A Study of Progressive Failure in Porous Rocks Using Numerical and Experimental Modeling

ABSTRACT: Physical mechanisms governing natural and induced earthquakes are still poorly understood and, despite the relevance, this lack of knowledge limits the usefulness of Geoenergy projects and severely undermines their societal acceptance. Due to the difficulty in retrieving direct measurements and observations to understand these important processes, we study laboratory acoustic emissions in combination with numerical models to shed light on the physics governing seismicity. A dry sample of Berea sandstone was tested to failure under a confining pressure of 40 MPa in a triaxial machine (LabQuake). Using in-house developed conical-type piezo-electric transducers, which are absolutely calibrated and exhibit a flat, broadband response between 100 kHz and 1.5 MHz, we are able to investigate the generated acoustic emission clouds by locating the single events and by inverting for their moment tensor solutions. Furthermore, numerical simulations are performed using a finite difference two-dimensional continuum-based fully coupled thermo-mechanical visco-elasto-plastic numerical modeling tool. We are able to image and locate regions of high stress and strain and, thus, to identify potential nucleation zones of seismic events and compare them at a first order with our experimental results. 1 INTRODUCTION Many scientists and engineers are seeking strategies and alternatives to counteract climate changes. Among others, green energies are being investigated by the research community; Geoenergy related applications, such as enhanced geothermal systems (EGS), are thought to have a critical role (e.g., Scheck-Wenderoth et al., 2013; Lu, 2018; Wu and Li, 2020). However, the injection or extraction of fluids in the subsurface is a well-known cause of perturbation of the effective stress of a rock formation (e.g., Hubbert and Rubey, 1959), which may eventually lead to induced seismicity (Majer et al., 2007). In the last decades, various geothermal reservoir projects were the cause of some damaging earthquakes, which subsequently gave rise to the abandonment of such projects (e.g., Deichmann and Giardini, 2009; Grigoli et al., 2018). The societal acceptance of geothermal projects is thus severely influenced by such induced earthquakes (Trutnevyte and Wiemer, 2017; Grigoli et al., 2017) which, furthermore, are source of outstanding economic impact on the society (Mignan et al., 2015; Trutnevyte and Wiemer, 2017). Despite significant research efforts, the physical mechanisms governing induced seismicity are still not entirely understood and this lack of knowledge severely limits the potential of Geoenergy applications.