Abstract
The investigation of permeable hydrothermally altered and fractured zones and their distribution is a key issue for the understanding of fluid circulation in granitic rocks, on which the success of geothermal projects relies. Based on the use of short-wave infrared (SWIR) spectroscopy applied to rock cuttings coupled with interpretation of geophysical logs, we propose an investigation of the clay signature of fault and fracture zones (FZ) inside the granitic basement. This methodology was applied to two geothermal wells: GRT-2 from the Rittershoffen and GPK-1 from the Soultz-sous-Forêts (Soultz) geothermal sites, both located in the Upper Rhine Graben (URG). A total of 1430 SWIR spectra were acquired and analysed. Variations in the 2200 nm absorption band area are correlated with hydrothermal alteration grades. The 2200 nm absorption band area is found to reflect the illite quantity and its variations in the granitic basement. Low, stable values are observed in the unaltered granite facies, showing good reproducibility of the method, whereas scattered high values are associated with high hydrothermal alteration and FZs. Variations in the 2200 nm absorption band area were correlated with the gamma ray and electrical resistivity logs. This procedure allowed us to confirm that illite mainly controls the resistivity response except inside the permeable FZs, where the resistivity response is controlled by the geothermal brine. Thus, the architecture of these permeable FZs was described precisely by using a combination of the 2200 nm absorption band area data and the electrical resistivity log. Moreover, by correlation with other geophysical logs (temperature (T), porosity, and density), paleo-permeable and currently permeable FZs inside the reservoir were distinguished. The correlation of SWIR spectroscopy with electrical resistivity logs appears to be a robust tool for geothermal exploration in granitic reservoirs in the URG.
Highlights
The mid-Carboniferous granitic basement of the Upper Rhine Graben (URG) has been affected by several extensional and compressional tectonic phases that developed a multiscale fracture network during the Tertiary [1, 2]
X-ray diffraction (XRD) measurements of the GPK-1 well’s cuttings were performed in this study; the results showed the presence of illite in fractured and altered zones as well as the presence of chlorite and biotite in hydrothermal alteration grades—low (HLOW) and GRAN
This petrographical log presents three major units: reddish, oxidized granite at the top of the granitic basement, an altered zone from 2535 to 3060 m measured depth (MD) with a highly altered core from 2737 to 2875 m MD, and unaltered granite at the bottom of the well from 3060 to 3196 m MD. These main sections correlate well with the short-wave infrared (SWIR) values; more precisely, we can distinguish four sections in which the variations in the 2200 nm absorption band area are indicative of the alteration grades around the permeable fracture zones (FZ)
Summary
The mid-Carboniferous granitic basement of the Upper Rhine Graben (URG) has been affected by several extensional and compressional tectonic phases that developed a multiscale fracture network during the Tertiary [1, 2]. In the URG, the current targets for geothermal projects are hydrothermal fractured reservoirs in granitic rocks lying under a thick sedimentary cover [13]. Benefitting from lessons learned during the Soultz project and gained on several deep geothermal wells in Northern Alsace, we decided to avoid drilling subvertical wells in nearly vertical fracture systems to maximize the intersection of permeable FZs. a new drilling approach consisting of targeting deviated well trajectories crosscutting highly dipped FZs in the granitic basement was developed. A new drilling approach consisting of targeting deviated well trajectories crosscutting highly dipped FZs in the granitic basement was developed These FZs can act either as permeable conduits and as paleo-permeable barriers for natural fluid transport (Figure 1). Due to primary mineral dissolution and/or secondary mineral precipitation related to fluid-rock interaction, the resulting permeability of FZs can increase or even decrease, increasing the challenge of obtaining successful geothermal wells [14,15,16,17,18]
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