增强地热系统中干热岩水力剪切压裂THMC耦合研究

发布时间：2018-06-29 22:15

本文选题：增强地热系统 + 干热花岗岩岩　；参考：《西南石油大学》2017年博士论文

【摘要】：地热是储存于地壳岩石、蒸汽或天然流体中,由同位素连续衰变提供的可重复利用、清洁且绿色的能源,地壳3km深度范围内热值储量约为42.7×106EJ,其中80%来自于增强地热系统(EGS)中的干热岩,其热值约为化石能源的300倍。利用水力压裂改造干热岩岩体原有天然裂缝,构建流体过流和换热的裂缝网络通道,是开发该类能源的主要技术手段。干热岩具有力学强度高、基质无渗透性的物理特性,水力压裂过程中天然裂缝又受到热应力、水力剪胀和化学溶蚀的综合影响,往往导致注采循环中的“注不进、采不出”。因此,本论文以干热岩天然裂缝水力剪切机理为基础,建立岩体弹-塑性本构模型,系统开展热力-水力-岩体变形-化学溶蚀(Thermal-Hydro-Mechanical-Chemistry:THMC)的耦合问题和诱导应力作用下的岩体基质剪切破裂稳定性研究。本论文根据深层干热花岗岩岩心测试,分析其力学、形变和破坏特征,结合地热井压裂开发实例,阐明了天然裂缝的水力剪切理论。同时考虑天然裂缝受剪条件下峰前、峰后的差异性永久形变特征,分别建立了描述裂缝法向剪胀、切向滑移的峰前弹性和峰后弹-塑性本构方程。揭示出裂缝粗糙度系数(JRC)是影响裂缝长期导流能力的关键,峰后的塑性滑移和剪胀占形变的90%以上。提出水力剪切压裂,不仅能增大裂缝的自支撑导流开度,在摩擦弱化作用下也能降低压裂地面泵注压力的新论断。在天然裂缝缝内流动模型基础上(H),耦合干热岩体与缝内流体的传热模型(T),分析热弹性作用机理下的裂缝开度变化特征,裂缝内流体和岩体基质的温度分布特征;耦合二氧化硅溶解/沉降模型(C),分析裂缝在长期稳定增压注入条件下的溶蚀开度变化特征;结合裂缝弹塑性本构方程(M),建立了分析水力剪切压裂和长期增注过程天然裂缝开度变化的THMC耦合模型。明确了热诱导开度是近井区域天然裂缝导流能力的主要组成,远井裂缝区域的导流能力主要由水力剪胀作用提供,化学溶蚀开度对短期水力剪切压裂的导流能力增加几乎没有贡献。根据位移不连续理论,在耦合天然裂缝开度变化的基础上,建立了裂缝形变诱导应力场模型;根据岩体基质与缝内冷流体的换热模型,建立了干热岩体基质的热诱导应力场模型,以此重新构建岩体基质和未沟通天然裂缝的应力状态。根据干热岩岩体和未连通天然裂缝的稳定性条件,分析了远离被改造天然裂缝区域的岩体基质稳定性,为注、采井天然裂缝网络的沟通半径扩展提供了分析理论。证明了形变诱导应力和热诱导应力仅仅影响到压裂井口端数米范围内。本论文耦合了干热岩水力剪切压裂中的热力-水力-岩体变形-化学溶蚀,与诱导应力的结合发展和完善了地下裂缝网络理论,对增强地热系统干热岩储层开发和优化设计具有一定的理论指导意义。
[Abstract]:Geothermal is a reusable, clean and green energy source stored in rock, steam or natural fluid in the crust by isotopic decay. The heat value reserves in the 3km depth range of the earth's crust are about 42.7 * 106EJ, of which 80% are derived from the dry hot rocks in the enhanced geothermal system (EGS), with a heat value of about 300 times the fossil energy. It is the main technical means to develop the natural cracks in the dry hot rock mass, and to construct the network channel of fluid overflow and heat transfer, which is the main technical means to develop this kind of energy. The dry hot rock has high mechanical strength and no permeability, and the natural cracks are influenced by the thermal stress, hydraulic dilatancy and chemical dissolution in the hydraulic fracturing process. In this paper, the elasto-plastic constitutive model of rock mass is established on the basis of the hydraulic shear mechanism of natural fractures in the dry hot rock, and the coupling problem of the thermal hydraulic rock deformation chemical dissolution (Thermal-Hydro-Mechanical-Chemistry:THMC) and the matrix shear of rock mass under the induced stress are systematically carried out in this paper. On the basis of the deep dry hot granite core test, this paper analyses the characteristics of its mechanics, deformation and failure, and illustrates the hydraulic shear theory of natural fractures, combining with the fracture development example of geothermal well. At the same time, the characteristics of the differential permanent deformation of the natural fissure under the peak and after the peak are considered, and the cracks are described respectively. The fracture roughness coefficient (JRC) is the key to the long-term conductivity of the crack, and the plastic slip and dilation account for more than 90% of the deformation after the peak. It is proposed that the hydraulic shear fracture can not only increase the opening degree of the self supporting flow in the crack, but also under the effect of the friction weakening. On the basis of the flow model in the natural fracture seam (H), coupled with the heat transfer model of the dry hot rock mass and the fluid in the seam (T), the characteristics of the fracture opening variation under the thermoelastic mechanism, the temperature distribution characteristics of the fluid in the fracture and the matrix of the rock mass, and the coupling silica dissolution / settlement model (C) are analyzed. The characteristics of the change of the dissolution in the long-term stable pressurized injection condition are analyzed. Combining the elastoplastic constitutive equation of the crack (M), a THMC coupling model is established for the analysis of the natural fissure opening changes in the hydraulic shear fracture and the long-term injecting process. It is clear that the thermal induced opening is the main composition of the natural fissure conductivity in the near well area, and the far well fissure. The diversion capacity of the seam area is mainly provided by the hydraulic dilatancy, and the chemical dissolution has little contribution to the increase of the conductivity of the short term hydraulic shear fracture. Based on the displacement discontinuity theory, the stress field model of the fracture deformation is established on the basis of the change of the coupling natural crack opening, and the cold fluid in the seam is based on the matrix of rock mass. The heat induced stress field model of the dry hot rock matrix is established to reconstruct the stress state of the rock mass matrix and uncommunicated natural cracks. According to the stability conditions of the dry hot rock mass and the unconnected natural cracks, the stability of the matrix of rock mass far away from the reformed natural fracture area is analyzed, and the natural fracture net of the well is used as the injection. The extension of the communication radius of the collaterals provides an analytical theory. It is proved that the deformation induced stress and the thermal induced stress only affect the range of several meters at the end of the fracturing wellhead. This paper coupled the thermodynamic hydraulic rock deformation chemical dissolution in the dry hot rock hydraulic shear fracture, the combination of the induced stress and the theory of the underground fracture network. It has certain theoretical guiding significance to enhance the development and optimization design of geothermal system dry hot rock reservoir.
【学位授予单位】：西南石油大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：P314;TK52

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