Abstract
Gravity measurements in the Upper Rhine Graben evidence spatial variations at the regional scale and close to the geothermal sites. They are classically interpreted as linked to the geology. We aim to bring new insights on another potential origin of these gravity changes. Our approach is to quantify gravity anomalies related to the deep hydrothermal circulation. A thermo-hydro-mechanical model is developed at the reservoir scale for the Soultz-sous-Forêts site (Soultz), France. A finite element method is used in 2D and 3D. The size of the representative elementary volume is assumed to be 100 m with no regional fault in the reservoir. Surface gravity profiles and maps associated to the large-scale hydrothermal circulation are computed from the variations of the effective density through the model. Synthetic spatial gravity variations in 2D and 3D models are shown to have an amplitude of 0.02 mGal. They are shown to be mostly linked to the convective system. Their wavelength is about 7.5 km, consistent with the width of the hydrothermal convection cells. The anomaly maximum is located at the top of the maximum surface heat flux. However, gravity anomaly observations show much higher amplitude and heterogeneity. Spatial gravity variations linked to the hydrothermal circulation are shown to be smaller than the observed gravity spatial variations, but still measurable with very sensitive instruments (absolute gravimeters).
Highlights
Comparison between the 2D and 3D models of hydrothermal circulation The simulated circulation has been obtained after inversion of the observed temperature and stress–depth profiles at Soultz and Rittershoffen site (Vallier et al 2018, 2019)
The simulated hydrothermal circulation from the 3D model is compared to the results of the 2D approach
The hydrothermal circulation occurs in the upper granitic reservoir as well as in most of the sediments
Summary
Measurements of the gravity anomalies in the Upper Rhine Graben (URG) have been conducted since the past century for potash and oil exploration and more recently for deep geothermal projects (Haas and Hoffmann 1929; Edel et al 1982; Rousset et al 1993; Papillon 1995; Rotstein et al 2006; Huenges and Ledru 2011; Baillieux et al 2013). The origin of the gravity changes is classically interpreted as an effect of lithology variations in the sedimentary cover and the basement (Edel et al 1982; Rotstein et al 2006; Papillon 1995). Vallier et al Geotherm Energy (2020) 8:13 Other contributions such as the hydrothermal circulation have been investigated to explain the origin of the spatial gravity changes in the URG. These recent studies include hybrid gravimetric surveys (Portier et al 2018) or reinterpretations of the current potential method database (Baillieux et al 2014; Abdelfettah et al 2018). The filtering shows fault parallel negative anomalies linked to regional faults identified as fluid pathways by geochemical tracers and MT measurements
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