Understanding fracture slip susceptibility in geothermal reservoirs is central to the control of fluid injection-induced seismicity. To investigate the role of regional fracture systems on induced seismicity, a coupled thermo-hydro-mechanical (THM) model containing fracture networks was developed, which features direct coupling between different physics for explicit fractures, fractured rocks (porous matrix blocks with small-scale fractures) and their interactions, as well as indirect coupling through changes of material properties, such as stress-dependent fracture and rock permeabilities. The model was applied to simulate geothermal fluid extraction and re-injection in a natural fracture system comprised of three dominant fracture sets at the Hellisheiði geothermal field over a 10-year period (2011–2021), utilising field recorded monthly production and re-injection rates. Based on the model results, the slip susceptibility of regional fracture systems was examined under reservoir conditions before and after the start of fluid re-injection across different time scales, i.e., over short (1 month), intermediate (1 year) and long-term (10 years). Two model scenarios, one with cooling contraction and one without, were considered to examine the relative contribution of cooling contraction and fluid overpressure on fracture slip susceptibility. Results have shown that fracture networks act as preferential fluid flow paths that influence fluid pressure and stress distribution and fracture slip susceptibility in geothermal reservoirs. NE-SW and N-S trending fractures at Hellisheiði are susceptible to slippage before the start of fluid re-injection. During fluid re-injection, the distribution of fractures with enhanced slip susceptibility gradually shifts from surrounding the re-injection region to forming a two-lobed pattern in the fault-normal direction around the re-injection region, indicating the dominant role of cooling contraction over fluid overpressure on the fracture slip susceptibility in the intermediate- and long-term.

中文翻译：

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自然裂隙岩石中地热流体开采和注入相关的裂缝滑移敏感性和地震活性


了解地热储层的裂缝滑移敏感性对于控制流体注入引起的地震活动至关重要。为了研究区域裂缝系统对诱发地震活动的作用，开发了一种包含裂缝网络的耦合热-水-机械 （THM） 模型，该模型的特点是显式裂缝、裂隙岩石（具有小规模裂缝的多孔基质块）及其相互作用的不同物理场之间的直接耦合，以及通过材料特性的变化进行间接耦合，例如应力依赖性裂缝和岩石渗透性。该模型用于模拟 Hellisheiði 地热田 10 年期间（2011-2021 年）由三个主要裂缝组组成的自然裂缝系统中的地热流体开采和回注，利用油田记录的月度生产和再注入率。基于模型结果，研究了区域裂缝系统在流体回注开始前后不同时间尺度上的储层条件下，即短期 （1 个月）、中期 （1 年） 和长期 （10 年） 的滑移敏感性。考虑了两种模型场景，一种有冷却收缩，一种没有冷却收缩，以检查冷却收缩和流体超压对裂缝滑移敏感性的相对贡献。结果表明，裂缝网络是影响地热储层流体压力和应力分布以及裂缝滑移敏感性的优先流体流动路径。Hellisheiði 的 NE-SW 和 N-S 走向骨折在液体再注射开始前容易滑脱。 在流体再注入过程中，滑移敏感性增强的裂缝分布逐渐从再注入区周围转变为在再注入区周围形成断层-法线方向的双瓣状模式，表明冷却收缩对流体超压对裂缝滑移敏感性在中长期起主导作用。