增强型地热系统开采过程中热储渗透率对温度场的影响

以西藏羊易地热田的温度信息为依据,假想激发不同渗透率的EGS热储,采用数值模拟的方法,观察开采50 a内系统温度场分布,分析热储的可持续开采能力、冷却影响范围等。共设计了9个EGS开采案例,根据模拟结果的温度场分布形状,可将模型划分为极高、高、低渗透率3种类型。结果表明,高渗透率模型在开采过程中的温度降低幅度不大,50 a后开采点温度为270℃,热储仍具有开采潜力,此案例适用于对热储可持续性和后期热恢复要求较高的地热开采;低渗透率模型在开采过程中出现了大面积低于100℃的冷却区域,模拟结束后开采点的温度基本不变,此案例适用于对地热能开采稳定性要求较高的情况;极高渗透率模型的开采寿命只有20 a。     

增强型地热系统热开采性能的数值模拟分析

增强型地热系统(EGS)作为一种极富发展潜力的可再生清洁能源利用技术,正逐渐成为世界各国新能源发展的重点关注方向之一。EGS地下采热过程直接影响EGS的产能和寿命。文章使用一套自主开发的三维动态数值模型对不同地质条件下双井EGS的长期热开采运行进行了模拟,额外引入平均产热速率和地热开采率作为采热性能评价指标,结合EGS运行寿命和产热速率综合分析了热储渗透率、循环流体流量和周围热储岩石的热补偿对采热的影响及其作用机理。     

热储分层影响EGS采热的数值模拟研究

由于岩石构造不同、天然裂隙的差异以及压裂过程的随机性等因素,增强型地热系统（EGS）人工热储通常具有较强的非均质性。探究热储的非均质性对EGS热开采过程的影响,对EGS性能预测与分析评价有重要意义。论文考虑到热储沿深度方向的非均质性,基于等效分层多孔介质物理模型,并使用自主开发的EGS数值模型,模拟了多个具有分层热储EGS的长期运行过程,发现热储深度方向上非均质性对热能的开采影响显著,而流量分布的不均匀性是导致系统采热性能下降的主要原因。为了方便分析和评价,我们建立一种新的定量化描述热储非均质性的方法,然后基于更多的非均质热储EGS算例结果,拟合得到EGS采热性能与热储非均质性的定量关系式。     

增强型地热系统热开采过程的数值模拟研究

增强型地热系统（EGS）是指采用人工方法在地下3 ~ 10 km内的干热岩体中形成储层、通过灌输采热流体以开采出干热岩中热能用于地面发电的地热利用系统,是一种极富潜力的可再生清洁能源利用技术。循环流体在地下热储中的流动与换热对EGS的采热性能有重要影响。本文首先对EGS数值模型进行了综合评述,然后基于一套自主开发的三维瞬态数值模型模拟了不同渗流条件下EGS地下热储内的热流过程。通过对模拟结果的分析,揭示了均匀压裂的人工热储中流体短路的形成机理,并通过对比双井和三井系统中流场和局部地热开采率分布,结合当前钻井工艺和裂隙激发技术水平,探讨了抑制流体短路、优化EGS采热性能的可能方案。     

地下热流固耦合对EGS热开采的影响

人工热储的孔隙率及渗透率在增强型地热系统（EGS）地下热开采过程中受温度（T）、水力（H）、应力（M）的综合影响。本文建立了EGS热开采过程THM耦合的三维计算模型,并采用局部非热平衡假设处理液岩对流换热。对一理想的五口井EGS系统采热过程进行了THM模拟计算,分析了岩石温度、孔隙压力对岩石应力场的作用机理,进一步研究了应力场对EGS采热性能的影响。结果表明,开采过程中岩石应力场为热储内孔隙压力和温差综合作用的结果,由孔隙压力造成的岩石应力为压应力,仅集中于注入井附近,由岩石温度变化引起的热应力为拉应力,随着热开采区域的扩展而扩展。液−岩温差是触发工质与岩石热交换的动因,同时也是产生热应力的根本。     

土酸注入速率对EGS热储层改造效果影响的实验研究

增强型地热系统(Enhanced Geothermal System,EGS)是一种通过人工形成地热储层的方法从低渗透性岩体中经济地开采相当规模深层热能的工程,对渗透率的改造体现了EGS热储层改造效果。文章进行了实验室条件下的岩心流动实验,选用4组不同土酸注入速率进行对比,根据注酸前后岩心裂隙渗透率的变化,分析研究土酸的注入速率对岩心裂隙渗透率的影响。研究结果表明,采用"高流量"的注入方式更有利于凝灰岩裂隙通道的改造。     

地热开采的裂隙渗透T-H-M耦合模型及模拟研究

地热开采是典型的热流固耦合问题。文章以裂隙岩体地热开采为背景,推导出含热储温度变量的孔隙率、渗透率方程,由此建立含裂隙热储的温度、渗流和变形场全耦合数学模型。利用上述模型求解了含简化裂隙热储的地热开采问题,得出裂隙开度等参数对热储温度场和地热产能的影响规律。研究结果表明:注入井和生产井之间的裂隙岩体是地热资源的主要供给区域;天然裂隙开度或人工裂隙开度是控制热储温度,提高地热开采效率的主要因素,裂隙开度扩大1倍,地热产能增加近6倍。     

U型井取热不取水地热开采潜力及影响因素分析

取热不取水井筒闭循环采热工艺是开采地热水低产区、地热尾水回灌难地区地热资源的有效方式。为了研究U型井式闭循环地热系统对中低温砂岩地热资源的可持续开采能力,文章以河北任县已有的地热地质数据和短期供暖数据为基础,使用井-储耦合模拟程序T2WELL对U型井采热能力进行数值分析,探究U型井式闭循环地热系统对该研究区地热供暖的可持续潜力。此外,定量分析了不同关键工程参数对U型井提热能力的影响,以指导设计合理的U型井取热不取水地热开采方案。结果表明：U型井地热开采系统的产流温度和提热功率在同一个供暖季内随时间降低,在连续多个供暖季内也随时间降低,其变化趋势为先快后慢。基于文章模型研究,确定U型井式闭循环地热系统最适合的注采流量、回灌温度、水平井长度分别为60 m3/h,10℃和400 m。     

增强型地热系统中液-岩化学作用数值模拟研究

增强型地热系统（Enhanced Geothermal System, EGS）利用深层岩石中连通的裂隙网络进行流体工质循环,从而实现地热能的持续开采。EGS运行时循环流体工质会与深层岩石产生化学反应,引起岩石中矿物的溶解/沉积,使热储中的裂隙网络形貌产生动态变化,对地下流动与传热过程造成影响。本文分析了EGS中液–岩化学作用特点,详细阐述了在多孔介质热流动模型中耦合入液–岩化学反应的方法,基于已开发成功的EGS传热传质数值模型初步建立了传热–流动–化学（Thermal-Hydraulic-Chemical, THC）多场耦合数值模型,并使用该模型对五井布局EGS的长期运行过程进行了模拟分析,模拟时仅考虑方解石在水流体中溶解和沉积。模拟结果显示,循环流体的注入温度以及注入流体中的矿物离子浓度的设定十分重要。如果二者没有达到较为合适的“平衡”,就会导致注入井附近渗透率和孔隙率的持续变化,对EGS的导流能力造成极大影响。     

增强型地热系统井筒-储层耦合数值模拟分析

干热岩是指地下3~10 km处低渗透性的高温岩体。增强型地热系统(EGS)是利用水力压裂等作业措施形成人工热储层,通过注入载热流体以经济地开采出干热岩中热能的人工地热开采系统。目前,关于干热岩储层开采潜力是否满足商业开采目标,以及如何提高EGS开采潜力是EGS研究的重点。文章首先对EGS的发展及技术可行性进行了概述,然后以松辽盆地为研究场地,以水为载热工质,采用井筒-储层耦合数值模拟程序T2WELL对储层开采潜力进行了定量研究,并通过不确定因素和参数分析探讨了优化EGS开采潜力的可行方案。不确定因素和参数分析表明,储层初始温度、裂隙间隔、布井方式是影响储层开采潜力的关键因素,渗透率对储层开采潜力影响较小。     

Influence of different fracture morphology on heat mining performance of enhanced geothermal systems based on COMSOL

Formation of enhanced geothermal systems (EGS) is the necessary approach to obtain geothermal energy efficiently. In-situ stress, nature of reservoir physical properties and fracturing methods will affect the artificial fracture morphology after reservoir stimulation. A three-dimension thermal coupled seepage model of fractured media was established to simulate the influence of fracture morphology on heat mining performance of EGS, considering the pressure- and temperature-dependent physical properties of working medium. The results indicate that formation of complex fracture network is favorable for heat mining. Production mass flow in Case1 with complex fracture network enhances nearly 2.5 times comparing to the unenhanced model at exploitation beginning. The total net energy rate will up to 44 MW and be maintained above 10 MW for 5 years. The system impedance can be effectively reduced, however the sustainable heat mining duration decreased to 30 years. The increase in length and number of branch fractures is expected. While increasing the width of branch fractures deliberately has little effect on the exploitation of EGS. Finally, we investigate the adaptability of employing supercritical CO₂ in EGS with complex fracture network. Production mass flow will be enhanced 3–5 times compared with water, but the stability is poor, total net energy rate decrease from 90 MW to 3 MW over the 10-year operation period.     

增强型地热系统水力压裂与双井连通实验仿真研究

增强型地热系统（Enhanced Geothermal System, EGS）作为未来新能源和清洁能源利用的一个重要方向,受到了世界各国的广泛关注。一直以来,野外试验场的工程实践和数值模拟分析是进行EGS研究的两种主要方式。本文通过实验室规模的小型试验系统,对EGS的水力压裂、裂隙监测、生产井定位和注水测试进行了仿真,成功实现了注入井−热储层−生产井的水力连通,分别以定井口压力和定注水流量进行水力测试。试验结果表明,热储层的裂隙开度会随着水力特性而发生变化,注水压力较大时热储层的裂隙具有更大的开度和渗流能力。从提升热储层经济性的角度考虑,实践中应当在较大注水压力时对热储裂隙结构进行加固处理。     

Modeling the coupling between free and forced convection in a vertical permeable slot: Implications for the heat production of an Enhanced Geothermal System

The aim of the hydraulic stimulations in the Soultz-sous-Forêts, France, Enhanced Geothermal System (EGS) project was to create, in crystalline rocks, a fractured reservoir 750 m high, 750 m long and 35 m thick interconnecting the injection and production wells. Increasing the permeability in a zone with a high geothermal gradient will trigger free convection, which will interact with the forced flow driven by pumping. A systematic numerical study of the coupling between forced and free convective flows has been performed by considering a large range of injection rates and Rayleigh numbers. The simulations showed that if there is weak or no free convection in an EGS reservoir, economic exploitation of the system will rapidly end because of a decrease in produced fluid temperature. The maximum injection rate preventing such a temperature drop increases with the Rayleigh number and the height of the stimulated domain. The model establishes constraints on the conditions for achieving optimal heat extraction at the Soultz-sous-Forêts EGS site. It was also shown that, although mineral precipitation may partially close or heal some open fissures, it does not lead to a major decrease of the hydraulic conductivity in the stimulated reservoir.     

苏尔士增强型地热系统的开发经验及对我国地热开发的启示

增强型地热系统是采用人工形成地热储层的方法,从低渗透性岩体中经济地采出相当数量深层热能的人工地热系统。法国苏尔士（Soultz）地热项目已有20多年开发研究历史,但前人尚未对开发过程中的关键问题进行深入探讨,对其成功经验也未进行系统总结归纳。本文通过回顾其发展历程,总结该项目在钻井、储层激发、水力循环测试和储层监测方面的成功经验,同时提炼出地热开发中遇到的储层建设和井下泵设备等方面的问题,并指出数值模拟在地热开发过程应用方面的启示。苏尔士地热项目开发吸取了其他早期地热田的经验和教训,成功地建造了商业规模的人工激发储层,产生了大量的科研成果和先进技术,对后续开发的地热项目有重要指导意义。     

Multifracture response to supercritical CO2‐EGS and water‐EGS based on thermo‐hydro‐mechanical coupling method

Hydraulic‐fracturing treatments have become an essential technology for the development of deep hot dry rocks (HDRs). The deep rock formation often contains natural fractures (NFs) at micro and macroscales. In the presence of the NF, the hydraulic‐fracturing process may form a complex fracture network caused by the interaction between hydraulic fractures and NF. In this study, analysis of carbon dioxide (CO₂)‐based enhanced geothermal system (EGS) and water‐based EGS in complex fracture network was performed based on the thermo‐hydro‐mechanical (THM) coupling method, with various rock constitutive models. The complexity of the fracture geometry influences the fluid flow path and heat transfer efficiency of the thermal reservoir. Compared with CO₂‐based EGS, water‐based EGS had an earlier thermal breakthrough with a rapid decline in production temperature. CO₂ can easily gain heat rising its temperature thus reducing the effect of a premature thermal breakthrough. Both CO₂‐based EGS and water‐based EGS are affected by in‐situ stress; the increase in stress ratio improved the fracture permeability but resulted in an early cold thermal breakthrough. When the same injection rate is applied to water‐based EGS and CO₂‐based EGS, water‐based EGS displayed higher injection pressure buildup. Water‐based EGS had higher reservoir deformation area than CO₂‐based EGS, and thermoelastic constitutive model for water‐based EGS showed larger deformed area ratio than thermo‐poroelastic rock model. Furthermore, higher values of rock modulus accelerated the reservoir deformation for water‐based EGS. This study established a novel discussion investigating the performance of CO₂‐based EGS and water‐based EGS in a complex fractured reservoir. The findings from this study will help in deepening the understanding of the mechanisms involved when using CO₂ or water as a working fluid in EGS.     

Pressure analysis of the hydromechanical fracture behaviour in stimulated tight sedimentary geothermal reservoirs

Hydromechanical phenomena in fractured sediments are complex. They control the flow in stimulated tight sediments and are crucial for the exploitation of geothermal energy from such rocks. We present the analysis of a cyclic water injection/production (huff–puff) process, a promising method to extract geothermal energy from tight sedimentary reservoirs. It uses a single borehole, which considerably reduces investment costs. A huff–puff test was performed in a 3800-m deep sedimentary formation (borehole Horstberg Z1, Lower Saxony, Germany). The analysis presented herein explains the downhole pressure measurements by a simplified reservoir model containing a single vertical fracture. The model addresses the flow behaviour between the fracture and the rock matrix in a layered formation, and the coupling between fluid flow and the mechanical deformation of the fracture. The latter aspect is relevant to predict the efficiency of the geothermal reservoir because cooled regions resulting from a particular injection/production scheme can be identified. The analysis methods include: (1) the curve-fitting code ODA used for a determination of different flow regimes (radial or linear), (2) an analytical solution for the calculation of the injection pressure, assuming a time-dependent fracture area, and (3) the simulator ROCMAS, which numerically solves the coupling between fluid flow and fracture deformation. Whereas each single approach is insufficient to explain the complete test data, a combination of the results yields an understanding of the flow regimes taking place during the test.     

Life cycle assessment of geothermal binary power plants using enhanced low-temperature reservoirs

Geothermal binary power plants that use low-temperature heat sources have gained increasing interest in the recent years due to political efforts to reduce greenhouse gas emissions and the consumption of finite energy resources. The construction of such plants requires large amounts of energy and material. Hence, the question arises if geothermal binary power plants are also environmentally promising from a cradle-to-grave point of view. In this context, a comprehensive Life Cycle Analysis (LCA) on geothermal power production from EGS (enhanced geothermal systems) low-temperature reservoirs is performed. The results of the analysis show that the environmental impacts are very much influenced by the geological conditions that can be obtained at a specific site. At sites with (above-) average geological conditions, geothermal binary power generation can significantly contribute to more sustainable power supply. At sites with less favorable conditions, only certain plant designs can make up for the energy and material input to lock up the geothermal reservoir by the provided energy. The main aspects of environmentally sound plants are enhancement of the reservoir productivity, reliable design of the deep wells and an efficient utilization of the geothermal fluid for net power and district heat production.     

Electricity generation using a carbon-dioxide thermosiphon

There is an opportunity to expand the baseload geothermal electricity generation capacity through the development of engineered geothermal systems (EGS). Carbon dioxide (CO₂) could be used as an alternative to water to extract heat from these systems considering its advantages of ease of flow through the geothermal reservoir, strong innate buoyancy that permits the use of a thermosiphon rather than a pumped system over a large range of fluid flow rates, and lower dissolution of materials that lead to fouling. However, the thermodynamics of EGS using CO₂ to extract heat from subsurface rock masses is not well understood. Here we show that the wellbore frictional pressure losses are the dominant factor in CO₂-based EGS. Wellbore friction is the major limiter on the amount of energy that can be extracted from the reservoir by CO₂, as measured by the exergy available at the surface. The result is that CO₂ is less effective at energy extraction than water under conditions similar to past EGS trials. Nevertheless, CO₂ can perform well in lower permeability reservoirs, or if the wellbore diameter is increased. Our results demonstrate that CO₂-based EGS need to be designed with the use of CO₂ in mind. We suggest this work to be a starting point for analysis of the surface infrastructure and plant design and economics of CO₂-based EGS.