Slip Tendency Evaluation of Fracture Systems Associated with Seismicity at the Hellisheiði Geothermal Field, Iceland

Abstract

ABSTRACT An understanding of 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, which features direct coupling between different physics for the rock matrix, fractures, and their interactions, as well as indirect coupling through changes of material properties, such as stress-dependent rock and fracture permeabilities, was developed. The model was applied to simulate the geothermal fluid extraction and re-injection over a 10-year period (2011-2021) at the Hellisheiði geothermal field, utilising field recorded monthly production and re-injection rates. Based on the model results, the slip tendency of regional fracture systems was examined under reservoir conditions before and after the start of fluid re-injection. Results have shown that fracture networks act as preferential fluid flow paths that influence fluid pressure and stress distribution and fracture slip tendency in geothermal reservoirs. NE-SW and N-S trending fractures are susceptible to slippage before the start of fluid re-injection, and the distribution of fractures with enhanced slip tendency shifts from surrounding the re-injection region at the onset of fluid re-injection, to a two-lobed pattern in the fault-normal direction around the re-injection region in the long term. INTRODUCTION Fluid extraction and injection-induced seismicity has become a major concern for deep geothermal exploitation. A clear understanding of the complex coupled behaviour involved in the geothermal fluid extraction and cold fluid re-injection process is crucial to mitigate against induced seismicity. In industrial geothermal operations, regional fracture systems that form preferential fluid flow pathways have a significant impact on heat extraction efficiency (Shi et al. 2019). Regional fracture systems also exert fundamental controls on the induced seismic occurrence (Jiang et al. 2022). The slip-tendency analysis for fractures provides a rapid evaluation of stress in terms of its potential to cause slip, or seismic susceptibility in seismic-prone regions (Morris et al. 1996; Moeck et al. 2009). However, fractures implemented in coupled numerical models have mostly been simplistic major fractures (Salimzadeh et al. 2018; Chen et al. 2022) or statistically generated discrete fractures (Gan and Elsworth 2016; Li et al. 2019; Wang et al. 2019; Sun et al. 2021), rather than field realistic fracture systems. On the other hand, conventional seismic susceptibility evaluation methods for realistic fracture systems or fault structures, such as deterministic and probabilistic fault slip potential analysis, were solely based on fluid overpressure, rather than coupled numerical models that reflect other causal mechanisms such as poroelastic and thermoelastic stressing (Walsh III and Zoback 2016; Hennings et al. 2021).