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 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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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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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 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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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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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 parameter optimization is of great significance to improve overall heat extraction performance from hydrothermal or enhanced geothermal systems. Injection flowrate/temperature and production pressure are relatively easy to control to optimize the exploitation of geothermal resources during the planned reservoir lifetime. The net heat power and flow impedance are two contradictory production indexes for describing the exploitation effect. One indicates energy efficiency from reservoirs, the other represents the mining difficulty or artificial energy input. In this study, a multi-objective optimization procedure is proposed and applied to a synthetic multilateral-well system for 30 years. Firstly, a multilateral-well geothermal model coupled with thermal and hydraulic parameters is established. Then, a multiple regression method is employed to obtain the net heat power and flow impedance functions with injection/production parameters and physical properties of the reservoir. Finally, a multi-objective genetic algorithm is used to gain a Pareto solution set of injection and production parameters. A comparison with the base case indicates the superiority, high efficiency, and intelligence of multi-objective optimization.

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Geothermal energy is clean and sustainable as an alternative to fossil energy. Hydrothermal resources are one of the most important geothermal resources in the world. A two-layer multilateral horizontal well system is proposed to exploit hydrothermal energy with circulating groundwater in this paper. One main wellbore is drilled into the geothermal reservoir. Then several injection and production multilateral horizontal wells are side-tracked from the main wellbore in the upper and lower formation, respectively. In addition, a central insulation tube is installed in the main wellbore. Low-temperature water is injected into the geothermal reservoir through the main borehole annulus and then the injection laterals, to extract heat from the reservoir. Finally, the hot water is produced from the production laterals and pumped to the ground through the central insulated pipe, to be used for residential heating and power generation. In this paper, a three-dimensional unsteady fluid flow and heat transfer model is established for the two-layer multilateral horizontal well hydrothermal geothermal system. Four typical parameters, which are average production temperature, average pressure on injection wells, average production rate and net heat power, are defined and analyzed to evaluate the extraction performance of the system. The whole development can be divided into the stable production stage and the reduction stage based on the variation trend of four characteristic parameters. Moreover, the temperature field, pressure field and velocity field of the formation at different times are compared and investigated to describe visually the dynamic change of the hydrothermal system. As a result, the low temperature region expands like a funnel and the high pressure region shows like a sphere. This paper provides a novel and efficient two-layer multilateral horizontal well system and reveals the production characteristics of the system for the exploitation of hydrothermal resources.@en