基于热-流-固全耦合的干热岩热采效率研究—以青海共和GR1井为例

Research on thermal recovery efficiency of dry hot rock based on a fully coupled thermal-hydro-mechanical model- Taking the GR1 well in Gonghe, Qinghai as an example

  • 摘要: 大力开发深层干热岩资源是缓解我国能源安全困境的钥匙,是我国实现“双碳”目标的重要抓手。干热岩处于高温、高压的复杂地质环境中,因此,发展热-流-固耦合模型,研究增强型地热系统(干热岩)各物理因素的相互作用及其对地热开采的影响具有重要的意义。本文建立了热-流-固全耦合模型,考虑了流体压力变化、温度变化引起的储层变形,受储层变形影响的储层孔隙率、渗透率和断层裂隙开度,受温度控制的流体性质(密度、粘度等)和流体流动引起的热传导。以青海共和GR1井为例,研究了定流量和定压力两种典型生产模式下,储层渗透率、断层类型对于地热资源(干热岩)开发过程的影响。研究结果表明:(1)在定压力生产模式下,阻隔型断层严重降低地热开采效率,提升热储层渗透率可以大幅度提高地热开采效率,但是过高的渗透率会造成资源浪费不利于持续性开采;(2)定流量生产模式下,地质因素对地热开采效率影响很小,开采效率主要由注水/生产速率决定;(3)从安全角度看,在定流量模式下,过小的渗透率和阻隔型断层会引起很大的孔隙压力响应,会导致一系列的施工安全问题;(4)对于青海共和地区的地热开采,综合考虑开采效率和施工安全,建议若采用定压生产,需将储层渗透率改造在1×10-13~1×10-12 m2范围内,若采用定流生产,需将储层渗透率改造为1×10-13 m2以上,且两种生产模式都应避免注水井与生产井之间存在阻隔型断层。

     

    Abstract: It is widely believed that developing deep dry hot rock resources is crucial for solving China's energy security issue and achieving its "dual carbon" goals. However, this type of energy is located in a complex geological environment with high temperature and pressure, making it challenging to extract. Therefore, it's essential to establish a thermal-hydro-mechanical coupling model that considers various physical factors to study the interaction and impact of enhanced geothermal systems (dry hot rock). This paper presents a fully coupled thermal-hydro-mechanical model, which takes into account reservoir deformation caused by fluid pressure and temperature changes, reservoir porosity and permeability, fault fracture opening that's affected by reservoir deformation, fluid properties controlled by temperature (density, viscosity, etc.), and heat conduction caused by fluid flow. The study aims to determine the effects of reservoir permeability and fault type on the development process of geothermal resources (dry hot rocks) under two typical production modes: constant flow rate and constant pressure. The research also discusses the applicability of four evaluation indicators. The results indicate that the reservoir permeability increases in the early stage of water injection in the water injection area, but decreases in the later stage. On the other hand, the reservoir permeability decreases in the pumping area due to the decrease in fluid pressure and temperature. The fault opening increases in the early stage due to the increase of fluid pressure, and decreases in the later stage due to the decrease of fluid pressure and temperature. In the constant pressure production mode, increasing the permeability of thermal reservoirs can significantly improve the efficiency of geothermal extraction. However, barrier-type faults can seriously reduce the efficiency of geothermal extraction. Under the constant flow production mode, a smaller permeability will bring higher intra-domain flow velocity, while under the condition of blocking faults, the inter-domain flow velocity is larger, and the impact of the two is not significant. Finally, the research shows that the average temperature of production wells, output heat power, heat recovery rate, and cumulative heat recovery are all applicable to constant pressure production conditions. However, for constant flow production, output heat power and heat recovery rate are more applicable.

     

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