A numerical study on the long term thermo-poroelastic effects of cold water injection into naturally fractured geothermal reservoirs
A numerical study on the long term thermo-poroelastic effects of cold water injection into naturally fractured geothermal reservoirs
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DOI：
10.1016/j.compgeo.2011.03.007
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发表时间：
2011-07-01
影响因子：
5.3
通讯作者：
Rahman, Sheik S.
中科院分区：
文献类型：
--
作者：
Koh, Joshua;Roshan, Hamid;Rahman, Sheik S.
裂隙系统和地应力对结晶岩中的流体流动和传热有重要影响。裂缝系统对有效应力变化的长期响应，特别是热应力对储层特性的长期地质力学效应是地热工业特别感兴趣的，本文提出了一个地热储层模型，其中热-孔隙弹性有限元模块耦合到裂缝地质力学模块。这将裂缝闭合描述为有效应力和参数变化的函数，如有效渗透率、孔隙度和离散裂缝孔径。这种方法的新奇在于它的动态处理的特征属性的单个裂缝在模拟流体流动和普遍的响应的天然裂缝系统冷流体注入。该模型使用成熟的经验弹性变形解来计算天然裂缝对有效应力变化的响应。该储层模型应用于典型的天然裂缝地热系统，裂缝密度为0.25 m（-1），注采井间距为565 m。计算了10年生产期内的流体流速和相应的压力损失、采出水温度、压降、有效应力和储层渗透率，研究结果表明，当热储层岩石中的热量通过渗透从热储层岩石中提取时，冰冷的液体由于岩石基质的热降深，裂缝性储层中的正常有效应力逐渐减小，使得天然裂缝打开，从而增加储层渗透率。热应力对地热储层长期渗透率的贡献是显着的。在生产期间，观察到对于给定压降的注入能力的大幅增加。还观察到由于朝向生产井的渗透率的动态和非均匀分布而导致的生产流体温度的变化。短期热效应（
The residing fracture system and the prevailing in situ stresses have a significant impact on fluid flow and heat transfer in crystalline rocks. The long term response of fracture systems to changes in effective stresses, in particular the long term geo-mechanical effects of thermal stresses on reservoir characteristics is of particular interest to the geothermal industry.In this paper, a geothermal reservoir model is presented, in which a thermo-poroelastic finite element module is coupled to a fracture geomechanical module. This describes fracture closure as a function of effective stress and the changes in parameters, such as effective permeability, porosity and discrete fracture apertures. The novelty of this approach lies in its dynamic treatment of the characteristic properties of individual fractures in simulating fluid flow and the pervasive response of the natural fracture system to cold fluid injection. The model uses well-established empirical elastic deformation solutions to calculate the response of natural fractures to changes in effective stress.The reservoir model is applied to a typical naturally fractured geothermal system with a fracture density of 0.25 m(-1) and an injector-producer separation of 565 m. Fluid flow rates and corresponding pressure losses, produced water temperature, thermal drawdown, effective stresses and reservoir permeability were calculated over a production period of 10 years, to evaluate the long term effects of cold water injection on geothermal reservoir properties.Results of this study have shown that tensile thermal stresses normal to the fracture surfaces are induced as heat is extracted from hot reservoir rock by pervading cold fluid. The gradual decrease of the normal effective stresses in the fractured reservoir due to thermal drawdown of the rock matrix allows the natural fractures to open and thus, increases reservoir permeability. The thermal stress contribution to the long term permeability of the geothermal reservoir was shown to be significant. Large increases in injectivity for a given pressure drop were observed over the course of the production period. Also observed was the variation in produced fluid temperature due to the dynamic and heterogeneous distribution of permeability towards the production well. Short term thermal effects (