World Geothermal Congress 2020+1
March - October, 2021

From Brittle to Ductile Deformation in the Continental Crust: Mechanics of Crystalline Reservoirs and Implications for Hydrothermal Circulation

Mateo ACOSTA, Benoit GIBERT, and Marie VIOLAY

[Ecole Polytechnique Fédérale de Lausanne, Switzerland]

Supercritical Geothermal Systems (SGS) and Enhanced Geothermal Systems (EGS) are virtually unlimited, clean and sustainable, sources of electric power. Aside from the current technology costs, their widespread development has yet been limited mainly due to 1) the difficulties of assessing hydrothermal circulation at depths close to the brittle-to ductile transition (BDT) in the case of SGS and 2) due to induced seismicity during deep reservoir stimulation in the case of EGS. Here, we present the results of novel laboratory experiments conducted in tri-axial stress conditions under high confining pressure (Pc=100-130 MPa), high temperature (300-1000 °C), and high-fluid pressure (Pf=25-30 MPa) on Westerly Granite (WG) samples. We first studied the mechanics of the BDT through experiments performed on intact WG cylinders (Pc= 130 MPa; T= 600 – 1000 °C) where the porosity and permeability evolution were measured during deformation (Violay et al., 2017). In these experiments, we observe that, at ambient temperatures from 600 to 800 °C the failure mode was brittle and the deformation was dilatant (increase in porosity with increasing deformation). At 900 °C, we observe a transition to compactant behavior (decrease in porosity with increasing deformation) due to an increase in the efficiency of crystal plastic processes but still with an overall brittle failure mode (localized shear fracture formation). Finally, at 1000 °C, the deformation is compactant with an overall ductile failure mode (no strain localization, but pore space reduction and strong evidence of crystal plastic processes). Once the BDT was identified, we studied the mechanics of brittle failure by reproducing EGS thermo-hydro-mechanic conditions (Pc= 100 MPa; T= 300-500 °C, Pf= 0-25 MPa) on saw-cut samples (Acosta et al., 2018). There, we studied the influence of fluid pressure, fluid chemistry, and ambient temperature on fault mechanics of dry, and water-and-argon pressurized samples that were reactivated by reproducing tectonic loading. Such fault reactivation experiments allow for the study of simplified fault systems which reproduce the mechanical behavior of idealized single fractures in EGS reservoirs. At a given effective pressure (Pc-Pf= 100 MPa) and ambient temperature (300°C) we observe that dry experiments fail at lower differential stresses (Δσ0=250 MPa) than Argon (Δσ0=300 MPa) and Water pressurized experiments (Δσ0=360 MPa). When studying experiments run at a higher ambient temperature (500 °C), we observe a temperature weakening of the samples so they fail at lower differential stresses. Such observation has strong implications for hydraulic transport in faults and associated seismicity. Our results suggest that the presence and nature of fluids present in the brittle crust will influence pre- and post-failure behavior of SGS and EGS reservoir rocks. At the BDT, there is a change between dilatant and compactant deformation of deep reservoir crystalline rocks. This implies that, the deeper the reservoir is located into the BDT, the lower the reservoir permeability. In that case, hydrothermal circulation will be diminished without other applied stimulations. In addition, because shear deformation of fault zones is usually related to dilation and increases in permeability (Kranz et al., 1979), the nature and pressure of fluids found in EGS will have strong influence on the reservoir mechanics. Our results will contribute to feasibility analysis for power extraction in both SGS and EGS across the brittle to ductile domains. Through laboratory experiments, we open the door to the study of coupled mechanical deformation and hydraulic transport properties as a function of depth in the continental crust

        Topic: Geophysics Paper Number: 13079

         Session 16C: Geophysics 7 -- Seismic Monitoring 3 - Supercritical Reservoirs [Tuesday 11th May 2021, 12:00 pm] (UTC-8)
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