will cause mineral precipitation which leads to the reduction of fracture aperture and the increase of pressure difference. The opening is reduced by 0.041 mm and the permeability goes down by 2 orders of magnitude. The maximum increases of average production temperature and differential pressure are 10 °C and 13.5 MPa respectively in the 20th year from undersaturation to supersaturation. Besides, it is figured out that reaction rate denotes the evolution of fracture aperture. Both of them are distributed as a band at a fracture plane, which is determined by saturation index and temperature together. The former controls the reaction pattern (dissolution/precipitation) while the latter controls the reaction speed. Furthermore, the evolution of opening is uneven around the production well at different exploitation stages. Fracture width is higher on the strong side in the first 8.7 years but is higher on the weak side in later development when undersaturated water (0 mol/kg) is injected. The phenomenon can be employed to judge the production status, guide the descaling, etc. Production analysis indicates that oversaturation delays the heat breakthrough and undersaturation reduces the surface pump input. This study will provide a significant reference for the geothermal sustainable production influenced by chemical reactions in an enhanced geothermal system.

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The efficient development of hydrothermal resource is an important focus in the global geothermal industry. A novel hydrothermal open-loop geothermal system in a multi-lateral horizontal well is presented to achieve the simultaneous exploitation of multi-layer reservoirs and accelerate the large-scale development of the hydrothermal resource. To investigate the detailed performance of the novel geothermal system, a novel 3D fluid flow and heat transfer numerical model is established based on the local thermal non-equilibrium and dual porosity theories. The flow and temperature fields are analyzed, while the pressure and temperature interferences between the lateral wellbores of the multi-lateral horizontal well system are comprehensively investigated. The influences of the injection flow rate, lateral-wellbore number, spacing between the injection segment and the main wellbore, fracture aperture on the heat extraction are studied. Furthermore, the thermal characteristics of the novel geothermal system and traditional doublet well system are compared for the first time. The investigation indicates that the flow impedance of the novel geothermal system can decrease by 75.38%, and its energy efficiency reaches 8 times that of the doublet well system. The findings can provide a key reference for scientific research and application of the novel and traditional geothermal systems.

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There are chemical dissolution and precipitation in geothermal formations with the long-term exploitation of hot dry rocks, which would induce the remarkable alteration of fracture transmissibility and evolution of thermal performance. The objective of this study is to identify the influence of various parameters incorporating chemical reactions to provide suggestions for the operation of the enhanced geothermal system (EGS). A 3D thermal-hydraulic-chemical coupling model is established for a fractured EGS considering silica-water reaction kinetics at fracture surface. Parametric study and sensitivity analysis are innovatively conducted to investigate the effects of key operation and formation parameters, including injection concentration (c_in), injection temperature (T_in), injection rate (Q_in) and initial formation silica reactant content (C_i), on thermal extraction performance. Results indicate that undersaturated injection causes chemical dissolution while oversaturated injection leads to silica precipitation. Higher c_in relieves the thermal breakout (10 °C) but enhances the injection-production pressure difference (13.5 MPa). The selection of c_in is an efficient way to adjust geothermal production, especially for oversaturated injection. The influence degrees of c_in on production temperature (T_out) and pressure difference (Δp) under oversaturated injection are 8 times and 2 times larger than those under undersaturated injection. T_in (<80 °C) exerts little influence on the chemical reaction. The remarkable water storage phenomenon will be induced with the increase of Q_in when precipitation occurs. Higher C_i results in lower T_out and Δp. The sensitivity analysis provides the rank of major parameters to thermal performance: Q_in, T_in, c_in and C_i. The standard sensitivities of Q_in to T_out, Δp and net thermal power N are 0.78, 0.63, and 0.88 respectively. As for the contribution ratio, c_in contributes the most (89%) to the variation of fracture aperture while T_in (<80 °C) contributes the least (nearly 0%). The research will provide a significant reference for the selection of parameters during the development of geothermal energy.

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Geothermal energy is gaining attention as an environmentally friendly and sustainable alternative to fossil fuels. There exist complicated coupling processes among fluid flow, heat transfer, mechanical deformation, and chemical reaction in fractured geothermal reservoirs. Understanding of thermo-hydro-mechanical-chemical (THMC) coupled process is of significance for the efficient development of enhanced geothermal systems. This research aims to quantify the contributions of mechanical and chemical behaviors to reservoir feature variation during geothermal production. Herein, a fully coupled THMC model is developed for the production of an enhanced geothermal system based on fracture aperture alternation. The key improvement lies in the inclusion of mechanical-chemical (MC) coupling where chemical behaviors cause stress to be redistributed. On a geographical and temporal scale, the unique proportion technique is employed to study ratios of mechanical to chemical effects and poroelasticity to thermoelasticity. The findings emphasize the necessity of incorporating the MC coupling when analyzing reservoir characteristic variations. The mechanical effect plays a major role near the injection well and in a short term. The chemical effect is dominated far from the injection well and its time scale of action is long-term. The intensity of thermoelasticity is stronger than that of poroelasticity. Moreover, the proportion analysis reveals that when stable production is achieved, there are primarily thermo-driven and chemical-driven regions. To improve production performance and adjust geothermal reservoir features, injection temperature and solute concentration should be optimized. This paper serves as a valuable reference for both the theoretical multi-physical coupling model and practical geothermal production.

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Global warming induced by excessive carbon dioxide emission generated from fossil fuels consumption is becoming increasingly severe. Development and utilization of clean and renewable energy is a significant measure to contribute for the carbon neutrality. Geothermal resource is an important clean and renewable energy, because of its advantages of abundant reserves, exploitation without restrictions from the climate and season. Extracting heat from the high temperature geothermal reservoir requires well drilling, completion and fracturing. However, due to some particular conditions in geothermal development, such as high temperature, erosion, leakage, and hard rocks, traditional petroleum well drilling and completion technologies are not well applicable in the geothermal industry. In this paper, the state-of-the-art geothermal drilling, and completion technologies as well as several fracturing methods, such as foam drilling, air hammer drilling, hydrothermal jet drilling, expansion casing technology, high temperature resistant drilling fluid and cement, chemical stimulation and thermal stimulation are comprehensively reviewed. Besides, heat extraction performances of water and CO₂ as working fluids in the geothermal system are discussed. A few novel heat extraction methods including closed loop, open loop, enhanced geothermal systems are compared and analyzed. This review is expected to provide suggestions for research fields worthy of study for geothermal drilling and exploitation.

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Geothermal energy is one of renewable and clean energy resources. Predicting geothermal productivity is an essential task for managing a continuable geothermal system, which is a huge challenge due to the highly non-linear relationship between the productivity and constraint conditions, such as reservoir properties and operational conditions. Using numerical simulation to predict the geothermal productivity is computationally expensive and very time consuming. Therefore, this study proposes a novel Long Short-Term Memory (LSTM) and Multi-Layer Perceptron (MLP) combinational neural network to effectively forecast the geothermal productivity considering constraint conditions. In the LSTM and MLP combinational neural network, MLP is trained to learn the non-linear relationship between the geothermal productivity and constraint conditions, while LSTM is used to memorize sequential relations within the production data. We comprehensively evaluate the geothermal productivity prediction performance of the LSTM and MLP combinational network. It indicates that the LSTM and MLP combinational neural network could accurately and stably predict the geothermal productivity and has a good generalization ability. Compared with original LSTM, MLP, Simple Recurrent Neural Network (RNN), the LSTM and MLP combinational network demonstrates the best geothermal productivity prediction accuracy, stability and generalization ability. This study provides a high precision and efficiency forecasting method for the geothermal productivity prediction.

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