Is There a Link Between Mineralogy, Petrophysics, and the Hydraulic and Seismic Behaviors of the Soultz‐sous‐Forêts Granite During Stimulation? A Review and Reinterpretation of Petro‐Hydromechanical Data Toward a Better Understanding of Induced Seismicity and Fluid Flow

Abstract

In the framework of the European Soultz‐sous‐Forêts enhanced geothermal system (EGS) in Alsace, France, 20 years of scientific and preindustrial tests had to be performed before the site began production of electricity in 2008. Stimulation tests were designed to enhance the permeability because most of the numerous natural fractures that crosscut the granite body were sealed by secondary minerals that crystallized as an effect of the circulation of local hot brines. The deep‐seated granitic reservoir is located between 4,500 and 5,000 m depths. Hydraulic stimulations were conducted in the four deep wells (GPK1, GPK2, GPK3, and GPK4) inducing different microseismic event patterns, which cannot be explained by tectonic structures alone. In the present work, we provide a review of the hydraulic tests and reinterpret them in the light of mineralogical data obtained along the boreholes. A clear relationship appears between mineralogy (mainly clay and calcite content) and the petrophysical, mechanical, and hydraulic behaviors of the rock mass. High calcite contents are correlated with an abundance of clay minerals, low Young's modulus, low magnetic susceptibility, and variation in spectral gamma ray. Microearthquakes are generated in the fresh granite zones, while clay and calcite‐rich zones, linked with hydrothermal alteration, might behave aseismically during hydraulic stimulations. These findings highlight the importance of a detailed knowledge of the petrography of a reservoir to conduct an effective stimulation while keeping the seismic hazard at a minimum.

Estimating in situ rock mass strength and elastic modulus of granite from the Soultz-sous-Forêts geothermal reservoir (France)

“…9b). These low rock mass elastic modulus values are within a similar range as the wireline elastic modulus values for the fractured rock mass shown in Meller and Ledésert (2017).…”

“…We also recommend that core and wellbore characterisation be undertaken such that fracture frequency, orientation, infilling and roughness for natural fractures are collected at a resolution that allows computation of the rock mass strength and stiffness criteria. Finally we recommend that wireline techniques, such as gamma ray (Bigelow 1992), be used to explore the possibility of identifying clay infilling signatures for joint condition assessment (as in Meller and Ledésert 2017).…”

“…It has been characterised, for example, in terms of its fracture density, structure, alteration, thermal conductivity, and permeability (e.g. Rummel 1992; Ledésert et al 1993Ledésert et al , 1999Ledésert et al , 2010Genter and Traineau 1996;Dezayes et al 2000Dezayes et al , 2010Surma and Géraud 2003;Sausse et al 2006Sausse et al , 2010Bartier et al 2008;Géraud et al 2010;Vidal et al 2017;Meller and Ledésert 2017). However, there are few published data on the mechanical behaviour, strength and stiffness (elastic modulus) of the URG reservoir granite rock mass.…”

“…These values are crucial input parameters for reservoir-scale modelling (e.g. Bérard and Cornet 2003;Yoon et al 2014;McClure and Horne 2013 and references therein) and guiding stimulation design (e.g., Cipolla et al 2008;Meller and Ledésert 2017). We selected the widely used generalised Hoek-Brown failure criterion (Hoek et al 2002) to estimate the strength of the rock mass as a function of depth.…”

How do the geological and geophysical signatures of permeable fractures in granitic basement evolve after long periods of natural circulation? Insights from the Rittershoffen geothermal wells (France)

“…In the GRT-2 well, 240 cutting samples collected in the basement were thoroughly observed and investigated in terms of primary and secondary mineralogy. According to the previous studies on the Soultzsous-Forêts geothermal site, the hydrothermal alteration grades in the granite are well known from core observations (Ledésert et al 1999;Meller and Ledésert 2017;Sausse et al 2006;Traineau et al 1992). The core mineralogy and hydrothermal alteration grades were cross-referenced with the minerals observed in the cuttings, although the texture information and the mineral assemblage were not reflected in the cuttings (Fig.…”