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。     

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。     

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。     

西藏羊易EGS开发储层温度场与开采寿命影响因素数值模拟研究

西藏羊易地区具有丰富的地热能,单井开发潜力接近10 MW,对其深部热储进行EGS开采,可缓解西部能源紧缺问题。本文建立二维理想EGS开发模型,探讨深层地热开采过程中开采流量、注采方式、注入温度等参数对热储温度场分布及开采寿命的影响。基于羊易温度信息设计了12个数值模型,对比研究发现,开采流量对EGS开采的影响较大,为保证开采50年内的商业利用价值,最大开采流量应控制在0.028 kg/s以下;考虑到钻井成本,注采方式的选择以高注高采和中注高采为最佳;注入温度对热储开采影响较小,可选择40℃ ~ 80℃之间任意温度的地热尾水进行回灌,实现地热资源梯级利用。     

增强型地热系统垂直裂隙热储热开采过程数值模拟

增强型地热系统(Enhanced Geothermal System,EGS)作为开采深层地热资源最为有效的方法已经成为国际研究热点。充分探究EGS运行时热储内热量的开采过程对评估EGS性能及今后EGS商业开采过程中工程优化控制有着重要意义。文章建立了平行相间的垂直裂隙系统EGS开采模型,运用FLUENT软件对多平行垂直裂隙情形下增强型地热系统热储热开采过程进行了数值模拟。同时,通过改变对热储热开采过程有影响的裂隙宽度和水流速度两个参数,对比研究了其对热储热开采过程的影响。研究结果显示,裂隙宽度和水流速度对热储热开采过程影响较大,且影响效应几乎一致。当裂隙宽度为1 mm、裂隙水流速度为1 cm/s时,开采20 a时间内无论是裂隙宽度扩大1倍还是水流速度提高1倍,对热储内经济可用热能的开采率提升均超过25%,对热储内热能开采速率提升达到252%。     

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。     

地热回灌井间压差补偿对回灌效率影响的分析

以天津滨海新区塘沽地区(533km2)馆陶组孔隙性地层为研究重点,建立地热热储概念模型和数学模型。利用TOUGH2软件拟合研究区内地热井的历史数据,模拟结果与监测数据吻合较好。在此基础上,进一步研究在60%和100%回灌率下,采灌井距从700m减至250m时地热流体的温度和压力变化。结果表明:随着井距的逐渐减小,开采井压力略有上升;当采灌井的间距由700m逐步减小至500m时,地热流体温度无明显变化;而当井距进一步缩小至250m时,发生热突破,地热流体温度在3～5a后出现明显下降。因此,综合考虑对井回灌压力补偿作用和温度场的影响,提出孔隙型热储地热采灌井的间距不宜小于500m。     

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。     

30/38柴油机活塞热力耦合有限元分析研究

采用有限元软件ANSYS,分析了30/38柴油机活塞的温度场、热应力及热变形;并通过热-机耦合的方法,分析了该机活塞的耦合应力场。研究结果表明:活塞顶部应力以热应力为主,温度对活塞的应力和变形起主导作用。在此基础上提出了有关活塞的研制建议。     

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.     

A novel three-dimensional transient model for subsurface heat exchange in enhanced geothermal systems

Understanding the subsurface heat exchange process in enhanced geothermal systems (EGS) is crucial to the efficiency of heat extraction and the sustainable utilization of geothermal reservoir. In the present work we develop a novel three-dimensional transient model for the study of the subsurface heat exchange process in EGS. The novelty of this model is embodied by a couple of salient features. First, the geometry of interest physically consists of multiple domains: open channels for injection and production wells, the artificial heat reservoir, and the rock enclosing the heat reservoir, while computationally we treat it as a single-domain of multiple sub-regions associated with different sets of characteristic properties (porosity and permeability etc.). This circumvents typical difficulties about matching boundary conditions between sub-domains in traditional multi-domain approaches and facilitates numerical implementation and simulation of the complete subsurface heat exchange process. Second, the heat reservoir is treated as an equivalent porous medium of a single porosity, while we consider thermal non-equilibrium between solid and fluid components and introduce two sets of heat transfer equations to describe the heat advection and conduction for fluid in rock apertures and the heat conduction in rock matrix, respectively, thus enabling the simulation and analysis of convective heat exchange between rock matrix and fluid flowing in the apertures. Case study with respect to an imaginary EGS demonstrates the validity and capability of the developed model.     

注入温度对增强型地热系统性能的影响研究

基于考虑增强型地热系统(EGS)流体损失的三维传热传质数值模型,将热提取速率定义为与注入温度无关的函数,对比分析注入温度对以CO₂为工质的EGS(CO₂-EGS)和以水为工质的EGS(H₂O-EGS)的性能的影响。结果发现:对于CO₂-EGS,较高的注入温度抑制热开采和CO₂损失(即CO₂封存);对于H₂O-EGS,较高的注入温度促进热开采,同时导致较大的水损失。当热提取速率被大部分研究者定义为与注入温度相关的函数时,注入温度对CO₂-EGS和H₂O-EGS的热开采性能的影响将分别被高估和低估。     

On production behavior of enhanced geothermal systems with CO2 as working fluid

Numerical simulation is used to evaluate the mass flow and heat extraction rates from enhanced geothermal injection–production systems that are operated using either CO₂ or water as heat transmission fluid. For a model system patterned after the European hot dry rock experiment at Soultz, we find significantly greater heat extraction rates for CO₂ as compared to water. The strong dependence of CO₂ mobility (=density/viscosity) upon temperature and pressure may lead to unusual production behavior, where heat extraction rates can actually increase for a time, even as the reservoir is subject to thermal depletion. We present the first ever, three-dimensional simulations of CO₂ injection–production systems. These show strong effects of gravity on the mass flow and heat extraction due to the large contrast of CO₂ density between cold injection and hot production conditions. The tendency for preferential flow of cold, dense CO₂ along the reservoir bottom can lead to premature thermal breakthrough. The problem can be avoided by producing from only a limited depth interval at the top of the reservoir.     

Fluid circulation and heat extraction from engineered geothermal reservoirs

A large amount of fluid circulation and heat extraction (i.e., thermal power production) research and testing has been conducted on engineered geothermal reservoirs in the past 15 years. In confined reservoirs, which best represent the original Hot Dry Rock concept, the flow distribution at any given time is primarily determined by three parameters: (1) the nature of the interconnected network of pressure-stimulated joints and open fractures within the flow-accessible reservoir region, (2) the mean pressure in the reservoir, and (3) the cumulative amount of fluid circulation—and therefore reservoir cooling—that has occurred. For an initial reservoir rock temperature distribution and mean fluid outlet temperature, the rate of heat extraction (i.e., thermal power) is at first only a function of the production flow rate, since the production temperature can be expected to remain essentially constant for some time (months, or even years). However, as reservoir circulation proceeds, the production temperature will eventually start to decline, as determined by the mean effective joint spacing and the total flow-accessible (i.e., heat-transfer) volume of the reservoir. The rate of heat extraction, which depends on the production flow rate, can also vary with time as a result of continuing changes in the flow distribution arising from reservoir cooling.The thermal power of engineered reservoirs can most readily be increased by increasing the production flow rate, as long as this does not lead to premature cooldown, the development of short-circuit flow paths, or excessive water losses. Generally, an increase in flow rate can be accomplished by increasing the injection pressure within limits. This strategy increases the driving pressure drop across the reservoir and the mean reservoir pressure, which in turn reduces the reservoir flow impedance by increasing the amount of joint dilation. However, the usefulness of this strategy is limited to reservoir operating pressures below the fracture extension pressure, and may lead to excessive water losses, particularly in less-confined reservoirs. Under such conditions, a downhole production-well pump may be employed to increase productivity by recovering more of the injected fluid at lower mean reservoir operating pressures.     

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.     

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。     

Effects of heat extraction on fracture aperture: A poro–thermoelastic analysis

Poroelastic and thermoelastic effects of cold-water injection in an enhanced (or engineered) geothermal system (EGS) are investigated by considering flow in a pre-existing fracture in a hot, rock matrix that could be permeable or impermeable. Assuming plane fracture geometry, expressions are derived for changes in fracture aperture caused by cooling and fluid leak-off into the matrix. The corresponding induced pressure profile is also calculated. The problem is analytically solved for the cases pertaining to a constant fluid injection rate with a constant leak-off rate. Results show that although fluid loss from the fracture into the matrix reduces the pressure in the crack, the poroelastic stress associated with fluid leak-off tends to reduce the aperture and increase the pressure in the fracture. High rock stiffness and low fluid diffusivity cause the poroelastic contraction of the fracture opening to slowly develop in time. The maximum reduction of aperture occurs at the injection point and become negligible near the extraction point. The solution also shows that thermally induced stress increases the fracture aperture near the injection point and, as a result, the fluid pressure at this point is greatly reduced. The thermoelastic effects are particularly dominant near the inlet compared to those of poroelasticity, but are pronounced everywhere along the fracture for large times. Although poroelasticity associated with leak-off does not change the fracture aperture significantly for low permeability rocks, it can lead to pore pressure increase and cause nearby fractures to slip.