The Influence of Stress Heterogeneity on Hectometer Scale Hydraulic Stimulation Experiments

ABSTRACT: From 2022 to 2023, a series of hydraulic stimulation experiments were conducted at the Bedretto Underground Laboratory in Switzerland (BedrettoLab) to study the creation of an engineered geothermal reservoir in crystalline rock. A 400 m long stimulation borehole was divided into 14 intervals using a multi-packer system. During the volume characterization, an azimuthal rotation of borehole breakouts was observed, indicating a stress perturbation caused by a major fault zone intersecting the experimental volume. In this study, we present the seismo-hydromechanical observations from seven hydraulic stimulations in four selected intervals. This is the first systematic comparison of repeated stimulations in different intervals at the BedrettoLab. Two of these intervals are neighboring the fault zone, while the other two are located further away. We utilize data from a dense multiparameter monitoring network to show that the stress perturbation affects the reactivation of similarly oriented pre-existing fractures. In our example, the stress perturbation caused by the fault zone prevents the reactivation of structures in the nearby interval, while similar structures are clearly reactivated in the three other intervals. Reactivation pressures from pressure-flow rate plots agree with the pressures encountered at the onset of seismicity. Given that the prevailing fractures are well oriented for shear reactivation within the far field stress field, hydraulic shearing is considered the probable reactivation mechanism. 1 INTRODUCTION Interest in engineered geothermal systems (EGS) as a low emission, renewable energy source (Lu, 2018; Aghahosseini and Breyer, 2020) has grown since the late 2000s. In Central Europe, EGS reservoirs with sufficiently high temperatures are located at depths of several kilometers, where the permeability of the crystalline basement rocks is insufficient for geothermal energy extraction. Permeability enhancement is achieved through hydraulic stimulation, which can involve hydraulic shearing of natural fractures or shear zones, hydraulic fracturing of intact rock, or a combination of both (McClure and Horne, 2014). This permeability enhancement goes hand in hand with induced seismicity, which can reach damaging levels if large fault zones are reactivated (e.g. Deichmann and Giardini, 2009; Evans et al., 2012; Ellsworth et al., 2019). To address this challenge, several scaled-down in situ hydraulic stimulation experiments have been conducted at decameter scale in underground research laboratories in representative crystalline rock types (e.g. Amann et al., 2018; Zimmermann et al., 2019; Schoenball et al., 2020; Fu et al., 2021).