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.

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.

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.

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.

Development and application of turbodrills in hot dry rock drilling

The strata of hot dry rock(HDR) are usually igneous rocks with high temperature which are challenging for drilling activities. This paper deals with the key technology and research of turbodrills, introduced the application of turbodrills in HDR drilling at home and broad, and analyzed the field application cases. With the advancement such as low speed high torque turbodrill, reduction turbodrills, independent spindlesection and PDC bearings and so on, the application of turbodrills has widely expanded. The application of high-temperature turbodrills in Fenton Hill, HDR geothermal wells, high-temperature formation in Tahe oilfield all proved that turbodrills are the best downhole motors in deep high-temperature HDR drilling, thus they deserve further research and generalization.

Overview on hydrothermal and hot dry rock researches in China

Geothermal energy is a precious resource,which is widely distributed,varied,and abundant.China has entered a period of rapid development of geothermal energy since 2010.As shallow geothermal energy promoting,the depth of hydrothermal geothermal exploration is increasing.The quality of Hot Dry Rock (HDR)and related exploratory technologies are better developed and utilized.On the basis of geothermal development,this paper reviews the geothermal progress during the "12th Five-Year Plan",and summarizes the achievements of hydrothermal geothermal and hot dry rocks from geothermal survey and evaluation aspects.Finally,the authors predict the development trend of the future geothermal research to benefit geothermal and hot dry rock research.

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.

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.

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.

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.

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.

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.

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.

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.

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.

美国FORGE计划犹他州干热岩开发示范项目进展综述

21世纪是全球能源获取方式发生重要变化的时代。现有以煤炭、石油、天然气等传统资源为基础的能源正在逐渐被风能、太阳能、生物质能、地热能等环境友好、低排放的绿色能源所取代。干热岩是一种不含原生水(或含少量水但不能流动)的高温地热资源,可通过水力刺激改造形成增强型地热系统储层,从而提取数量相当可观的地热能。本文以美国能源部正在组织实施的FORGE计划犹他州干热岩示范项目为研究对象,综述了项目在场地地质条件表征、基础设施建设等方面取得的主要进展与成果,总结了项目在高温硬岩钻井、储层建造、微震监测等方面取得的广泛且深入认识,从统筹部署、组织实施、场地表征和设施建设、关键技术装备突破、数据共享等方面提出了值得我国借鉴参考的具体实践。

内蒙古中部干热岩地热资源成因机制研究

内蒙古中部地区处于板块结合部位,经历了多期构造活动和岩浆活动,浅层水热型地热资源丰富,但对其深部干热岩地热资源仍缺少深入研究。基于区域地质与构造背景、大地电磁探测结果与地热异常显示,系统探讨内蒙古中部地区干热岩地热资源成因机制与地球动力学过程,建立干热岩成因模式。研究表明:内蒙古中部地区深层干热岩型地热资源的热源主要为深部局部熔融体和残余高温岩浆囊,热流通道包括板块缝合带、区域深大断裂带及其交汇部位、次级断裂及塑性流变韧性剪切带等壳内薄弱层;干热岩储层主要为新生代基性侵入岩,即辉绿岩和辉长岩体,被高温岩浆和侵入体加热的花岗岩类也可作为研究区潜在干热岩储层;干热岩区域性盖层为白垩系、新近系和第四系沉积地层。西伯利亚板块与华北克拉通板块之间的大陆碰撞、拼合,形成易于破坏的碰撞带,晚中生代-新生代以来的古太平洋板块对华北板块的西向俯冲作用,导致岩石圈底部熔融,岩石圈伸展、减薄,地幔软流圈热物质上涌,并伴随强烈的新生代断裂与断陷活动,共同导致晚中生代-新生代岩浆、火山活动强烈。内蒙古中部地区存在3种聚热模式:Ⅰ.新生代辉绿岩、辉长岩储层高温干热岩系统,为研究区优势干热岩储层;Ⅱ.新生代以前花岗岩储层中高温干热岩系统,为潜力干热岩储层;Ⅲ.浅部碎屑岩、花岗岩或变质岩储层水热系统,为浅层水热型优势储层。内蒙古中部地区浅部高温水热型地热系统与深部干热岩地热系统存在同源共生关系,浅层高温异常区的圈定对于深部干热岩的发现具有重要的指示意义。

渤海湾盆地干热岩开发利用前景评估——基于开采优化数值模拟的认识

渤海湾盆地的大地热流高,5000 m埋深地层平均温度为175℃,热储岩性以低孔低渗的变质岩、火成岩为主,具备形成干热岩资源的条件。基于大地热流、岩石热导率、生热率等热参数,利用COMSOL软件建立三维水热耦合的干热岩开采模型、分析不同井间距、注采速率、布井方式等差异开采方案下在100 a内对热储层温度的影响随开发时间的变化,选取最优方案并估算干热岩资源。结果表明:注采速率一定时,随着开采时间的增加,开采井水温度下降速率与井间距成反比;当注采井间距一定时,注采速率越大,开采井水温度下降越快,发生“热突破”的时间越早;其他条件相同的情况下,“两采两注”布井方案比“一采一注”布井方案获得的热量更多,开采效率更高。基于上述认识,确定研究区最优开采方案为:年限50 a、井间距400 m、注采量90 m^(3)/h、“两采两注”方式。此方案下,可获得开采井水平均温度为172℃,对应全渤海湾盆地可采资源量为3.28×10^(19)J/a。以河北任丘市为例,按照民用住宅热负荷指标100 W/m^(2)计算,利用最优方案进行干热岩的开采,仅需157.75 km^(2)干热岩有利区即可满足全市居民供暖需求。因此,开发利用研究区干热岩资源,可增强华北地区能源供应保障能力,打造可持续发展的绿色低碳能源体系。

冷冲击作用下干热岩井井筒裂纹扩展数值模拟研究

干热岩(HDR)是一种清洁可再生能源,主要通过增强型地热系统(EGS)进行开发。在EGS工程中,无论是注入井还是生产井的建设都需要采用地热钻井技术,而高温高压钻井过程中地层破裂、井壁坍塌等现象是干热岩钻井施工面临的重要问题。在温差作用下,岩石的矿物颗粒由于其热膨胀、冷收缩特性的差异性,矿物颗粒之间产生温度应力,导致岩体发生热破裂。本文借助RFPA数值模拟软件,对干热岩开采过程中井筒花岗岩冷冲击作用下的裂纹扩展进行研究。结果表明:井筒模型在冷冲击过程中,岩石表面拉应力随着冷冲击时间的增加,先升高至峰值后缓慢下降。裂纹扩展可以大致分为前、中、后期。冷冲击前期,井筒周围出现环形拉应力区,开始出现均匀的微小裂纹。冷冲击中期,随着时间的增加,拉应力区逐渐向井筒外围扩展,裂纹随着拉应力区向外扩展。冷冲击后期,拉应力大小逐渐降低直至小于模型抗拉强度,裂纹扩展速度减缓直至停止扩展。围压、井径、温度对井筒围岩冷冲击时的破坏损伤效果影响显著,其中,温度对冷冲击裂纹扩展起到促进作用,围压对冷冲击裂纹扩展起到抑制作用,井径对冷冲击裂纹扩展起到促进作用。

增强地热系统停止运行后温度恢复过程的数值模拟

在增强地热系统(EGS)运行过程中,干热岩(HDR)人工储层的温度会迅速下降,外界的热量补充速度远远跟不上系统热量提取的速度,同时也会对人工储层的流体压力产生巨大的影响,EGS停止运行以后人工储层温度和压力场会逐渐的恢复到初始状态。前人对干热岩开采过程中的温度场和流体压力场变化做了大量研究,但很少关注停止开采以后人工储层的温度恢复过程和流体压力场变化,而停止开采以后的温度场恢复过程对于新的地热井选址和废弃地热井的重新利用具有重要意义。利用数值模拟软件COMSOL Multiphysics建立双井EGS运行过程和停止运行之后两个阶段的二维热-孔隙流体耦合数值模型,模型考虑了放射性生热的贡献和不同热导率的影响。研究表明:在EGS运行期间,热对流是热量交换的主要方式,人工储层温度下降过程很快,19 a后EGS已经到达运行寿命,68 a后HDR人工储层已经失去开采价值。而地热开采停止以后,人工储层温度的自然恢复过程长达万年,热传导是主要的热量传递和温度恢复方式,热导率越高,人工储层温度恢复越快。