Hot Dry Rock Research Articles

Heat extraction performance analysis of a multilateral wells exploitation system with super-long gravity heat pipe

Super-long gravity heat pipe (SLGHP) system has previously been proven as an environmental and efficient technology in exploiting hot dry rock. This study proposed a multilateral-well SLGHP system to achieve the purposes of improving heat extraction performance and fully utilizing fractured reservoirs. The heat extraction performance was investigated by establishing a 3D thermal–hydrologic (TH) coupling model. It is found that, compared to traditional single-well SLGHP system, after 30 years of operation, multilateral-well SLGHP system has improved the heat extraction rate by 38.1 kW and increased the heat compensation by 61×1012 J. The utilization ratio of geothermal reservoirs has been improved by 8.1%. In addition, the effects of different well layouts on heat extraction performance are also studied. Injection wells at the top of fractured reservoir increase the heat extraction rate by 6.4 kW compared to placing them in the bottom area. The relative changes of wellbore location in fractured reservoirs have slight effects on heat extraction performance. This study could provide good suggestions for the SLGHP system to obtain better heat extraction performance.

Key Technology of 240°C High-Temperature Drilling Fluid

Hot dry rock (HDR) is a renewable green and clean energy source from the depths of the earth. It has the characteristics of high efficiency, environmental protection, safety, and stability, and is not restricted by seasons and climates. Difficulties in exploration and development include deep burial, formation fragmentation, abnormal pressure, high well temperature gradient, and high hole bottom temperature (usually >200°C), key technical issues such as sticking. This paper analyzes the difficulties of high-temperature resistant drilling fluid in dry hot rock: temperature resistance of treatment agents and systems, foaming, leakage, wellbore instability, and other problems. The optimization and experiment of treatment agents such as loss reducers, high-temperature protective agents, etc., and research on drilling fluid technology resistant to high temperature of 240°C in hot dry rock drilling has been carried out, through the optimization experiment of pulping materials and high temperature treatment agent. The main formula is: 3%-4% Sodium bentonite + 2%-3% HPS + 3%-4% SPNH + 2%-4% GJA + 3%-4% JSJ-1 + 0.5%-1.0% GGDP + 1% GBHJ. The drilling fluid formula has good rheology after hot rolling at 240°C, API filtrate loss<5ml, the viscosity recovery rate is over 90%. It provides effective technical support guarantee for promoting the exploration and development of hot dry rock resources in China.

Development and Application of the Large‐Scale Simulation Experimental System for the In Situ Long‐Term Injection and Production of EGS

With the proposal of the double carbon target, the exploration, development, and efficient utilization of geothermal energy is an important field for the energy development. Meanwhile, there also exists a series of challenges. With regard to the injection and production process of enhanced geothermal system, researchers mainly focus on the numerical simulation. As the conceptual model is developed based on several assumptions, the results are unable to truly reflect the heat transfer and seepage process of the fluid. Unfortunately, there are few reports on the large‐scale physical simulation experiments. Considering the present research situation, we have developed a large‐scale physical simulation experiment system in which the in situ long‐term injection and production process of hot dry rock (HDR) can be simulated. The experimental system includes several components: high‐temperature and high‐pressure true triaxial chamber and loading system, multiwell injection system, acoustic emission monitoring system, and flow and temperature monitoring system. By designing the experiments, the performance evaluation of the temperature variation, fluid flow, and heat energy extraction in the long‐term HDR injection and production process can be completed. Taking the granite outcrop rock sample, which is from the Gonghe Basin in Qinghai Province, as an example, we have conducted the long‐term injection and production experiment. The production temperature and flow rate were monitored and discussed. The results showed that the experimental system can simulate the heat transfer and seepage process of the fluid. More working conditions can be designed for the research of the long‐term injection and production process. Hopefully, this will provide an experimental platform for the efficient development of HDR.

Experimental Study on the Evolution of Fracture Aperture of Single‐Fracture Granite during Liquid Nitrogen Cold Shock Cycling

Fractures in hot dry rock (HDR) reservoirs are the locations where heating fluid flows exchange heat with the HDR matrix. Cold shock with liquid nitrogen is one method for stimulating cracks. This study investigates the evolution law of fracture aperture under cold shock with liquid nitrogen. The real‐time high‐temperature triaxial servo control rock testing machine was used to conduct permeability experiments to examine the fracture aperture of single‐fracture granite during liquid nitrogen shock cycles at various temperatures. The effects of pore pressure, temperature, and shocking cycles on the fracture aperture are analyzed, and the difference in fracture aperture variation under liquid nitrogen cooling and natural cooling modes is compared. The results showed that (1) during liquid nitrogen cooling, the fracture aperture expands as pore pressure rises; the effect of pore pressure on the fracture aperture becomes more robust as the number of liquid nitrogen shocking cycles and initial temperature increases; (2) under 1‐2 soaking cycles, fracture aperture decreases as the temperature rises. Under two or more soaking cycles, the fracture aperture first increases and then decreases with increasing temperature; (3) when the initial temperature of fractured granite is 100°C, the fracture aperture is not significantly changed by repeated cold soaking cycles. However, with a higher initial temperature, the fracture aperture develops with more liquid nitrogen cold soaking cycles. The liquid nitrogen cooling method is more conducive to increasing the fracture aperture than natural cooling. The experimental results can provide primary experimental data for future research into controlling the evolution of granite cracks.

Study on the mechanical properties of granite responses of cyclic heating and water cooling considering microcosmic and energy.

Investigating the coupling effects of temperature levels and heating-water cooling cycles on the physical and mechanical responses of HDR (hot dry rock) is a vital issue during the exploitation of geothermal energy. In this study, the physical properties of granite specimens were measured first after each heating and water-cooling cycle. Then, uniaxial compressive tests were conducted on those granites to obtain their mechanical properties. With the increase in heating temperature (T) and cycles of heating and water cooling (N), P-wave velocity, uniaxial compression strength (UCS), and elastic modulus (E) showed a decreasing tendency, and the decrease of those four properties corresponding to T variation is greater than N variation. Due to the α-β phase transition of quartz happening at 573°C, the density UCS and E of granite decreased rapidly when the heating temperature increased from 450°C to 600°C at N = 1. With the increase of T and N, the failure mode of granite gradually changes from tensile failure to shear failure and, finally, comminute failure. The failure mechanism of granite gradually transfers from brittleness-dominated to ductility-dominated due to accumulated thermal damage. Finally, X-ray diffraction (XRD) and scanning electron microscope (SEM) were used to determine the damage mechanism of cyclic heating-cooling. The micro test results show that the high-temperature treatment changes the mineral composition and the microcracks number of the granite and finally affects the macroscopic physical and mechanical properties. The study conclusions of this manuscript are important for exploiting geothermal resources.

A novel experimental setup for axial-torsional coupled vibration impact-assisted PDC drill bit drilling.

The geological conditions of hot dry rock (HDR) reservoirs are complex. The geothermal mining of HDR faces major challenges in the drilling and construction of wells, fracturing to create storage, and flowing to extract heat. Vibration impacts help improve the rock-breaking efficiency, where the axial-torsional coupled vibration impact technology can increase the bit penetration depth and reduce the stick-slip effect. To study the feasibility and efficiency of the axial-torsional-coupled vibration impact-assisted Polycrystalline Diamond Compact (PDC) bit to break high-temperature and high-pressure rocks, a new experimental setup was designed. The system includes a drilling fluid circulation system, an axial-torsional coupled impact drilling system, a formation simulation system, and a data acquisition and control system. This setup can produce a rock-breaking torque of 2000 N·m, a drilling speed of 200rpm, a weight on bit of 100kN, an axial vibration frequency of 100Hz, and a torsional vibration frequency of 50Hz. It can simulate the formation pressure of 70 MPa and the rock temperature of 400 °C. A series of rock-breaking drilling experiments were successfully conducted using this setup. The results show that the axial-torsional coupled vibration-impact assisted PDC bit has a good performance in breaking high-temperature and hard rocks, which can accelerate the application of this new technology in deep formation drilling.

Simulation of Temporary Plugging Agent Flow State in Fractures of Hot Dry Rock Considering Environmental Changes

Simulation of Temporary Plugging Agent Flow State in Fractures of Hot Dry Rock Considering Environmental Changes

Real-time automatic identification methods for downhole whirling based on mechanical specific energy model of drill bit

In the drilling exploitation of hot dry rock for geothermal energy, whirling is one of the main low-efficiency conditions that affect the efficiency of Polycrystalline Diamond Compact (PDC) bit in horizontal well drilling. Realizing automatic and real-time identification with whirling is of great significance to save non-productive time and ensure drilling safety and benefit. In this paper, we first constructed a real-time drilling mechanical specific energy (MSE) model combined with while-drilling testing to reflect the real-time drilling conditions. The MSE model is used to normalize multi-source parameters. Secondly, we constructed a Back Propagation (BP) artificial neural network (ANN), and then the normalization effect verification, optimization of network parameters, and identification effect verification of whirling were carried out. The final results show that the established MSE model has a favourable effect on data normalization, which could also reduce the complexity of the required network model, and shorten the training time by 20–30 s/step. The optimal algorithm is Trainscg, whose optimal number of hidden layer nodes is 5, and the optimal maximum number of iteration steps is 1000. The established ANN model can accurately identify whirling based on MSE, the accuracy is about 0.94, and the average relative error is 1.3%. The method established in this paper provides a reference for the automatic identification of various low-efficiency conditions based on MSE.

Two stages power generation test of the hot dry rock exploration and production demonstration project in the Gonghe Basin, northeastern Qinghai-Tibet plateau, China

Two stages power generation test of the hot dry rock exploration and production demonstration project in the Gonghe Basin, northeastern Qinghai-Tibet plateau, China

A Distributionally Robust Optimization Scheduling Model for Regional Integrated Energy Systems Considering Hot Dry Rock Co-Generation

A Distributionally Robust Optimization Scheduling Model for Regional Integrated Energy Systems Considering Hot Dry Rock Co-Generation

Numerical simulation of the thermo-hydro-chemical coupling in enhanced geothermal systems: Impact of SiO2 dissolution/precipitation in matrix and fractures

The hydraulic properties of reservoir matrix and fractures, as well as the production performance of enhanced geothermal systems, can be significantly affected by chemical dissolution and precipitation. Therefore, chemical reaction effects must be considered when exploiting dry hot rock resources. To investigate the impact of chemical effects, this study incorporates the dissolution/precipitation process of silica in the reservoir matrix and fractures and establishes a thermo-hydro-chemical coupling dual-well model that includes a main fracture and multiple secondary fractures. Results indicate that under-saturation injections increase the mechanical aperture of the main fracture by up to 0.066 mm and permeability by up to 217.3 % over a 30-year production period. Oversaturation injections decrease the mechanical aperture by up to 0.035 mm and permeability by up to 66.1 %. Reservoir matrix permeability exhibits a noticeable zonal distribution, with a maximum increase of 22.6 % in the under-saturation injection and a maximum decrease of 4.1 % in the saturation injection over a 30-year production period. Chemical reactions have a significant impact on the average production pressure. Neglecting chemical reactions in the matrix may result in overestimation or underestimation of changes in production pressure. These results provide a reference for the sustainable production of EGS in the long term.

Investigation of Rock Mechanical Properties under Liquid Nitrogen Environment

In order to promote sustainable energy development and reduce the impact of fossil fuels on the environment, it is crucial to strengthen the development and utilization of clean and renewable geothermal energy. Liquid nitrogen fracturing, as an emerging waterless fracturing technology, has outstanding advantages in rock fracturing effect and thermal exchange ability with hot dry rock and is more environmentally friendly. In order to evaluate the influence of liquid nitrogen on the mechanical properties, acoustic emission characteristics, and cross-sectional crack propagation characteristics of granite at different initial temperatures, this paper carried out three-point bending tests and acoustic emission detection on granite treated by high-temperature heating and liquid nitrogen cooling. Finally, based on the cross-sectional scanning test, the expansion characteristics of microcracks in granite were analyzed. The results show that the higher the initial temperature of granite, the stronger the cold impact of liquid nitrogen on granite, and the faster the rock’s mechanical performance declines. The acoustic emission ringing count is closely related to the development of microcracks in granite, and as the initial temperature of granite increases, the more ringing counts there are, indicating that the huge temperature difference induces more microcracks inside the rock. In addition, the cold impact of liquid nitrogen can effectively promote the fracturing of granite. After liquid nitrogen treatment, the fractal dimension of the granite cross-section increases, the shape of the cross-section becomes rough, and many micropores appear. This study can provide a scientific basis for the engineering application of liquid nitrogen fracturing technology.

Modeling rock damage during the long-term production process for hot dry rocks: Effects of additional conductivity on the production performance and economic efficiency

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.

Heat Extraction Evaluation of CO2 and Water Flow through Different Fracture Networks for Enhanced Geothermal Systems

Enhanced geothermal system (EGS) technologies have been developed to improve geothermal energy production from hot dry rock (HDR). In this study, discrete fracture network models for geometric topological networks that consider different parameters (the fracture density and the fracture length index) were built on the basis of fractal geometry theory. The heat extraction processes of CO2 and water as the working fluid through different discrete fracture networks were simulated with the application of the thermal–hydraulic–mechanical (THM) coupled method. A series of sensitivity analyses were carried out to reveal the influences of fracture parameters on heat transfer processes. Based on the simulation results, heat extraction efficiencies and temperature distributions in the reservoir of CO2 and water as the working fluid were compared, which showed that CO2 as the working fluid can bring a faster thermal breakthrough. It was found that the fracture length index a = 2.5 and the fracture density I = 5.0 can provide the highest heat extraction rate compared with other cases. This study provides a detailed analysis of fracture parameters and working fluids, which will contribute to the optimized management of geothermal energy production.

Simulation of thermo-hydro-mechanical coupled heat extraction process of geothermal reservoir under varied geological conditions with CO2 as working fluid

The use of supercritical CO2 as a working fluid instead of H2O in an enhanced geothermal system (EGS) for extracting heat from hot dry rock geothermal resources has garnered significant attention. The objective of this study is to investigate how different geological conditions affect the heat extraction behavior of supercritical CO2 and to reveal the efficiency differences between supercritical CO2 and H2O as working fluids in each geological scenario, providing deeper insight into reservoir management. A series of 3D coupled thermo-hydro-mechanical models were established. Besides, the impact of initial temperature, pressure, porosity, permeability, and stress field on the heat extraction of EGS with different working fluids are also analyzed and compared. Results indicate that, at the same circulating volumetric flow rate, CO2-EGS generally exhibits an earlier thermal breakthrough time, lower net energy production, and reduced heat extraction rate compared to H2O-EGS. Taking into account the extended reservoir lifespan and the relatively small pressure difference between injector and producer in CO2-EGS, CO2 is a more suitable working fluid than H2O. Furthermore, the performance of CO2-EGS is primarily affected by the initial temperature, porosity, and permeability distribution pattern, rather than the initial pressure. On the one hand, a reservoir with low initial temperature, porosity and homogeneous permeability were found to be more conducive to enhancing the average net energy production. On the other hand, reservoir vertical permeability anisotropy can have a more profound impact on CO2-EGS efficiency compared to horizontal y permeability anisotropy. Additionally, this study also examined the impact of in situ stress field. It was found that, compared to H2O-EGS, both isotropic and anisotropic in situ stress fields have a weak effect on the performance of CO2-EGS, which can be attributed to the similarity of the channelization degree of the flow field under different stress conditions.

Numerical analysis on the failure characteristics of hot dry rock subjected to axial-torsional coupled percussion with single cutter

High-hardness and poor-drillability of hot dry rock (HDR) impose great challenges on drilling-rate improvement, limiting the economic exploitation of HDR geothermal. Axial-torsional coupled percussive drilling (ATCPD), as a potential drilling-rate improvement method, might provide solutions for achieving efficient breaking of HDR. To determine the rock breaking performance of ATCPD in HDR, we investigate the failure behaviors of high temperature rocks under percussion loads. Three loading modes, i.e., static loading, axial percussion loading, and torsional percussion loading, are compared with axial-torsional coupled percussion loading (A-TCPL) mode, with respect to rock breaking capacity and failure behaviors. The effect of some important engineering parameters, e.g., rock temperature, impact amplitude and frequency, is put particular emphasis to obtain the great rock breaking performance. Results show that ATCPD is characterized by the higher amplitude force, lower torque and larger cracking volume as compared to other loading modes, thereby resulting in a growth drilling efficiency. A-TCPD shows greater performance to high temperature rocks in stress response and material damage. The maximum principal stress, tensile and compressive damage factors are positively correlated with rock temperatures under A-TCPL mode. Increasing the impact amplitude and frequency are beneficial for inducing damage on rocks with temperatures ranging from 200 to 600 °C. By using the rock breaking total displacement as the evaluation index, the optimal ratio of axial to torsional impact frequencies for greater rock breaking capacity is obtained. For rocks at 200 °C, 400 °C, and 600 °C, the optimal ratio of axial to torsional impact frequencies is 1:2, 1:3, and 2:3, respectively. The results are expected to provide new insights to enhance drilling rate for geothermal well and HDR excavation.

Review of discrete fracture network characterization for geothermal energy extraction

Geothermal reservoirs are highly anisotropic and heterogeneous, and thus require a variety of structural geology, geomechanical, remote sensing, geophysical and hydraulic techniques to inform Discrete Fracture Network flow models. Following the Paris Agreement on reduction of carbon emissions, such reservoirs have received more attention and new techniques that support Discrete Fracture Network models were developed. A comprehensive review is therefore needed to merge innovative and traditional technical approaches into a coherent framework to enhance the extraction of geothermal energy from the deep subsurface. Traditionally, statistics extracted from structural scanlines and unmanned aerial vehicle surveys on analogues represent optimum ways to constrain the length of joints, bedding planes, and faults, thereby generating a model of the network of fractures. Combining borehole images with seismic attributes has also proven to be an excellent approach that supports the stochastic generation of Discrete Fracture Network models by detecting the orientation, density, and dominant trends of the fractures in the reservoirs. However, to move forward to flow modelling, computation of transmissivities from pumping tests, and the determination of hydraulically active fractures allow the computation of the hydraulic aperture in permeable sedimentary rocks. The latter parameter is fundamental to simulating flow in a network of discrete fractures. The mechanical aperture can also be estimated based on the characterization of geomechanical parameters (Poisson’s ratio, and Young’s modulus) in Hot Dry Rocks of igneous-metamorphic origin. Compared with previous review studies, this paper will be the first to describe all the geological and hydro-geophysical techniques that inform Discrete Fracture Network development in geothermal frameworks. We therefore envisage that this paper represents a useful and holistic guide for future projects on preparing DFN models.

Characterization for 3D pore-fracture systems in Hot-Dry-Rock samples from Gonghe Basin, China

As a new type of renewable energy, Hot-Dry-Rock (HDR) is gaining increasing attention. The physical properties of rock influenced by the pore-fracture structure are vital for the hydraulic and chemical characteristics of HDR reservoirs. However, the natural pore-fracture structure in HDR has not been sufficiently studied. To fulfill this gap, we used focused ion beam-scanning electron microscopy (FIB-SEM) to construct three-dimensional (3D) digital models for four granite samples from the deep HDR reservoir in Gonghe Basin, Qinghai Province, China, and obtained the nanoscale structure of the pores and fractures in HDR reservoir for the first time. Our analysis based on the 3D models indicates that the heat transfer area in the HDR reservoir is mainly from fractures and crack-like pores, but a large amount of small but regular pores could be neglected when heat transfer in HDR is considered. The permeability of HDR reservoirs at nanoscale varies from 9.414 nD to 0.082 mD, and the high permeability in this study is accompanied by throughgoing fractures. The variation of porosity and permeability of deep HDR at nanoscale in this study is much more severe than that of shallow granites previously studied at microscale. The performance of geothermal field is significantly influenced by the size of throats and the average size and tortuosity of the interconnected pore-fracture network in HDR. The hydraulic aperture could be 32-58% lower than the mechanical aperture of the natural fractures in HDR reservoirs, which is a vital clue for the numerical simulation associated with enhanced geothermal systems.

Numerical investigation and optimization on dynamic power generation performance of enhanced geothermal system

The Enhanced geothermal system (EGS) is a crucial technology for the development and utilization of hot dry rock (HDR). However, current research typically overlooks the strong coupling characteristics of fracture seepage, wellbore transport, and thermoelectric conversion, leading to a lack of precision in describing the dynamic operational performance of EGS throughout its entire lifecycle. A dynamic numerical model of evaporator and condenser is established by the moving boundary (MB) method. Subsequently, the dynamic power generation process is integrated with a coupled thermal-hydraulic-mechanical (THM) model of the fractured reservoir, proposing a methodology for calculating the dynamic performance of EGS throughout its lifecycle.Traditional EGS models exhibit significant deviations between calculated results and actual performance due to the neglect of instantaneous changes in injection temperature and the failure to consider the nonlinear characteristics of heat exchangers. The production pressure difference is a primary factor influencing the net output power of EGS. Therefore, selecting an appropriate injection pressure is crucial to ensuring continuous power generation throughout the EGS lifecycle. By optimizing the operating parameters of the actual EGS project, the optimal injection flow rate, fracture aperture, initial fracture permeability and well spacing are 50 kg/s, 0.03 m, 1.5 × 10−10 m2 and 350 m respectively.

Heat transfer characterizations of water flow through a single fracture with integrating water dissolution effects for the Enhanced Geothermal Systems

The Enhanced Geothermal Systems (EGS) is the technology that generates connected fracture networks in the Hot Dry Rock (HDR) reservoir with the application of water as the working fluid for hydraulic fracturing and geothermal energy production. A good understanding of water flow behaviors in the single fracture is critically important because connected fracture networks are composed of individual fractures. It should be noticed that the dissolution reaction between the reservoir rock and the working fluid (water) will change the fracture surface directly and consequently affect the mass and heat transfer process of water through the rock fracture in the HDR reservoir. In this study, the heat transfer process of water flow in the single fracture by considering dissolution reactions between water and fracture surfaces of reservoir rock have been investigated under high pressure and temperature conditions. The experiments of dissolution reaction and convective heat transfer have been performed separately for different temperature conditions with the rock sample of 0.2 mm fracture aperture. The influences of reaction duration, mass flow rates and initial temperature have been presented and analyzed. It is found that mass flow rates and temperature have significant effects on measured temperature of fracture surfaces. The effects of dissolution reaction on fracture surface morphology exists but it is extremely small for heat transfer. Based on experimental and simulation data, a modified correlation that integrates reaction duration, mass flow rates and changes of thermophysical properties has been proposed and its maximum error that compares with experimental results is less than 7%. The results of this study provided an accurate correlation for simulations and characterizations of the Hot Dry Rock reservoirs, which is of great importance for geothermal energy production and optimization.