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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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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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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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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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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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 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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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

The effective exploitation of deep hydrothermal resource is an emphasis in the global geothermal industry. A horizontal-well coaxial closed-loop geothermal system (CCGS) is presented to realize its efficient development by significantly increasing the heat exchange area based on a long horizontal well. However, few researchers focus on the complex convection process in the hydrothermal reservoir, making it difficult to effectively evaluate the thermal characteristics of a geothermal system. Therefore, a 3D transient flow and heat transfer model considering the forced and natural convection in the reservoir is established to achieve a comprehensive CCGS analyzes. Then, the flow and temperature fields in the formation with forced and natural convection are compared. The thermal characteristics of the horizontal-well CCGS with and without convection are discussed. Also, the influences of key factors on the heat production of the horizontal-well CCGS are investigated. The differences between the horizontal-well and vertical-well CCGSs in exploiting the hydrothermal resource are analyzed. The results show that the fluid convection in the reservoir, particularly the forced convection, can greatly enhance heat transfer by affecting heat convection. The temperature field in the reservoir may return to the original state before the next heating season when a high convection velocity occurs, which is beneficial to realize 100% sustainable heat production. The horizontal length, permeability, and reverse circulation should be the primary considerations in improving heat extraction. Compared with the limited performance of vertical-well CCGS, the horizontal-well CCGS can fully extract the heat stored in the hydrothermal reservoir via a long horizontal well. The findings provide a vital reference for the CCGS scientific study and engineering application.

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Operational parameters optimization is of great significance to improve overall heat extraction performance from the hydrothermal or enhanced geothermal system. Injection flowrate, injection temperature, and production pressure are several of the easily human-controlled parameters to make the best of limited geothermal resources during the planned life. The net heat power and flow impedance are two contradictory production indexes to be optimized for efficient exploitation of long-term geothermal production. In this study, an integrated approach of finite element, multiple regression, non-dominated sorting genetic algorithm, and the technique for order preference by similarity to ideal solution is proposed and applied to the multilateral-well system to realize the optimization of geothermal development. Firstly, parametric cases coupling with thermal and hydraulic models are analyzed. Then, multiple regression is employed to obtain the net heat power and flow impedance functions considering human-controlled operational parameters and reservoir physical properties. Afterward, the multi-objective optimization algorithm is used to gain the Pareto solution set of injection and production parameters. Finally, the technique for order preference by similarity to ideal solution is employed to select the optimal combination of operational parameters for geothermal extraction. It is concluded that water loss and thermal drawdown are necessarily considered in the optimization process. The proposed approach represents global optimization. Operational parameters for the optimal case are (Q_in, T_in, p_out) which equal (49.98 °C, 62.21 kg/s, 27.44 MPa) under the conditions of this study. From the comparison between the base case and optimal case, it is observed that horizontal spread length, 2D swept area and 3D swept volume of the low-temperature scope is reduced by 77.9 m, 10 × 10⁴ m², and 16 × 10⁶ m³. Besides, the thermal breakthrough has been delayed by 1.1 years. The water loss decreases by 36.23%. The optimal case demonstrates a great improvement in production performance. Sustainable exploitation is achieved through multi-objective optimization for operational parameters. The proposed method reflects superiority, efficiency, and intelligence in geothermal development.

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