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Accurate surge pressure prediction during tripping is significant to ensure drilling safety. Based on the theory of wellbore hydraulics and heat transfer, a surge pressure prediction model considering the influence of HTHP and joints is established in this paper. The finite difference method is used to solve the wellbore flow model. Compared with the measured surge pressure, it was found that the error between the predicted and measured values after considering the above factors was only 0.89%. The influence of dimensionless joint outer diameter, joint length and drill string tripping speed on the surge pressure was further analyzed. The results show that the existence of joints increases the surge pressure. When the dimensionless joint outer diameter was increased from 0.70 to 0.91, the surge pressure increased by 76%. Neglecting the effect of the joint will cause a large error in calculating the surge pressure. In addition, the surge pressure is positively correlated with the drill string tripping speed. However, with the decrease in drill string tripping speed, the surge pressure will gradually tend to a stable value. This study provides a theoretical reference for the hydraulic design of HTHP well tripping. [ABSTRACT FROM AUTHOR]

We propose a novel multilateral-well coaxial closed-loop geothermal system (CCGS) to help realize the commercial exploitation of hot dry rock. Compared with the enhanced geothermal system, it avoids the high-cost fracturing and environmental problems by closed-loop circulation. Also, the multilateral-well CCGS can significantly increase heat production than traditional vertical-well or horizontal-well CCGSs via lateral wellbores. Then, we establish a new 3D transient model and validate it with experimental data. The temperature field is studied. The Influences of injection flow rate, number of lateral wellbores, wellbore size, thermal conductivity of insulation pipe, and reservoir temperature are analyzed. Results show that the lateral wellbores are vital to improving heat extraction, and a viable lateral-wellbore spacing is determined. The total energy output rises linearly with time, while the extracted heat in the initial stage is limited though the thermal power is high. There is an appropriate injection flow rate by comparing the total energy output and dissipation. A smaller wellbore size is preferred due to the minor effect. A reasonable thermal conductivity exists for the insulation pipe. Particularly, the high-temperature geothermal resources at various depths can be efficiently developed through the multilateral-well CCGS given the effect of reservoir temperature. • A multilateral-well coaxial closed-loop geothermal system (CCGS) is proposed. • A new 3D transient flow and heat transfer model is established. • The temperature field in the multilateral-well CCGS is discussed. • The effects of important factors on heat extraction are investigated. • Multilateral-well CCGS realizes the efficient development of hot dry rock. [ABSTRACT FROM AUTHOR]

The complex fracture network is necessary to provide flow and heat transfer channels for working fluid of an enhanced geothermal system (EGS). It is significant to compare heat extraction performances of different complex fracture networks and to provide suggestions for fracturing operation of an EGS. Based on a 3D thermal-hydraulic-mechanical coupled model, this paper studies heat extraction performances of 11 different complex fracture networks with natural and hydraulic fractures for a multilateral-well EGS. Effects of natural and non-planar fractures on multilateral-well EGS performance are investigated. Influences of primary fracture stage quantity on multilateral-well EGS performance are studied. Contributions of primary and secondary fractures to heat extraction of multilateral-well EGS are compared. Results indicate that natural fractures should be considered when accurately estimating multilateral-well EGS performance. The model with planar fracture networks overestimates production temperature and thermal power of multilateral-well EGS. It is suggested that 3 stages of primary fractures are beneficial for multilateral-well EGS performance. Fracturing operation of multilateral-well EGS should focus on generating long fractures to connect natural fractures far from lateral wells rather than inducing numerous fractures around lateral wells. Simulation results provide significant suggestions for the fracturing operation of multilateral-well EGS. • Effects of complex fracture networks with natural & hydraulic fractures are studied. • A thermal-hydraulic-mechanical coupling model of multilateral-well EGS is presented. • Model with planar fracture networks overestimates multilateral-well EGS performance. • 3 stages of primary fractures are beneficial for multilateral-well EGS performance. • Long hydraulic fractures connecting with more natural fractures should be created. [ABSTRACT FROM AUTHOR]

• A 3D multiscale model couples flow and thermal processes of borehole and reservoir. • The model is validated by field data and an analytical solution respectively. • The flow field and temperature field in formation are analyzed comprehensively. • Thermal performances under various key factors for OLGS are investigated. With the increasing concern on drilling costs in geothermal industry, more attention has been paid to the open loop geothermal system (OLGS) in a single well. For the OLGS, a vertical wellbore is generally drilled to the geothermal reservoir. Then, the injection and production intervals are perforated in the upper and lower wellbore and an insulated inner tube is installed in the wellbore. The working fluid is injected into the reservoir from the injection interval, then extracts heat from the reservoir, and finally returns to the ground surface through production interval and insulated tube. Thus, the injection and production can be accomplished in a single well, which avoids drilling an injection well and reduces drilling costs. However, few studies were conducted to investigate the heat extraction performance of OLGS comprehensively. Hence, a 3D unsteady-state multiscale model considering fluid flow and heat transfer processes of the borehole and reservoir is established. The model is verified by field data and an analytical solution respectively in various computational regions. Subsequently, the results indicate that a large flow rate is beneficial to obtain higher thermal power. The injection-production interval affects the OLGS performance significantly, and a longer interval could avoid the early breakthrough of circulating fluid effectively. And the permeability plays a remarkable role in heat production, while the influence of porosity is negligible. Furthermore, the fractures distribution near the wellbore may result in a more serious thermal breakthrough. And it is crucial to employ the insulation tube with low thermal conductivity to reduce the heat loss. The temperature field analysis depicts that the OLGS has a larger impact scope than conventional BHE, which acquires greater heat extraction performance. The main findings can offer guidance for the field application and optimal design of the OLGS. [ABSTRACT FROM AUTHOR]

Abstract The borehole heat exchanger (BHE) has been widely used for geothermal development, but its heat production is limited. To enhance the heat transfer, an open loop geothermal system (OLGS) is proposed to exploit hydrothermal resources. For OLGS, the injection and production intervals are perforated in upper and lower wellbore and an insulated inner tube is installed in the wellbore. The OLGS provides a hydraulic circulation in a single well. The working fluid is injected into the reservoir from the injection interval, then extracts heat from the reservoir, and finally returns to surface through production interval and insulated tube. Currently, few studies were conducted to comprehensively investigate the performance of OLGS. Hence, a 3D unsteady-state numerical model considering fluid flow and heat transfer processes of borehole and reservoir is established. Subsequently, the dynamic response of temperature field, pressure field and velocity field are analyzed. The results indicate that the OLGS has a larger impact scope than conventional BHE, which acquires greater heat extraction performance. And a longer injection interval is beneficial to reduce the injection pressure. Then, the elastic energy plays a remarkable role in the fluid production and the change of reservoir pressure. Furthermore, a high-velocity region occurs near the wellbore, which can lead to the serious thermal breakthrough. Therefore, there is a need to improve the local permeability away from the borehole. The main findings of this study can offer guidance for the optimal design of OLGS. [ABSTRACT FROM AUTHOR]

Abstract The coaxial borehole heat exchanger (CBHE) can effectively develop deep geothermal resources with larger heat exchange area than traditional U-tube BHE. However, few studies focus on its parameters optimization and temperature field dynamic response. Hence, an unsteady-state heat transfer model is established for a deep CBHE. The finite difference method is adopted to solve the model, and the model is validated with experiment data. The effects of key factors on the CBHE performance are comprehensively analyzed. Simulation results depict that the outlet temperature decreases greatly at the initial stage, and then remains relatively stable. There is a critical value for flow rate to obtain higher thermal power with appropriate pressure drop. The insulated pipe can significantly reduce heat dissipation of working fluid in central pipe. There is an optimal insulation length in terms of heat insulation effect and costs. The thermal conductivity of cement sheath has remarkable influence on the thermal process and partial cementing with high thermal conductivity cement is favorable. The temperature field analysis shows that the impact scope is limited, and the temperature in the reservoir before the next heating period would almost recover its original condition, verifying the sustainability of CBHE system. Highlights • An unsteady-state heat transfer numerical model for CBHE is presented. • The simulation values show particularly good agreement with field data. • The temperature field dynamic response of CBHE is analyzed comprehensively. • Influences of key factors on the heat production performance of CBHE are studied. [ABSTRACT FROM AUTHOR]

Abstract A CO 2 enhanced geothermal system (EGS) with multilateral wells is proposed to exploit hot dry rock (HDR) in this paper. For this EGS, several upper injection and lower production multilateral wells are sidetracked from one main wellbore in HDR. CO 2 is injected from the injection multilateral wells into the reservoir and then extracts heat. Subsequently, high temperature CO 2 is produced from production multilateral wells and returns to the surface through the insulated tubing in the main wellbore. In this study, a 3D fluid flow and heat transfer model for this CO 2 -EGS is established. The effects of key factors on the heat extraction performance of this CO 2 -EGS are studied. The performances of double-well and multilateral-well CO 2 -EGS are compared. The results indicate that multilateral-well CO 2 -EGS has a greater heat extraction performance than conventional double-well CO 2 -EGS. The lower production pressure, more multilateral wells and longer multilateral wells can improve CO 2 -EGS performance, while the lateral-well diameter has negligible effects on CO 2 -EGS performance. The injection mass flow rate and well spacing should be optimized to obtain a stable output thermal power and long enough service-time. This work offers good guidance for the optimization design of CO 2 -EGS with multilateral wells. Highlights • A novel CO 2 -EGS with multilateral wells is proposed. • A fluid flow and heat transfer model for multilateral-well CO 2 -EGS is presented. • Effects of injection rate & production pressure on CO 2 -EGS performance are studied. • Effects of lateral-well geometrical parameters on CO 2 -EGS performance are studied. • Performances of double-well and multilateral-well CO 2 -EGS are compared. [ABSTRACT FROM AUTHOR]

The downhole coaxial heat exchanger (DCHE) is expected to exploit medium-deep geothermal resources because of its large heat transfer area. For this geothermal system, the working fluid is injected from the annulus and produced from the central insulated tubing. There have been many studies on the heat extraction performance of DCHE. However, to the best of our knowledge, most previous heat transfer models did not consider the fluid flow in the reservoir, which has a significant effect on DCHE performance. Thus, an unsteady-state heat transfer model considering heat conduction and heat convection of reservoir is presented. The finite difference method is employed to solve the mathematical model. The temperature distribution in the wellbore and nearby reservoir during the exploitation process are analyzed. Subsequently, the effects of the key factors, including flow velocity in reservoir, aquifer thickness, and thermal conductivity of cement on the heat extraction performance are studied. The simulation results depict that the temperature decreases sharply near the wellbore. The temperature impact scope in the reservoir without geothermal fluid is about 20 m, while it reaches 40 m in the aquifer. This indicates that the fluid flow in the reservoir can enhance the heat transfer of DCHE and improve the heat extraction performance. The increase of flow velocity in reservoir will increase the outlet temperature and thermal power. As the aquifer thickness increases, the outlet temperature and thermal power increase. Besides, the outlet temperature and thermal power have a remarkable decrease at the initial stage, but then remains relatively stable. The findings can offer guidance for optimal design of DCHE geothermal system. [ABSTRACT FROM AUTHOR]

A novel enhanced geothermal system with multilateral wells is proposed to extract heat from hot dry rock in this study. For this EGS, one main wellbore is drilled to hot dry rock. Several injection and production multilateral wells are side-tracked from the main wellbore in upper and lower formation, respectively. An insulated tubing is installed in the main wellbore. The working fluid is injected from the annulus and injection wells and then extracts heat from the hot dry rock reservoir. Subsequently, the working fluid is produced from production wells and returns to surface through the insulated tubing. In this study, an unsteady-state fluid flow and heat transfer 3D model is presented to investigate the heat extraction performance of the multilateral-well EGS. The model is verified by a known analytical solution. The temperature and velocity fields of the multilateral-well EGS are analyzed and heat extraction performances of four various well types are compared. The results indicate that the output thermal power, production temperature, heat extraction ratio and accumulative thermal energy of the multilateral-well EGS are higher than those of conventional double vertical wells EGS. This study provides a better alternative for EGS to obtain greater heat extraction performance. [ABSTRACT FROM AUTHOR]

The Closed Loop Geothermal System (CLGS) is a novel method for exploiting geothermal energy. However, to the best of our knowledge, few studies have investigated the heat production performance of CLGS, which is particularly important for its field application. Hence, in this paper, an unsteady-state heat transfer model for CLGS is presented that considers variation of convection heat transfer coefficient. The finite difference method is employed to solve the mathematical model. The proposed model is verified with an analytical solution. Subsequently, the temperature field of CLGS is analysed, and influences of key parameters on CLGS performance are studied. The results indicate that the production process of CLGS can be divided into three sections: decreasing region, transition region and stable production region. The flow rate, inlet temperature and horizontal section length have significant influences on CLGS performance. To obtain the higher thermal power and appropriate outlet temperature, there is a critical value for flow rate. Additionally, the longer horizontal section length and larger temperature difference between the working fluid and formation are beneficial for improving CLGS performance. Finally, small wellbores should be drilled for CLGS. The main findings of this work can offer guidance for the design and optimization of CLGS. [ABSTRACT FROM AUTHOR]

Heat extraction from a geothermal reservoir using a single U-tube in a reservoir has been applied in the field. The advantages include extracting only heat instead of water. Based on the geological data of Bazhou geothermal field, China, a three-dimensional steady state numerical model, including the U-tube, wellbore and reservoir is established to analyze the entire flow field comprehensively. The performance of the exchanger is studied through investigation on the influences of four important geological parameters of depth, porosity, permeability and heterogeneity of the formation. Simulation values are validated by results obtained from field tests. Results indicate that the overall flow velocity in the geothermal reservoir is relatively low compared with the flow in the wellbore mainly due to the high flow resistance. In addition, the mass flow rate can hardly affect the flow in the reservoir. Under the conditions of this study, the depth of the geothermal field, the reservoir porosity and the heterogeneity of the geothermal formation may hardly affect the performance of the U-tube. However, if it is possible, it is better to install the single U-tube in the homogeneous area of the reservoir, which may obtain better heat extraction effect. The results in this paper could provide implications for further study of geothermal energy exploitation. [ABSTRACT FROM AUTHOR]

A downhole heat exchanger (DHE) geothermal system has been widely used for space heating in commercial and residential buildings. The DHE is important in heat extraction performance of the geothermal system. Nevertheless, to the best of our knowledge, the single U-tube is the most common configuration for a DHE. An investigation of the heat extraction performances of various DHE configurations is important. In this study, a 3D unsteady-state numerical model was established to describe the fluid flow and thermal processes of a DHE system. The model was validated by field experimental data. Three DHE configurations were evaluated, including a spiral tube, multiple-tubes with a parallel connection, and multiple-tubes with a serial connection. Subsequently, the influences of the key geometrical parameters for the three configurations on the DHE performance were analyzed. The DHE performances of the three configurations were compared. The simulation results showed that the outlet temperature and thermal power of the serial connection DHE were higher than those of the parallel connection with an equal number of tubes. Among the three configurations, the spiral tube DHE showed the greatest heat extraction performance. The key findings of this study could offer guidance for the optimisation and design of a DHE. [ABSTRACT FROM AUTHOR]

Hydrothermal jet technology is a novel drilling technology and has just been studied for several years. This technology uses a high temperature and high velocity jet to break rocks and has the potential to be more efficiently advantageous than conventional techniques for deep hard formations and geothermal well drilling. This paper presents a multi-orifice hydrothermal jet model with cooling water to investigate the features of the flow field by CFD methods. First, a transient impact flow field is analyzed. Besides, influences of jet temperature, cooling water temperature and cooling water pressure on the flow field and annular cooling effect are predicted and compared. Predictions are found to be in good agreement with the published heat transfer theory for hydrothermal jet temperatures ranging from 650 K to 800 K. Results indicate that ambient cooling water envelops the high temperature hydrothermal jet in the middle. There is a second peak of bottomhole temperature and the pseudo-critical point is located. Controlling the hydrothermal jet temperature at the vicinity of the pseudo-critical point or far larger than the pseudo-critical temperature can have a better heat transfer effect. The simultaneous effect of both the hydrothermal jet and cooling water results in the non-uniform distribution of bottom temperature, which leads to higher rate of penetration. It is recommended that the cooling water pressure should be larger than the hydrothermal jet pressure, which might result in higher rate of penetration and cool the coiled tubing and borehole wall simultaneously. Results in this paper are beneficial to the parameters design for the hydrothermal jet drilling technology. [ABSTRACT FROM AUTHOR]

Geothermal energy is one of the most promising renewable resources and an alternative to conventional fossil fuels. Downhole heat exchanger is the most common form used for exploitation. Based on the geological data of Xinji geothermal field, China, the entire flow field of a single U-tube downhole heat exchanger inside the borehole is analyzed comprehensively by a three-dimensional steady state model. Thermo-physical properties of working fluid are considered specifically. The performance of the exchanger is studied through investigation on the influences of mass flow rate, degree of depth, length of the U-tube and temperature difference between working fluid and geothermal field. Finally, conversion rates are calculated and comparison of influences between four parameters has been made quantitatively via grey correlation analysis. Simulation values are validated by results obtained from field tests. For geothermal fields with high temperature, it is better to apply a large volume of circulating water to obtain higher heat extraction rate. The length of the U-tube is the most significant impact factor on the conversion rate, with the following sequence being temperature difference, mass flow rate and degree of depth. Therefore, lengthening the U-tube is the most efficient and cost-effective way for the exploitation of geothermal energy. [ABSTRACT FROM AUTHOR]

A unified model considering heat source/sink effects is developed to investigate flow and thermal behavior of pure CO 2 injection for sequestration, EOR, and fracturing. The model is based on mass conservation, momentum, and energy balance and employs a state-of-the-art equation of state and transport models of CO 2 to calculate in-situ thermodynamic and transport properties, respectively for a certain section of wellbore. Temperature, pressure, and properties of CO 2 are coupled in the depth direction using an iteration scheme. CO 2 , the annular fluid, and formation are coupled to calculate the overall heat transfer coefficient in the radial direction. The model is validated using data from both field and numerical simulation. Heat transfer mechanism involving thermal resistance and heat source/sink in wellbore is analyzed. A sensitivity study is also conducted to investigate the influence of factors on bottomhole pressure and temperature. Results indicate that annular fluid is the most important thermal resistance in the wellbore. The natural convection of liquid in annulus is much stronger than that of gas, which can greatly promote heat transfer. The heat source/sink effects consist of gas expansion, frictional heat, and Joule–Thomson effect. Their contributions to wellbore heat transfer vary intensely with mass flow rate or Reynolds number. Wellbore heat transfer is dominated by the formation at low flow rates, in which the equilibrium state can be attained at shallow depths. Both bottomhole pressure and temperature change linearly with injection pressure and temperature; there is a non-linear relationship between bottomhole pressure/temperature and time. [ABSTRACT FROM AUTHOR]

Hydrothermal jet technology is a new drilling method in which the rocks are broken by the combination of thermal spallation and high velocity impact effect. This technology uses a high temperature and high velocity jet to break rocks and has the potential to be more economically advantageous than conventional techniques for geothermal well drilling. Previous related studies primarily examine numerical simulation on one hydrothermal jet flow field, and to the best of our knowledge, no specific study addresses the thermo-physical interaction between wellbore fluid and ambient rock. This paper presents a multi-orifice nozzle model to investigate the features of a flow field with multiple hydrothermal jets and the heat transfer to ambient rocks. A transient impact flow field with multiple hydrothermal jets is analyzed in terms of axial temperature, bottomhole temperature, and bottomhole pressure. Also, downhole velocity field is specifically investigated. Next, influences of time, jet temperature, and jet pressure difference on the flow field and heat transfer between wellbore fluid and ambient rocks are predicted and compared. The results indicate that the bottomhole central temperature and pressure are higher than the two sides under multiple hydrothermal jets conditions, which is similar to the flow pattern with a single jet. Moreover, the distribution of axial velocity has three peaks at cross-sections, and the centerline axial velocity is the highest. There is a negative relationship between the maximum radial velocity and the ratio between axial distance and nozzle diameter (L/D). Also, the position of the maximum radial velocity moves to the side wall as the ratio (L/D) increases. Second, the bottomhole temperature increases uniformly with increase in jet temperature. The bottomhole temperature becomes decreasingly sensitive to the variance of pressure difference, and the bottomhole pressure exhibits a ladder distribution. Finally, while an increase in either jet temperature or pressure difference can enhance the multiple hydrothermal jets heat transfer effect, increasing the jet temperature is considerably more effective. All of these results are beneficial to the parameters design for the hydrothermal jet drilling technology. [ABSTRACT FROM AUTHOR]

• A horizontal-well CCGS is presented to develop the deep hydrothermal resource. • A 3D model considering complex convection in the reservoir is established. • The influence of convection in the reservoir on heat extraction is investigated. • The performances of horizontal-well CCGS with various key factors are analyzed. • The difference between the horizontal-well and vertical-well CCGSs is compared. 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. [ABSTRACT FROM AUTHOR]

The EGS (enhanced geothermal system), particularly the multi-fracture EGS, is expected to be a key method to effectively develop the deep geothermal resource. The accurate modeling of flow and heat transfer in an EGS reservoir, particularly the fracture, is challenging. Therefore, an improved model considering the LTNE (local thermal non-equilibrium) and non-Darcy flow in a rough fracture is proposed. Then, the flow and temperature fields in the matrix are comprehensively investigated. The relationship between matrix and fracture is analyzed. The LTNE and LTE (local thermal equilibrium) are compared to determine the specific thermal process in the fracture. The difference between non-Darcy and Darcy flow in the fracture is studied to evaluate the actual flow behavior. The influences of the rough and smooth fractures are discussed. The results show that the direct contribution of the fracture to production is close to 100%, while the matrix mainly plays an indirect role as a heat source. There is a noticeable local low-pressure region induced by fluid density change in the matrix, while the heat conduction, especially rock conduction, dominates the heat transfer of the matrix. The intense heat convection in the fracture leads to a heat transfer rate difference of up to 5000 times compared with the heat conduction of the matrix, causing a marked thermal breakthrough. The non-Darcy flow represents the actual flow in the fracture, and the velocity is significantly lower than the Darcy flow velocity when a high injection flow rate is employed in the EGS production. The total extracted heat difference between rough and smooth fractures reaches 7.51%. • A model considering LTNE and non-Darcy flow in a rough fracture is established. • The heat conduction of the rock dominates the heat transfer process in the matrix. • The direct contribution of the EGS fracture to production is close to 100%. • The heat transfer rate in EGS fracture can be thousands of times that in the matrix. • The non-Darcy flow may well represent the actual flow behavior in EGS fracture. [ABSTRACT FROM AUTHOR]

Aquifer thermal energy storage (ATES) is an effective time-shifting thermal energy storage technology. Considering the enormous technical and economic input of the well pattern layout, the storage volume of a single well needs to be improved. This study proposes a novel aquifer thermal energy storage system in which several multilateral wells are side-tracked from the vertical well in the aquifer. Radial branches can enhance the connectivity of the wellbore to the aquifer. The research creates an unsteady-state 3D model to analyze the novel ATES system's fluid flow and heat transfer performance. The model is verified using experimental data. The temperature and pressure fields of the multilateral-well ATES are analyzed. Additionally, the effects of injection rate, radial branch length, number of radial branches, aquifer temperature, and aquifer thickness on system performance are studied. According to the research results, compared with the vertical wells, the performance coefficients of the multilateral hot well and cold well increase by 113.06 and 39.25, respectively, and the economic efficiency increases by 14.19% and 23.87%, respectively. The findings can provide a crucial reference for scientific research and the application of aquifer thermal energy storage systems with multilateral wells. ● A novel multilateral-well aquifer thermal energy storage (ATES) system is proposed. ● A 3D fluid flow and heat transfer model for multilateral-well ATES is presented. ● Pressure and temperature fields in aquifer of multilateral-well ATES are discussed. ● The influence of various factors on the operational performance of multilateral-well ATES is investigated. ● The behaviors of vertical-well ATES and multilateral-well ATES are compared. [ABSTRACT FROM AUTHOR]

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. • We present tools for estimation of statistical properties of fracture networks from cross-hole tests. • Particle-based algorithms are used to generate synthetic data for deep neural network (DNN) training. • Regionalized training strategy reduces the cost of data generation over the full parameter space. • We provide a balance analysis between DNN's generalization, accuracy and training cost. [ABSTRACT FROM AUTHOR]