Temperature dependence of mechanical properties and damage evolution of hot dry rocks under rapid cooling

Understanding the differences in mechanical properties and damage characteristics of granitoid under high temperatures is crucial for exploring deep geothermal resources.This study analyzes the evolution of the acoustic emission(AE)characteristics and mechanical parameters of granodiorite and granite after heating and water cooling by uniaxial compression and variable-angle shear tests under different temperature gradients.We identify their changes in mesostructure and mineral composition with electron probe microanalysis and scanning electron microscopy.Results show that these two hot dry rocks have similar diagenetic minerals and microstructure,but show significantly different mechanical and acoustic characteristics,and even opposing evolution trends in a certain temperature range.At the temperatures ranging from 100℃to 500℃,the compressive and shear mechanical properties of granodiorite switch repeatedly between weakening and strengthening,and those of granite show a continuous weakening trend.At 600℃,both rocks exhibit a deterioration of mechanical properties.The damage mode of granite is characterized by initiating at low stress,exponential evolutionary activity,and intensified energy release.In contrast,granodiorite exhibits the characteristics of initiating at high stress,volatile evolutionary activity,and intermittent energy release,due to its more stable microstructure and fewer thermal defects compared to granite.As the temperature increases,the initiation and propagation of secondary cracks in granodiorite are suppressed to a certain extent,and the seismicity and brittleness are enhanced.The subtle differences in grain size,microscopic heterogeneity,and mineral composition of the two hot dry rocks determine the different acoustic-mechanical characteristics under heating and cooling,and the evolution trends with temperature.These findings are of great significance for the scientific and efficient construction of rock mass engineering by rationally utilizing different rock strata properties.

Large-scale physical simulation of injection and production of hot dry rock in Gonghe Basin,Qinghai Province,China

Based on the independently developed true triaxial multi-physical field large-scale physical simulation system of in-situ injection and production,we conducted physical simulation of long-term multi-well injection and production in the hot dry rocks of the Gonghe Basin,Qinghai Province,NW China.Through multi-well connectivity experiments,the spatial distribution characteristics of the natural fracture system in the rock samples and the connectivity between fracture and wellbore were clarified.The injection and production wells were selected to conduct the experiments,namely one injection well and two production wells,one injection well and one production well.The variation of several physical parameters in the production well was analyzed,such as flow rate,temperature,heat recovery rate and fluid recovery.The results show that under the combination of thermal shock and injection pressure,the fracture conductivity was enhanced,and the production temperature showed a downward trend.The larger the flow rate,the faster the decrease.When the local closed area of the fracture was gradually activated,new heat transfer areas were generated,resulting in a lower rate of increase or decrease in the mining temperature.The heat recovery rate was mainly controlled by the extraction flow rate and the temperature difference between injection and production fluid.As the conductivity of the leak-off channel increased,the fluid recovery of the production well rapidly decreased.The influence mechanisms of dominant channels and fluid leak-off on thermal recovery performance are different.The former limits the heat exchange area,while the latter affects the flow rate of the produced fluid.Both of them are important factors affecting the long-term and efficient development of hot dry rock.

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

The Hot Dry Rock(HDR)is considered as a clean and renewable energy,poised to significantly contribute to the global energy decarbonization agenda.Many HDR projects worldwide have accumulated valuable experience in efficient drilling and completion,reservoir construction,and fracture simulation.In 2019,China Geological Survey(CGS)initiated a demonstration project of HDR exploration and production in the Gonghe Basin,aiming to overcome the setbacks faced by HDR projects.Over the ensuing four years,the Gonghe HDR project achieved the first power generation in 2021,followed by the second power generation test in 2022.After establishing the primary well group in the initial phase,two directional wells and one branch well were drilled.Noteworthy progress was made in successfully constructing the targeted reservoir,realizing inter-well connectivity,power generation and grid connection,implementing of the real-time micro-seismic monitoring.A closed-loop technical validation of the HDR exploration and production was completed.However,many technical challenges remain in the process of HDR industrialization,such as reservoir fracture network characterization,efficient drilling and completion,multiple fracturing treatment,continuous injection and production,as well as mitigation of induced seismicity and numerical simulation technology.

Development status and research recommendations for thermal extraction technology in deep hot dry rock reservoirs

Based on a comprehensive review of domestic and foreign literature, this article discusses the technical difficulties and development status of enhanced geothermal system(EGS) concerning the thermal energy extraction of deep hot dry rock(HDR) reservoirs and proposes suggestions for the research focus of numerical simulation of HDR reservoir stimulation. Additionally, it summarizes the existing methods and mainstream working fluids for HDR reservoir stimulation. The article emphasizes the significance of factors such as well location, production well depth, artificial fracture orientation, and complexity in optimizing the thermal production efficiency of the EGS. Furthermore, this article delves into a detailed discussion on the influence of fracture spacing, fracture permeability,fracture length, fluid injection rate, and injected fluid temperature on the performance of the EGS. In light of the thermo-hydro-mechanical coupling challenges associated with high-temperature reservoirs, it is suggested that future research efforts should focus on investigating the impact of thermo-induced stresses on the stability of the artificial fracture network within the EGS during long-term(>30 years) circulation of hot and cold fluids.

A thermal stress loading technique for large-sized hot dry rock mechanical tests

Testing of large-sized specimens is becoming increasingly important in deep underground rock mechanics and engineering.In traditional mechanical loading,stresses on large-sized specimens are achieved by large host frames and hydraulic pumps,which could lead to great investment.Low-cost testing machines clearly always have great appeal.In this study,a new approach is proposed using thermal expansion stress to load rock specimens,which may be particularly suitable for tests of deep hot dry rock with high temperatures.This is a different technical route from traditional mechanical loading through hydraulic pressure.For the rock mechanics test system of hot dry rock that already has an investment in heating systems,this technology may reduce the cost of the loading subsystem by fully utilizing the temperature changes.This paper presents the basic principle and a typical design of this technical solution.Preliminary feasibility analysis is then conducted based on numerical simulations.Although some technical details still need to be resolved,the feasibility of this loading approach has been preliminarily confirmed.

Enhance liquid nitrogen fracturing performance on hot dry rock by cyclic injection

Producing complex fracture networks in a safe way plays a critical role in the hot dry rock (HDR) geothermal energy exploitation. However, conventional hydraulic fracturing (HF) generally produces high breakdown pressure and results only in single main fracture morphology. Furthermore, HF has also other problems such as the increased risk of seismic events and consuption of large amount of water. In this work, a new stimulation method based on cyclic soft stimulation (CSS) and liquid nitrogen (LN2) fracturing, known as cyclic LN2 fracturing is explored, which we believe has the potential to solve the above issues related to HF. The fracturing performances including breakdown pressure and fracture morphology on granites under true-triaxial stresses are investigated and compared with cyclic water fracturing. Cryo-scanning electron microscopy (Cryo-SEM) tests and X-ray computed tomography (CT) scanning tests were used for quantitative characterization of fracture parameters and to evaluate the cyclic LN2 fracturing performances. The results demonstrate that the cyclic LN2 fracturing results in reduced breakdown pressure, with between 21% and 67% lower pressure compared with using cyclic water fracturing. Cyclic LN2 fracturing tends to produce more complex and branched fractures, whereas cyclic water fracturing usually produces a single main fracture under a low number of cycles and pressure levels. Thermally-induced fractures mostly occur around the interfaces of different particles. This study shows the potential benefits of cyclic LN2 fracturing on HDR. It is expected to provide theoretical guidance for the cyclic LN2 fracturing application in HDR reservoirs.

A 3D analysis of the occurrence of fractures in hot dry rock reservoirs based on the spatial distribution of natural fractures

Hot dry rock(HDR)geothermal energy is a kind of widely distributed clean energy with huge reserves.However,its commercial development has been constrained by reservoir stimulation.In the early stage of HDR geothermal energy development,properly determining spatial distribution patterns of natural fractures in HDR reservoirs can effectively guide reservoir stimulation.This study analyzes the spatial distribution of natural fractures by using FracMan software based on the actual geological data and log data of well M-2 in the Matouying Uplift area,Hebei Province.The fracture parameters are counted and Monte Carlo simulation technique is introduced to optimize the parameters,which makes the natural fracture model more accurate and reliable.Furthermore,this study simulates hydraulic fracturing using the model combined with the actual in-situ stress parameters and the construction scheme.As verified by fitting the changes in simulated wellhead pressure during hydraulic fracturing with the actual wellhead pressure data detected during construction,the methods for natural fracture modeling used in this study are scientific and reasonable.The preliminary prediction results show that the displacement design scheme with a pump displacement of 2.0-3.0 m^(3)/min,4.0-5.5 m^(3)/min and 6-7 m^(3)/min in the early,middle and late stages,respectively,has good fracturing effect.The results of this study can be utilized as a reference for preparing development schemes for HDR reservoirs.

Lessons learned from hydrothermal to hot dry rock exploration and production

In this paper,we investigate geothermal exploration and production in 189 hydrothermal projects and 42 hot dry rock projects around the world.The hydrothermal fields for a working hydrothermal system to generate electricity should have the elements of heat source,water-saturated porous or fractured reservoir,caprock,heat transfer pathway,and good heat preservation condition and geothermal power energy intensity of 10-20 MW per km^(2)within at least 5 km^(2)area in tectonically active region.The hot water or steam flow rate in this hydrothermal system is normally larger than 40 L/s with temperature of 150℃or above.The power generated from enhanced geothermal system(EGS)in hot dry rock projects are generally less than 2 MW because the flow rate in most cases is much less than 40 L/s even with the hydraulic fractures using the modern stimulation technology learned from the oil and gas industry.The natural fracture in the subsurface is generally beneficial to the hydraulic fracturing and heat recovery in the hot dry rock.Moreover,the hydraulic fracture parameters,injection rate and well spacing,drilling strategy should be properly designed to avoid the short-circuit between injector and producer and low heat productivity.In the future,CO^(2)enhanced geothermal recovery associated with CO^(2)sequestration in the high temperature oil,gas,and geothermal fields maybe a good choice.On the other hand,both nearreal-time seismic monitoring to limit the pumping rate and the closed-loop of the Eavor-Loop style system without hydraulic fracture can contribute greatly to heat recovery of hot dry rocks and mitigate the risks of the hydraulic fracturing induced earthquake.Furthermore,the hybrid solar and geothermal system performs better than the stand-alone geothermal system.

Heat Aggregation Mechanisms of Hot Dry Rocks Resources in the Gonghe Basin, Northeastern Tibetan Plateau

Hot dry rock(HDR) is an important geothermal resource and clean energy source that may play an increasingly important role in future energy management. High-temperature HDR resources were recently detected in deep regions of the Gonghe Basin on the northeastern edge of the Tibetan Plateau, which led to a significant breakthrough in HDR resource exploration in China. This research analyzes the deep temperature distribution, radiogenic heat production, heat flow, and crustal thermal structure in the Qiaboqia Valley, Guide Plain, and Zhacanggou area of the Gonghe Basin based on geothermal exploration borehole logging data, rock thermophysical properties, and regional geophysical exploration data. The results are applied to discuss the heat accumulation mechanism of the HDR resources in the Gonghe Basin. The findings suggest that a low-velocity layer in the thickened crust of the Tibetan Plateau provides the most important source of constant intracrustal heat for the formation of HDR resources in the Gonghe Basin, whereas crustal thickening redistributes the concentrated layer of radioactive elements, which compensates for the relatively low heat production of the basal granite and serves as an important supplement to the heat of the HDR resources. The negative effect is that the downward curvature of the lithospheric upper mantle caused by crustal thickening leads to a small mantle heat flow component. As a result, the heat flows in the Qiaboqia Valley and Guide Plain of the Gonghe Basin are 106.2 and 77.6 m W/m2, respectively, in which the crust-mantle heat flow ratio of the former is 3.12:1, indicating a notably anomalous intracrustal thermal structure. In contrast, the crust-mantle heat flow ratio in the Guide Plain is 1.84:1, which reflects a typical hot crust-cold mantle thermal structure. The Guide Plain and Zhacanggou area show the same increasing temperature trend with depth, which reflects that their geothermal backgrounds and deep high-temperature environments are similar. These results provide important insight on the heat source mechanism of HDR resource formation in the Tibetan Plateau and useful guidance for future HDR resource exploration projects and target sites selection in similar areas.

The first power generation test of hot dry rock resources exploration and production demonstration project in the Gonghe Basin,Qinghai Province,China

Hot dry rock(HDR)is a kind of clean energy with significant potential.Since the 1970s,the United States,Japan,France,Australia,and other countries have attempted to conduct several HDR development research projects to extract thermal energy by breaking through key technologies.However,up to now,the development of HDR is still in the research,development,and demonstration stage.An HDR exploration borehole(with 236℃ at a depth of 3705 m)was drilled into Triassic granite in the Gonghe Basin in northwest China in 2017.Subsequently,China Geological Survey(CGS)launched the HDR resources exploration and production demonstration project in 2019.After three years of efforts,a sequence of significant technological breakthroughs have been made,including the genetic model of deep heat sources,directional drilling and well completion in high-temperature hard rock,large-scale reservoir stimulation,reservoir characterization,and productivity evaluation,reservoir connectivity and flow circulation,efficient thermoelectric conversion,monitoring,and geological risk assessment,etc.Then the whole-process technological system for HDR exploration and production has been preliminarily established accordingly.The first power generation test was completed in November 2021.The results of this project will provide scientific support for HDR development and utilization in the future.

China First Discovered Massive Available Hot Dry Rock Resources

Hot Dry Rock （HDR） refers to those generally deeply buried impermeable metamorphic or crystallized biotite gneiss, granite and granodiorite rock bodies, with a burial depth greater than 2000 m, containing no water or vapor. HDR is as implied, hot and dry, with temperatures ranging from 150 to 650℃, and it can be utilized as a heat resource. Because this type of heat system does not require porosity, permeability and fluids and is widespread at the drillable depth range of 10 kin, it is thereby an infinite resource type. By contrast, thermal power generation results in copious pollution; wind power is irregular and unstable; solar energy occupies large areas.

Hydraulic Conductivity Characteristics of Hot Dry Rocks under Different Fracture Modes

Enhanced geothermal system(EGS) is an effective method for developing and utilizing hot dry rock(HDR). The key to the effectiveness of EGS is the construction of an artificial fracture network. The permeability of fractures has severe effects on the heat transfer efficiency and sustainability of geothermal energy. However, the evolution characteristics of hydraulic conductivity under different failure modes have not been adequately studied for HDR. To clarify this, rocks with different failure modes were investigated by conducting thermal triaxial compression experiments, and the fluid seepage related to different rock failure modes was comprehensively investigated. The results showed that the characteristic stresses and crack surface roughness of the rock increased as the confining pressure increased. The permeability in the composited failure mode was the largest(11.4 μm;), followed by that in the Y-shaped shear failure mode(9.7 μm;), and that in the single-shear failure mode was the smallest(7.2 μm;). The confining pressures had an inhibitory effect on permeability. As the confining pressure increased from 5 to 30 MPa, the permeability decreased by 88.8%, 88.4%, and 89.9%, respectively. In contrast, the permeability was significantly enhanced by 128.3%, 94.6%, and 131% as the flow rate increased from 3 to 7 m L/min.

Fracture Network Volume Fracturing Technology in High-temperature Hard Formation of Hot Dry Rock

It is more difficult for a hot dry rock to form a fracture network system than shale due to its special lithology, physical and mechanical properties under high temperature. The essential characteristics, rock mechanics and in-situ stress characteristics of a hot rock mass have been systematically studied by means of laboratory tests and true tri-axial physical simulation. The fracture initiation and propagation characteristics under different geological and engineering conditions are physically simulated, and the main controlling factors for the formation of a complex fracture network are revealed. The technology of low displacement for enhancing thermal cracking, gel fluid for expanding fracture and variable displacement cyclic injection for increasing a fracture network has been applied in the field, and good results have been achieved. Microseismic monitoring results demonstrate that complex fractures were formed in the field test, and the stimulation volume for heat exchanging reaches more than 3 million cubic meters. The research results play an important role in the stimulation technology of an enhanced geothermal system(EGS) and realize a breakthrough for power generation.

Hydraulic Fracturing-induced Seismicity at the Hot Dry Rock Site of the Gonghe Basin in China

Hot dry rock is becoming an important clean energy source. Enhanced geothermal systems(EGS) hold great promise for the potential to make a contribution to the energy inventory. However, one controversial issue associated with EGS is the impact of induced seismicity. In August 2019, a hydraulic stimulation experiment took place at the hot dry rock site of the Gonghe Basin in Qinghai, China. Earthquakes of different magnitudes of 2 or less occurred during the hydraulic stimulation. Correlations between hydraulic stimulation and seismic risk are still under discussion. Here, we analyze the hydraulic stimulation test and microseismic activity. We quantify the evolution of several parameters to explore the correlations between hydraulic stimulation and induced seismicity, including hydraulic parameters, microseismic events, bvalue and statistical forecasting of event magnitudes. The results show that large-magnitude microseismic events have an upward trend with an increase of the total fluid volume. The variation of the b-value with time indicates that the stimulation experiment induces small amounts of seismicity. Forecasted magnitudes of events can guide operational decisions with respect to induced seismicity during hydraulic fracturing operations, thus providing the basis for risk assessment of hot dry rock exploitation.

Progress on Heat Transfer in Fractures of Hot Dry Rock Enhanced Geothermal System

Hot Dry Rock(HDR)is the most potential renewable geothermal energy in the future.Enhanced Geothermal System(EGS)is the most effective method for the development and utilization of HDR resources,and fractures are the main flow channels and one of the most important conditions for studying heat transfer process of EGS.Therefore,the heat transfer process and the heat transfer mechanism in fractures of EGS have been the hot spots of research.Due to the particularity of the mathematical models of heat transfer,research in this field has been at an exploratory stage,and its methods are mainly experimental tests and numerical simulations.This paper introduces the progress on heat transfer in fractures of Hot Dry Rock EGS in detail,provides a comparative analysis of the research results and prospects for future research directions:It is suggested that relevant scholars should further study the mathematical equations which are applicable to engineering construction of seepage heat transfer in irregular fractures of the rock mass,the unsteady heat transfer process between multiple fractures of the rock mass and the heat transfer mechanism of the complex three-dimensional models of EGS.

Micro-seismic Event Detection of Hot Dry Rock based on the Gated Recurrent Unit Model and a Support Vector Machine

Micro-seismic monitoring is one of the most critical technologies that guide hydraulic fracturing in hot dry rock resource development. Micro-seismic monitoring requires high precision detection of micro-seismic events with a low signal-to-noise ratio. Because of this requirement, we propose a recurrent neural network model named gated recurrent unit and support vector machine(GRU;VM). The proposed model ensures high accuracy while reducing the parameter number and hardware requirement in the training process. Since micro-seismic events in hot dry rock produce large wave amplitudes and strong vibrations, it is difficult to reverse the onset of each individual event. In this study, we utilize a support vector machine(SVM) as a classifier to improve the micro-seismic event detection accuracy. To validate the methodology, we compare the simulation results of the short-term-average to the long-term-average(STA/LTA) method with GRU;VM method by using hot dry rock micro-seismic event data in Qinghai Province, China. Our proposed method has an accuracy of about 95% for identifying micro-seismic events with low signal-to-noise ratios. By ignoring smaller micro-seismic events, the detection procedure can be processed more efficiently, which is able to provide a real-time observation on the types of hydraulic fracturing in the reservoirs.

Numerical Simulation for Hot Dry Rock Geothermal Well Temperature Field

Study on temperature distribution simulation during cementing of hot dry rock (HDR) geothermal well is rare. It has important guiding significance to simulate the construction process of temperature distribution of hot dry rock on site construction. Based on numerical simulation of HDR considering heat-fluid-solid coupling, the influence of temperature distribution on well cementing is analyzed when the drilling fluid cycles and reaches stable state, respectively, and when the cement slurry is injected during the cementing process. It is found that the seepage at the well bottom accelerates the flow velocity of wellbore;the stable temperature change is less than the cyclic temperature change;and the upper and lower temperature variation of the stratum is greater when the cement slurry is injected. Therefore, as to cement retarder involved, the influence of temperature variation on concretion should be considered during cementing of the hot dry rock geothermal well.

马头营干热岩开采试验场地人工注水诱发地震探讨

结合马头营干热岩区7个临时测震台站的地震观测资料和M1井2022-06~09的加压注水数据,研究地震活动与注水压力、水流量的关系,同时分析诱发地震震源机制解及其对周边应力场的影响。结果表明,注水作业可能诱发了201个地震,震源深度主要集中在0~4 km,这个区段也是地围压较小的安全区域,发生中强震的几率较小;水流量、水压力的改变不仅影响诱发地震的时空分布特征,而且影响地震活动频度,应力积累到一定水平诱发地震发生,之后水流量、水压力与地震频度呈现正相关关系;注水量和震级之间关系不明显,压力和诱发地震震级呈线性相关;在持续注水量达到11 821 m^(3)、压力达到23.46 MPa时,有断层失稳、诱发地震的可能;最大诱发地震的震源机制解为走滑型,主压应力NNW向,与华北区域应力分布有一定偏差,可见M1井开采对局部应力分布有影响;从地震震中位置迁移规律推测,有新的地裂缝产生。

基于综合物探的济阳坳陷潜山型干热岩靶区优选

济阳坳陷蕴藏着丰富的地热资源,开展深层干热岩的系统评价对于坳陷地热资源综合利用具有重要意义。本文利用收集的1∶20万重力数据38870 km^(2)、1∶5万航磁数据26630 km^(2)和胜利油田以往实施的钻井、地震勘探资料,采用Parker-Oldenburg反演迭代算法、重力-地震联合剥层方法、重磁正反演等方法,开展莫霍面、居里面、壳断裂-岩石圈断裂、基底构造和基底岩性等地热地球物理研究。通过实施大地电磁测深剖面116 km,验证了壳幔过渡带熔融体发育特征。从居里面深度、莫霍面深度、地温梯度、热储层岩性、盖层厚度、潜山规模6个方面评价优选了5处干热岩靶区。济阳坳陷具“壳幔生热、构造控热、潜山储热、盖层保温”四位一体地热成因特征,是以“传导型干热-水热共存、对流型水热补充”为主要特征的盆地复合型地热系统,潜山型干热岩靶区具有进一步勘探和开发的前景。

济阳坳陷深层地热资源研究进展与下步展望

济阳坳陷地热资源分布范围广、资源潜力大,是胜利油区未来地热资源开发利用的重要方向。为科学、高效地开发利用济阳坳陷地热资源,系统梳理了关于济阳坳陷水热型和干热岩型地热资源的热储层、热盖层、热源等地热地质条件,分别计算了2类地热资源的资源量,揭示其地热资源潜力。研究结果表明,济阳坳陷水热型地热资源主要赋存在馆陶组、东营组及下古生界等,其中馆陶组为水热型地热资源潜力最大的层系,分布广、资源量大,是下步重点勘探开发利用层系。济阳坳陷属于“冷壳热幔型”沉积盆地,具有较高的大地热流值,干热岩型地热资源热储层主要为太古宇的岩浆岩和变质岩,深部地幔热流是其主要热源,热储层分布面积大,温度均较高,其资源丰度为渤海湾盆地中最高。济阳坳陷地热资源丰富,为加快其开发利用步伐,建议水热型地热资源要加强地热资源与油气的关系及资源分区评价2项关键工作;干热岩型地热资源的勘探开发仍处于起步探索阶段,今后需要重点加强其热储层类型的选择与热源机制2个方面的研究。